---
title: Exterior Trim Materials
description: "On this page[Introduction](#)[Before You Begin](#)[Prerequisites](#)[What You Need To Know](#)[Naturally Durable Wood: What the Code Means by It](#)[Exterior Part Names: Fascia, Soffit, Frieze, Corner Board, Rake Board, Water Table, Column Base, Deck Skirt Board](#)[Cedar for Exterior Work](#)[Redwood and Cypress: Rare Here,…"
url: "https://letsbuildstuff.co/learn/exterior-trim-materials/"
updated: "2026-10-02"
category: General Carpentry
---

# Exterior Trim Materials

On this page[Introduction](#)[Before You Begin](#)[Prerequisites](#)[What You Need To Know](#)[Naturally Durable Wood: What the Code Means by It](#)[Exterior Part Names: Fascia, Soffit, Frieze, Corner Board, Rake Board, Water Table, Column Base, Deck Skirt Board](#)[Cedar for Exterior Work](#)[Redwood and Cypress: Rare Here, and What They Are For](#)[Cellular PVC Trim](#)[Polyurethane and Polystyrene Molding](#)[Fiberglass and Composite Columns: Load-Bearing versus Split-Wrap](#)[Composite Decking and Railing](#)[Aluminum Trim Coil](#)[Exterior Trim Quick Reference](#)[Related Guides](#)

[General Carpentry](/learn-category/general-carpentry/) Skill Level: [Intermediate (Level 2)](/learn/difficulty-levels/#level-2) · Estimated Time: 35 Minutes To Read

## Introduction

Exterior trim has to survive weather, so the material list outside a house looks nothing like the one inside it. The [Western Red Cedar Lumber Association](https://www.realcedar.com/) is the authority on the cedar half of exterior trim; the synthetics beside it play by other rules.

This guide covers the material vocabulary for the work this crew does outside a house: porch columns, the exterior part names on a wall and a roofline, cedar and its exterior grades, the non-wood systems that increasingly sit next to it, and the code language that decides when wood needs help to survive the weather.

Be honest about where this comes from. This crew's own published guide library is almost entirely interior work, casing, [baseboard](/learn/install-baseboard/), stairs, closets, cabinets. So a good part of what follows is what a MATERIAL actually is and what the trade generally does with it, sourced to the makers who build it and the code that regulates it, not to a house rule this crew wrote for itself. Where this crew's own practice is genuinely documented, in the field record or the rulebook, this guide says so plainly. Where it is not documented, it says that too, instead of guessing.

The crew lead once captioned a finished composite step with the line that this is what happens when you have a trim carpenter do an outside guy's job. That is one line in one text, not a saying, but it is a fair frame for what follows. Seven hand-offs point elsewhere for real subject matter this guide does not repeat. Treated lumber and the code that governs it live in a separate guide. Cedar's identity as a species lives in another. The interior version of a column wrap lives in a third. Brick mould as a profile, and brick mould on a door unit, each live in their own guide. Stair vocabulary, including the word riser, lives in its own guide. And [sheet goods](https://letsbuildstuff.co/learn/sheet-goods-and-engineered-materials/), MDF, plywood, OSB, live in yet another. This guide points at each of those instead of repeating them.

## Before You Begin

### Prerequisites

Know [nominal versus actual sizing](/glossary/materials/#nominal-vs-actual-size), the name on the tag is not what your tape shows, before you start here; [a separate guide in this series](/learn/lumber-sizes-grades-and-milling/) covers it in full. Know which cedar is which; [a separate guide in this series](/learn/wood-species-field-guide/) sorts out the three trees that hide under that one name. And know how this work gets scheduled: the house rule puts specialty items, porch columns among them, on their own plan, completed separately from the rest of the trim and often ahead of it, while garage and patio steps are grouped with stairs and [handrails](/learn/mount-handrail-wall/) instead. No house rule says where porch ceilings fall.

### What You Need To Know

Two ideas run under everything below. Get them straight here, and the rest of this guide will refer back to them instead of re-explaining them section by section.

Outside, every material moves, and the movement is what the construction details below are for. Wood moves with [moisture](/glossary/materials/#moisture-content): it swells, shrinks, and cups as it trades water with the air around it, the same behavior [a separate guide in this series](/learn/lumber-sizes-grades-and-milling/) covers for interior trim. [Cellular PVC](/glossary/materials/#pvc-composite-trim) and aluminum move with temperature instead, expanding and contracting as they heat and cool; neither one swells or rots the way wood does, though aluminum still corrodes where it sits against treated wood or the wrong metal. Composite decking moves with temperature too, and its gaps exist for drainage as much as for movement. Each material's own maker publishes a number for how much it moves, and that number shows up in its own section below, attributed to the maker who published it.

