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Introduction #
A plinth block sits at the base of a door or window casing, wider and thicker than the casing itself, so the casing leg lands square on top of it instead of running all the way to the floor. It is a traditional detail you will see called out on formal entries, craftsman-style openings, and any job where the plans specify it, the same styling logic covered in This Old House's guide to trim and moulding. This guide covers installing a plinth block that is already made, whether it came from the supplier pre-milled or you built it yourself. If you need to build one from raw stock first, see How to Make a Plinth Block.
Once a plinth block is in, it changes how the rest of the room's trim lands at that opening. The casing leg butts square onto the top of the block instead of running to the floor, and the baseboard or base shoe butts square into the side of the block instead of mitering or coping around the casing.
Before You Begin #
Prerequisites #
- The jamb is set and the door or window is hung, with a consistent 3/16" reveal established on the jamb legs
- You have the plinth block on hand, either pre-made or built per the companion guide, matched to the width and grade of the casing it will sit under
- You know whether flooring is already down in this room, or whether it is coming later
What You Need To Know #
- The plinth block's outer face should read as a clean shadow line against the casing above it. Confirm the exact projection the plans or the customer wants before you set the block, since this is a style detail, not a fixed house standard
- If flooring has not gone in yet, hold the bottom of the plinth block up off the subfloor by the thickness of the future finish flooring. Use a scrap of the actual flooring material as a spacer if you have one on-site, the same way you would set a door jamb
- The casing leg lands on top of the plinth block with a square cut, not a mitered or reveal-set joint. Treat that seam like any other wood-to-wood joint: glue, putty, and sand it flush
- Baseboard and base shoe butt square into the side of the plinth block. Per How to Install Base Shoe, base shoe never gets nailed into the floor; the same rule applies here, nail into the block or the baseboard, not the floor
Tools Required #
Power Tools #
- 15-gauge finish nailer: for anchoring the block into the jamb and framing
- 18-gauge brad nailer: for the casing-to-block joint
- Compressor or a charged battery backup for the nailers
Hand Tools #
- Torpedo or 2-foot level: to check the block is plumb and sitting square to the floor
- Combination square: to set and check the block's reveal against the casing
- Tape measure: for the flooring-thickness spacer and the block's position
- Nail set: for any brads that do not sink flush
Supplies #
- Wood glue: for the casing-to-block joint
- Wood putty, matched to the finish grade, for the wood-to-wood joint
- 15-gauge finish nails and 18-gauge brads
Materials #
- Plinth block, matched to the casing width and finish grade for this opening
How to Install a Plinth Block Step by Step #
Step 1: Set the Bottom Height #
If flooring is already down, the block sits directly on the finished floor. If flooring has not gone in yet, hold the block up off the subfloor by the thickness of the flooring that is coming, using a flooring sample or a scrap as a spacer while you position it.
Step 2: Position and Check the Block #
Set the block against the jamb leg at the height from Step 1. Check it is plumb with your torpedo or 2-foot level, and check the reveal against the casing with your combination square so the block reads consistent with the reveal above it.
Step 3: Fasten the Block #
Nail the block into the jamb leg and, where you can reach it, into the framing behind it, with the 15-gauge finish nailer. Glue the back face where it contacts the jamb for extra hold. Check the block is still plumb and tight after nailing.
Step 4: Set the Casing Leg #
Cut the bottom end of the casing leg square, at the length that lands the leg's reveal correctly on top of the block. Dry-fit it before you glue anything.
Step 5: Glue and Nail the Casing-to-Block Joint #
Apply glue to the joint where the casing leg meets the top of the block. Nail the casing into the jamb as usual with the 18-gauge brad nailer, and pin the casing-to-block joint itself to hold it tight while the glue sets. Wipe off any squeeze-out before it dries.
Step 6: Putty and Sand the Joint #
Once the glue has set, putty the casing-to-block seam and sand it flush, the same as any other wood-to-wood joint. Check the joint by running your finger across it; it should feel like one continuous piece, not two pieces meeting.
Close-Out #
Sweep up sawdust and cutoffs around the opening; do not shop vac. Set any proud brads below the surface. Confirm the baseboard or base shoe for this room still has a clean, square line to butt into on both sides of the block.
Quality Check #
- Block is plumb and sits square to the floor
- Reveal on the block reads consistent with the casing reveal above it
- Casing leg lands square on top of the block with no gap
- Casing-to-block joint is glued, puttied, and sanded flush
- Block is solid with no movement when pressed
- Baseboard and base shoe have a clean, square line to butt into on both sides of the block
- Area is swept clean
Troubleshooting #
Block is not plumb against the jamb #
Check the jamb itself for plumb first; a plinth block installed plumb against a jamb that is slightly out of plumb will still look wrong from across the room. If the jamb is good, shim behind the block before you nail it, the same way you would shim any other trim piece against an out-of-plane wall.
Casing leg length keeps coming up short or long #
Dry-fit before you cut a final piece. If you are cutting several matching openings with plinth blocks, cut one, confirm it fits and reads correctly, then use that piece to set a stop block on the miter saw for the rest.
Floor is uneven under the block #
A block with a visible gap under one edge should be scribed to the floor rather than shimmed on top, the same principle as fitting baseboard to an out-of-level floor. Mark the gap, trim the bottom edge to follow it, and re-check plumb after the fit.