Outside, the metal and the wood argue with each other. Copper in treated wood eats plain steel and aluminum on contact, which is why treated wood fasteners have their own short list of acceptable metals. Cedar's own extractives, the natural compounds inside the wood, discolor when they meet iron, a different chemistry with a similar kind of result. Whether stainless steel belongs against bare aluminum is a third question, and the sources below do not agree on it. Most sections below that name a fastener also say why that fastener and not a different one, and where the sources disagree, this guide says so plainly instead of picking for you.

## Naturally Durable Wood: What the Code Means by It

The model code gives a builder more than one way to protect wood from decay in the places where decay is a real risk: treat it, use a species the code already trusts, or in some of those places keep the wood up off the ground and away from the moisture instead, on a pier, a pedestal, or a moisture barrier. Preservative treatment is one path, wood pressure-forced full of a preservative chemical. A separate guide in this series [covers treatment chemistry, the AWPA Use Category system](/learn/framing-and-treated-lumber-for-trim-carpenters/), and the treated lumber end tag in full; none of that belongs here.

Naturally durable wood is the code's other option, and it is defined by species, not by looks. The heartwood of redwood, cedar, black locust, or black walnut counts as naturally durable against decay. Only the heartwood counts, with one allowance: an occasional piece with a corner of sapwood still qualifies if 90 percent or more of each face it shows on is heartwood. The code also carries a separate, narrower list for termite resistance: redwood, Alaska yellow cedar, eastern red cedar, and western red cedar heartwood.

This guide needs only three items off the code's own list of where treated or naturally durable wood is required (up.codes, R317.1):

- Exterior siding, sheathing, and wall framing with less than 6 inches of clearance from the ground, or less than 2 inches measured vertically from a concrete step, porch slab, patio slab, or similar horizontal surface exposed to the weather.
- A wood structural member supporting a moisture-permeable floor or roof, such as a concrete or masonry slab, unless an impervious moisture barrier separates them.
- A structural member supporting a building, balcony, porch, or similar permanent appurtenance exposed to the weather without a roof, eave, overhang, or other covering to keep moisture off it.

A separate guide in this series [carries the code's full list, the treatment chemistries](/learn/framing-and-treated-lumber-for-trim-carpenters/), and the Use Category system; the other six items on that list belong there, not here.

The code also carves out an exception for a wood column that would otherwise need to be treated or naturally durable: a column exposed to the weather can stay untreated when it lands on a concrete pier or metal pedestal that clears the concrete floor or the exposed earth by the code minimum. So "it is outside, so it has to be treated" is not right on its own; what the column stands on matters. A separate guide in this series [states that exception with its clearance figures](/learn/framing-and-treated-lumber-for-trim-carpenters/) in full.

Fasteners and connectors in contact with preservative-treated wood have their own short list too: hot-dipped, zinc-coated galvanized steel, stainless steel, silicon bronze, or copper (up.codes, R317.3.1). A separate guide in this series [covers that requirement and its exceptions](/learn/framing-and-treated-lumber-for-trim-carpenters/).

On this crew's jobs garage and patio steps are built in regular pine or cedar, never pressure-treated, and [a separate guide in this series](/learn/framing-and-treated-lumber-for-trim-carpenters/) sets that house rule beside the code text and carries the open question about it. See [How to Build Garage Steps: Safe, Code-Compliant Stairs](/learn/build-garage-steps/).

Naturally durable is a decay-resistance category the code defines, not a promise that a species lasts forever. The crew lead's own way of putting it to a customer is blunt: every wood rots eventually, and a naturally durable species like cedar just buys more time before it does.

## Exterior Part Names: Fascia, Soffit, Frieze, Corner Board, Rake Board, Water Table, Column Base, Deck Skirt Board

These are the exterior part names this crew runs into most, each one defined by where it sits and what it does, not by what it happens to be made of, since the same part gets built in wood, cellular PVC, or aluminum depending on the job.

Fascia runs along the roof edge, the board that finishes the eave and gives a gutter something to mount to. On a deck the same word names a different board: one decking maker's own glossary calls fascia the board around the outside edge that finishes the sides and front. This crew's own project texts use it more narrowly, ordering a fascia board for a step's risers and a separate skirt piece for its sides, so read which face is meant before you cut.

Soffit sits underneath the eave, covering the underside of the roof overhang, and it usually carries the ventilation that keeps heat and moisture from building up in the attic.

Frieze board sits directly below the soffit, running horizontally along the exterior wall. It makes the transition between the siding and the roofline and helps block wind-driven moisture from reaching the upper courses of siding.

Corner board covers a corner where two siding runs meet, finishing it and adding a layer of protection against water getting in behind the siding there. One manufactured-trim maker notes on its own reference page that solid wood trim can be prone to cracking and swelling, which is part of the pitch for the manufactured alternatives later in this guide; read that as a maker making its own case, not as an independent test.

Rake board runs along the sloped gable edge, doing along the rake what the fascia does along the eave; the trade also calls it a bargeboard. A rake board sits fully exposed at the roof edge, where wind-driven rain and moisture hit it directly, while a sheltered eave fascia is more protected. That generally makes the rake the harder service position of the two, though it does not run one way only: an eave fascia rots too, behind an ice dam or under a gutter that overflows, and this crew has been on a job replacing exactly that. On some houses a rake board is also doing real structural work, tying roof outlookers together and backing the roof sheathing at the gable, a framing job a trim carpenter did not put there and should not assume is purely cosmetic before replacing one. Note the word itself collides with a completely different part: a stair also has a "rake", [a separate guide in this series](/learn/stair-parts-and-railing-vocabulary/) covers that sense, and it has nothing to do with a roofline.

Water table is a sacrificial band installed at the base of the wall, so rain splashing back off the ground hits it instead of rotting the bottom ends of the siding above it. It is usually capped with a sloped or kerfed profile that sheds water forward and starts the siding course above it. A cellular PVC maker sells a stocked version of exactly that part, a tapered board with a built-in nailing flange, cemented to the trim board below the siding. The same two words mean something different underground, in soil science, where "water table" names the level groundwater sits at; this guide means only the exterior trim band.

Porch column base is where the wood-column pier exception from the section above actually lands: a column base set on a proper concrete pier or pedestal can stay untreated even outside.

Deck [skirt board](/learn/install-skirt-boards/) closes the gap between the ground and the deck surface, hiding the substructure and keeping debris and animals out from underneath. The framing behind it still needs airflow, so a skirt board is commonly hung with a gap between boards, roughly 1/4 inch, rather than run tight.

Watch this one: "skirt board" names two different parts on this crew's own jobs, and a cellular PVC maker uses the same two words for a third, unrelated wall product. This guide is the only place that sorts all three out. This section's deck skirt board is one. A cellular PVC maker files a similar-sounding part, a board with an integrated drip edge and a rubber gasket the siding compresses against, under "skirt boards and bands", but that product sits at the base of a wall, not at the base of a deck, and it is not evidence for how this crew's own deck skirting behaves. The stair skirt board, the trim board that runs along the side of an interior staircase, against a wall or down an open stringer, is a third and completely different part; [a separate guide in this series](/learn/stair-parts-and-railing-vocabulary/) covers it, and it is also what this crew's own project texts mean when they order one piece of skirt for an exterior step. Keep all three apart.

[Brick mould](/glossary/doors/#brick-mould) and a drip cap are two more part names worth having here, since both show up on an [exterior door](/learn/install-exterior-door/) or window opening. Brick mould is the exterior trim that covers the gap between a door or window frame and the siding around it, doing outside the job that casing does inside, and it often ships attached to the unit already. Some units get a flat exterior casing there instead; see [How to Case One Side of an Exterior Door](/learn/install-exterior-door-casing/). A drip cap is a different piece, installed above the opening and shaped so water sheds off its front edge instead of running down behind the frame. Separate guides in this series cover brick mould as a profile and brick mould on a door unit. The two definitions above are deliberately the same ones those guides use, so the word means one thing across the series; everything past the definition lives there, not here.

On this crew's jobs, a house-to-garage door gets one sealing step that belongs to this crew alone, and it is not a weather step, because the garage side sees no weather: a low-expansion, door-and-window-rated spray foam, run between the jamb and the rough opening. The foam is the whole seal there. See [How to Install a House-to-Garage Door](/learn/install-exterior-door/). A weather-exposed exterior door adds a [sill pan](/glossary/doors/#sill-pan), a shallow watertight tray under the threshold, and [flashing](/glossary/doors/#flashing) tape run up the jambs and across the head; see [Installing a Weather-Exposed Exterior Door: The Extra Steps](/learn/weather-exposed-exterior-door/) for that method in full.

The curriculum for this series flags a common mistake: running standard fascia stock end-grain-up as rake board, said to shorten its paint and rot life. This session's research could not confirm that specific claim anywhere. What the trade press does support: end-grain cuts soak up moisture fast and start to rot if they are not primed, and moisture gets into exterior trim through any unsealed surface, which is why exterior trim generally gets sealed front and back before it goes up. On this crew's jobs that priming and sealing is the painter's step, not the carpenter's. Which face goes up on a board, bark side or pith side, is a genuinely unsettled question even in the trade press itself, industry groups favor bark-side-up, researchers have found other factors can override that difference, and the whole discussion is about deck boards, not exterior trim.

## Cedar for Exterior Work

Western red cedar is the cedar most likely to end up on a porch column or an exterior step here: soft, reddish, and naturally rot resistant. Inland and coastal western red cedar are the same species grown in different climates, inland running tighter-ringed and knottier. Aromatic cedar, the closet-lining wood, is eastern red cedar, a juniper, and not the same tree at all. A separate guide in this series [covers how to tell all three apart](/learn/wood-species-field-guide/); this guide is about what this crew builds out of cedar outside, and how to buy it.

What this crew actually builds in cedar outside is well documented in its own project texts:

- Porch columns wrapped in cedar 2x2s, mass cut and nailed up.
- Porch and patio ceilings built in tongue-and-groove cedar carsiding.
- Porch beams wrapped in cedar.
- Cedar accent and slat walls, one run beside an exterior fireplace, another scoped for a hot tub area.
- Cedar patio steps, often paired with composite treads, covered later in this guide.
- Cedar 1x8 and 2x12 stock ordered by the piece for these jobs, the same sizes this series covers for nominal versus actual sizing.

Cedar suppliers publish real grade tiers, and knowing the names beats guessing at what "good cedar" means. On the clear side, roughly in descending order of how clear the face runs, as the cedar association's own grading page describes them: Clear V.G. Heart (all vertical grain, heartwood, kiln dried), Clear Heart (heartwood on the exposed face, many pieces fully clear), A Clear (slightly more growth character than Clear Heart), and C and Better Clear (still an appearance grade, listed for trim, cabinetry, doors, and windows). On the knotty side: Select Knotty (sound, tight knots), Architect Knotty, Rustic, and Standard and Better, which that same page calls suitable for exterior construction. STK is the knotty grade you will hear most on a porch post, and here is the thing worth knowing about it: it is trade shorthand, not an official grade name. It gets read as Select Tight Knot, and also as sound tight knot and some tight knots, but STK is not in any cedar grade rule book. The rule-book grade for that knot allowance is Select Knotty, written by the NLGA and the WCLIB. Suppliers who sell STK describe it as allowing sound, tight knots plus star-checked or slightly chipped knots, below the pricier clear grades, but none of them names a grading agency behind the STK label itself. So STK on a quote tells you roughly what you are getting; Select Knotty on an order tells the mill exactly. No source backs a rule that trim always gets clear cedar and posts always get STK; the grade, like the species itself, is the customer's call.

Cedar's own extractives, the natural compounds inside the wood, react with iron to leave a blue-black discoloration, a completely different reaction from the copper problem in the section above. The cedar trade association's own guidance is to fasten cedar with hot-dipped galvanized, aluminum, or stainless steel nails only, never plain steel.

Cedar moves. This crew's own texts describe outside cedar work as something that is simply going to grow and shrink, and separately note that cupped, not-quite-straight cedar boards make a tight miter harder to close. Both are facts about the material this crew has already run into, not a defect on any one job.

Where cedar does NOT go on this crew's jobs: on this crew's jobs an interior ceiling beam is clear pine or white oak, almost never cedar. See [How to Install Ceiling Beams](/learn/install-ceiling-beams/). That rule is about the box built over an interior cleat; it says nothing about a beam wrapped outside on a porch, which this crew's own texts show does get built in cedar.

Cedar heartwood earns its place on the naturally durable wood list above because it is one of the code's four accepted species. Ordinary pine is not.

## Redwood and Cypress: Rare Here, and What They Are For

Redwood and cypress are two more species that travel in the same naturally durable company as cedar, and neither one turns up anywhere in this crew's own record. Redwood heartwood sits on the same naturally durable species list as cedar heartwood, which is the only reason it matters to this guide at all. Cypress does not appear on that code list at all, and its durability depends on the tree it came from: old-growth bald cypress is rated durable to very durable for decay resistance, while wood from younger trees rates only moderately durable, so the word cypress on a tag does not settle the question by itself. Pecky cypress is the decorative grade of it, scattered dark pockets and voids where the wood has been attacked by fungi, sold as a specialty material rather than a structural or trim default, and priced as one.

Here the honest finding is an absence, and it is worth stating as one rather than dressing it up as a practice. Nothing in this crew's published guides, its rulebook, or its own project texts names redwood or cypress as a material used on a job. Every "redwood" hit found in this crew's own field record turned out to be a street address, not a material order. A separate guide in this series [already asks the owner whether either species ever actually shows up](/learn/wood-species-field-guide/) on a job here; that question is not repeated in this guide.

## Cellular PVC Trim

Cellular PVC is a closed-cell PVC board with no wood in it at all, so it absorbs almost no water, does not rot, and will not support mold or mildew growth. It does not need paint to hold those problems off, which is also true of a naturally durable species like cedar left to weather, and not true of most wood trim. (The trade sometimes files this whole family under "PVC and composite trim", one glossary entry covering both.)

Expansion gaps. Two makers give two different rules, and both are worth having because they do not fully agree. One maker's own installation guide says that when a board is properly fastened along its whole length, allow 1/8 inch of expansion space per 18 feet, and glue the joints between pieces to stop them separating, leaving room at the ends of a glued run for that expansion. The other maker's own guide takes a narrower view: expansion is only really an issue on long runs, rake, fascia, or frieze, built from three or more 18 foot boards; a short run, around a window for example, can and should be built with tight joints. That second maker also notes that screws restrict movement more than nails do, that movement shows up worst on southern exposures and in direct sun, and that the material and the air should sit around 60 to 70 degrees before a long run gets installed.

The cement joint. The first maker's own cement has a 10 minute working time and cures fully in 24 hours; ordinary PVC cements cure much faster, which limits how long you have to work a joint. The second maker names two specific PVC cements, plus a two-part methacrylate as another option, and says most PVC cements cure in 3 to 5 minutes. Both makers say close to the same thing about a board or molding joint: it can never be held by adhesive or sealant alone, a fastener goes in too. Column wrap is the one common exception. One maker's own wrap panels are bonded to each other with adhesive, and fastening the finished wrap to the post is listed as an optional extra step.

The glue-then-fasten sequence. Glue the joint first, then fasten on each side of it so the bond has time to set, two fasteners on each side of a board joint. The two makers part ways on the cut: one calls for a [scarf joint](/glossary/joints-and-cuts/#scarf-joint-joints-and-cuts) rather than a butt joint, while the other never uses the word and pushes a shiplapped joint as the better one on a long run. Check which brand is on the truck before you decide how to cut it. This is the same shape as gluing every [miter](/glossary/joints-and-cuts/#miter-joint) and never cross-nailing through it, a rule this crew already follows on wood; the makers' own literature agrees on cellular PVC for the same reason, a bonded joint holds better than a fastener driven through it.

Sealant compatibility is the maker's specification and the painter's step, the same as everywhere else in this guide. One maker's preferred sealant is polymer based and contains solvents; that maker says plainly not to use silicone, and names urethane and tripolymer sealants as acceptable instead. (Sealant and caulk are not quite the same product family, and the difference matters to the maker's own specification. The finish bead that seals a gap after install is the painter's to buy and run. Construction adhesive that bonds the joint itself comes out of a gun too, and that one stays this crew's own material, the same as it already is on a glued wood joint.)

Fasteners run into a real collision with what this crew carries. Both makers want the fastener deep in solid substrate, one calling for a minimum of 1-1/2 inches of penetration and the other for a full 1-1/4 inches, and both want one within 2 inches of each board end. One maker calls for two fasteners at every framing member and bars staples, small brads, and wire nails outright; the other publishes a schedule by board width, two across a 4 or 6 inch board up to four or five across a 16, at a maximum of 16 inches on center, and says that if you can bend the fastener in your fingers it is too thin. Both name a screw for real holding power, a #8 or a 7d [trim head screw](/glossary/materials/#trim-head-screw-pilot-hole), in stainless or hot-dip galvanized. This crew runs 15, 18, and 23 gauge nailers, see [Nail Guns and Air: What We Actually Run](/learn/nail-guns-and-air/), and one of those makers names 18 gauge galvanized trim nails, this crew's own default trim gauge, among the fasteners it does not recommend for its PVC. One trade-press article on cellular PVC soffits, fascias, and rakes shows a builder gluing every miter and tacking it with 18 gauge stainless or hot-dip galvanized brads, then coming back with concealed trim screws for the final fastening once the glue has set, the same shape as this crew's own rail-to-newel rule, where the pins tack and the screws do the real holding.

Paint is the maker's own specification here, not a builder's preference. Both makers set the same standard for finish color, a light reflective value of 55 or higher, because a darker color absorbs more heat and moves the board more, and one of them says a dark color applied outside that standard voids its warranty. Read the direction carefully: 55 or greater means lighter, not darker. Both also publish a way past it, a vinyl-safe color palette in one case and a heat-reflective paint line in the other, so 55 is the standard rather than an absolute ceiling.

Where it is not structural. One maker's own guide says its trim must not be used in load-bearing applications, though it can span a fascia, soffit, or ceiling; it requires a minimum 1/2 inch plywood or OSB backer, set with [construction adhesive](/glossary/materials/#construction-adhesive) and mechanical fastening, behind anything spanning more than 16 inches on center. That is the maker's own structural spec for its product, not a house rule about backers.

What this crew's own record shows is thin. A builder picked up cellular PVC trim from a supply house, not this crew's own purchase. A crew walkthrough list names PVC base in a garage once. That is the entire record, two mentions, in a field archive of more than ten thousand messages.

## Polyurethane and Polystyrene Molding

Polyurethane molding is a closed-cell cast foam that takes a wood-grain texture and resists water, and the maker's own claim for that closed-cell structure is that it helps prevent the material from splitting, flaking, chipping, or bloating the way wood can. Helps prevent, not cannot: it is still a foam, and a hard knock or a fastener driven too near an edge will mark it. A large decorative profile, including a [corbel](/glossary/specialty/#corbel) (a bracket that reads as though it is supporting a beam or a shelf, more decorative than structural on most jobs), can be cast in one piece instead of being built up out of layered wood. One maker's product ships double-primed with an exterior-grade coating already on it, ready for finish, and its exterior pieces still need an exterior-grade 100 percent acrylic latex paint on top, which is the painter's step, the same as everywhere else in this guide. Joints get a polyurethane-compatible adhesive at every decorative molding joint, plus noncorrosive fasteners.

Polystyrene is the lighter, softer foam of the two. The only comparison this session's research could verify comes from a polyurethane maker's own marketing page, which calls standard polystyrene foam a softer material that can be prone to different kinds of damage, against its own durable polyurethane foam that it says resists dings and dents. That is a competitor's own comparison, stated here as exactly that, not an independent test. No density figure, no price per foot, and no verdict on which one is the economy option could be confirmed anywhere this session, so none of the three appears here.

Flexible molding is the bendable version of the same cast material, made to follow an arch or a curve rather than run straight. No bend radius or specific maker could be confirmed this session, so none is stated.

## Fiberglass and Composite Columns: Load-Bearing versus Split-Wrap

Get this one wrong and something over your head is not being held up by what you think is holding it up. So read this section slowly, and state it no wider than the makers state it themselves.

One fiberglass column maker's own product page states the distinction plainly: all of its fiberglass columns are load bearing, with one non-load-bearing exception, split columns, which the factory splits on request specifically to wrap around an existing column or post. A cellular PVC maker states its own column wraps flatly as not structural and not intended for load-bearing applications, and where its wrap goes around a post in a railing application, the maker requires solid blocking attached between the post and the wrap.

Why confusing the two is a structural mistake, stated plainly: a split or wrapped column is a cover, nothing more. The post or the framing inside it is what actually carries the roof or the porch above it. If a split-wrap product gets set where a load-bearing column was specified, nothing under that cover is holding the load it needs to hold. That maker's own FAQ page says the same thing in different words: its load-bearing answer is written about the unsplit columns, and its split-column answer describes a split column going around an existing structural post. Read either page and the split column is the wrapper, not the support. None of this is generalized to any other maker's line.

Two more facts from that same maker, both worth carrying onto a job. Never pour concrete inside a fiberglass column: the column and the concrete expand and contract at different rates, and the column will split. And filling the column at all restricts the airflow the column relies on to let condensation dry out. Installation is the maker's own, not this crew's: pre-drill before driving a screw, use a non-acetone construction-type adhesive, and where a wrap kit gets used, the corner joints get bonded and the last panel snaps or glues into place.

The exterior wood alternative to a fiberglass or split-wrap column is this crew's own documented practice: porch posts wrapped in cedar, mass cut and nailed up, with the corners trimmed out. A separate guide in this series [covers the interior version of the same idea](/learn/wall-treatment-and-fireplace-trim-vocabulary/), a box built around a structural member indoors; this guide does not resolve or repeat that guide's open question about method.

The choice itself often starts with the customer, and the field record shows exactly that: one homeowner asked this crew about fiberglass round columns specifically because they wanted the columns to stay white to match the trim, as an alternative to composite or wood. The archive carries no recorded reply to that question, so none is invented here.

## Composite Decking and Railing

The model code sends plastic composite exterior deck boards, stair treads, guards, and handrails to a single testing standard, ASTM D7032, and to the maker's own instructions (up.codes, R507.2.2). The product or its packaging has to carry a label showing D7032 compliance, plus the allowable load and the maximum allowable span. A separate guide in this series [shows the stair code pointing the same way](/learn/stair-parts-and-railing-vocabulary/) for composite treads, rails, and guards. What that means on a job: for this material, the maker's own instructions are not a suggestion, they are what the code itself points at.

Gaps. One decking maker's own installation guide requires gapping both end-to-end and width-to-width, for drainage, for the material's own thermal movement, and for shrinkage of the wood joist system underneath it. Width-to-width runs a minimum of 1/4 inch at all temperatures, 3/8 inch for docks and heavily wooded areas, and never more than 1/2 inch. End-to-end runs 1/8 inch above 40 degrees and 3/16 inch below it. Where a board abuts a wall or another solid object, that gap grows to 1/4 inch above 40 degrees and 1/2 inch below. Maximum perpendicular overhang is 1/2 inch. On a stair, the gap between boards on a tread runs 1/4 to 3/8 inch. Every one of these numbers belongs to that maker, not to a house standard.

Hidden fasteners. A hidden fastener clip sits in a groove milled into the board's edge and screws down to the joist, doing two jobs in one motion: it holds the board down and it sets the gap at the same time, so the gap becomes the fastener's own dimension rather than something measured separately. The clips have to land at every joist. A square-edge board can be routed for them from the bottom side with the maker's own bit. Where surface screws get used instead, the maker calls for two per joist, driven flush and never countersunk.

Composite decking cannot be used for structural applications, in the maker's own words. Joists, beams, posts, and stringers stay code-approved structural material regardless of what the decking on top of them is made of.

On an exterior step, the word riser means the structural board or the open gap itself, not the interior trim sense of the same word, which the painters finish; [a separate guide in this series](/learn/stair-parts-and-railing-vocabulary/) keeps the two senses apart.

What this crew actually does with composite is well documented in its own record. Patio steps get built with a cedar frame and composite treads, and the record also carries [garage steps](/learn/build-garage-steps/) called out as composite outright. Grooved deck boards, ordered with color-matched composite screws and a skirt piece, get called out for a step. A composite step gets a return edge piece on its open end, a 45 degree return back to the house, and this crew's own texts call that the normal way it finishes a composite step end now. And composite tread, riser, and fascia is a combination builders ask this crew to quote by name; note that "fascia" here is the board on the face of a step, the same word as the roof-eave fascia earlier in this guide, doing a similar job in a different place.

Composite railing gets one paragraph here because this guide is about the material, not the build. The makers who publish composite decking guides publish separate railing guides too, and none of those was read for this guide, so nothing about building one is stated. On the code side, a plastic composite guard or handrail goes to the same standard and the same maker's instructions as the decking, and it also has to meet a flame spread limit tested to ASTM E84 or UL 723 unless the plastic is rated noncombustible. Note one difference in the labeling: a deck board or a stair tread has to show its allowable load and its maximum allowable span, while a guard or a handrail has to show only the maximum allowable span. A separate guide in this series [covers rail vocabulary in full](/learn/stair-parts-and-railing-vocabulary/).

## Aluminum Trim Coil

Aluminum trim coil is painted aluminum sheet sold in a roll and bent to shape on site. One maker's published spec lists a nominal 0.019 inch thickness, in 14 inch and 24 inch widths, on 50 foot rolls. (The trade also calls this material coil stock; the tool that bends it is a brake; none of these three terms are in this crew's own glossary yet.)

Forming. A brake clamps the sheet and a hinged leaf bends it to whatever angle the job needs. A hem, where the edge gets bent all the way back on itself, stiffens the piece and hides the cut edge; it is typically the first bend made on a fascia piece. Thicker metal sometimes needs that hem hammered flat to finish closing it.

Cutting. Snips cut a shape directly, or the sheet gets scored with a utility knife along a line and flexed back and forth until it breaks cleanly at the score.

Fastening is one place two real sources disagree, and both are worth having rather than picking a winner. One trade-press fascia article calls for painted aluminum nails, one inch long, about every two feet, and says plainly not to use stainless steel nails, because stainless corrodes the aluminum coil stock it is driven into. The coil maker's own installation article instead lists painted aluminum or stainless steel trim nails, says to drill oversized nail holes, and says the nail should reach at least 3/4 inch into the wood behind it. Different sources, different answers; both are stated here rather than resolved.

Expansion. Aluminum lengthens when it warms and shortens when it cools, and if it gets pinned too tight it will bulge or buckle instead of moving freely. The coil maker's own guidance is to leave around 1/16 inch of space between the end of a bent piece and a fixed object, like an inside corner, and not to nail it down too tightly. Lap neighboring panels by about 2 inches, and think about which direction they will actually be seen from before deciding which panel laps over which.

This crew's own record shows nothing here. A full scan of this crew's field texts found zero mentions of trim coil, coil stock, a brake, or brake-forming, anywhere. So this is trade knowledge stated as trade knowledge, not this crew's own documented practice, and the question below asks the difference directly.

## Exterior Trim Quick Reference

One row per material family: what it moves with, the maker's own number for that movement, the fastener metal and why, and the one thing that goes wrong. Every number here already appears, attributed, above.

| Material | Moves With | The Maker's Own Number | Fastener Metal, and Why | What Goes Wrong |
| --- | --- | --- | --- | --- |
| Cedar | moisture, cups and shrinks | no movement number; the association names the fastener metal only | hot-dipped galvanized, aluminum, or stainless; cedar's own extractives discolor against plain iron | a cupped board makes a tight miter hard to close |
| Cellular PVC | temperature only, not moisture | one maker: 1/8 inch per 18 feet, properly fastened. the other: movement mainly on runs of three or more 18 foot boards | stainless or hot-dip galvanized, long enough to reach solid substrate; one maker bars staples, small brads and wire nails outright, the other bars anything you can bend in your fingers | a butt joint glued but not fastened on each side separates |
| Polyurethane molding | resists water; no movement number found | none confirmed this session | noncorrosive fasteners plus a polyurethane-compatible adhesive at every joint | a plain steel fastener in it, or a joint closed with no adhesive |
| Fiberglass and composite columns | expands and contracts at a different rate than concrete | no movement figure; the maker's own warning is about concrete poured inside the column | pre-drilled screws only, per the maker | treating a split-wrap column as load bearing |
| Composite decking | temperature, plus joist shrinkage | one maker: 1/4 inch width-to-width minimum, capped at 1/2 inch. end-to-end 1/8 inch above 40 degrees, 3/16 inch below | hidden fastener clips, or two surface screws per joist, never countersunk | using composite decking as structural framing |
| Aluminum trim coil | temperature | about 1/16 inch of clearance at a fixed object, per the coil maker | painted aluminum nails (one source) or stainless (another); the two sources disagree | pinning it too tight so it buckles |

The eight part names in one line each: fascia runs the roof eave, and on a step it is the board on its face; soffit covers the underside of the eave; frieze board sits below the soffit against the wall; corner board finishes where two siding runs meet; rake board runs the sloped gable edge, generally more exposed than the eave fascia though not always the first to go; water table is the sacrificial band at the base of the wall; porch column base is where the pier exception can keep a column untreated; deck skirt board closes the gap under a deck, with room left for the framing to breathe.

## Related Guides

- [How to Build Garage Steps: Safe, Code-Compliant Stairs](/learn/build-garage-steps/)
- [How to Install a House-to-Garage Door](/learn/install-exterior-door/)
- [Installing a Weather-Exposed Exterior Door: The Extra Steps](/learn/weather-exposed-exterior-door/)
- [How to Case One Side of an Exterior Door](/learn/install-exterior-door-casing/)
- [How to Install Ceiling Beams](/learn/install-ceiling-beams/)
- [Nail Guns and Air: What We Actually Run](/learn/nail-guns-and-air/)

Written by **David Hospodka**, Sun’s Up Construction

David Hospodka learned finish carpentry from his father, alongside his brother Joel, and worked his way through college trimming houses. He then spent about ten years as a railroad bridge engineer, handling design, construction, maintenance and project management. He came back to finish carpentry full time in 2017.

[About Let’s Build Stuff](https://letsbuildstuff.co/about/)