Introduction #

Building 3 stringer garage steps with closed risers creates the widest, most finished stair configuration. This is a supplemental guide for building garage or patio steps with 3 stringers and closed risers (3SC) — the widest and most finished-looking stair configuration. Before starting this guide, make sure you’ve read the core guide: Build Garage Steps. That guide covers the full process for a standard 2-stringer, open-riser stairway. This 3 stringer garage steps guide assumes you already know that process — including prerequisites, building codes, rise/run math, and every step from measuring to final verification.

Read the core guide first. This page for 3 stringer garage steps builds directly on it by adding two modifications:

• A third (center) stringer — for stairs wider than about 36″. The center stringer adds structural support and prevents tread bounce across the wider span
• Closed risers — vertical boards that cover the open face between each step, giving the stairway a cleaner, more finished look

Everything you need to build 3 stringer garage steps is covered between the core guide and this page. You do not need to read any other supplemental guides first. If you want additional detail on a single topic, these are available as references:

• 3 Stringers, Open Risers — center stringer details without closed risers
• Closed Risers, 2 Stringers — riser board details without a center stringer

Key Terms for 3 Stringer Garage Steps #

The core guide defines the basic stair terms (stringer, tread, riser, rise, run, kicker plate, etc.). Here are additional terms used in this supplemental:

• Blocking: Critical for 3 stringer garage steps — short horizontal pieces of lumber (typically 2×4 or matching stringer material) installed between stringers at each tread level. Blocking locks the stringers in position and prevents them from twisting, racking, or spreading apart. Think of them as cross-braces inside the stairway frame.
• Center stringer: The defining feature of 3 stringer garage steps — a third stringer installed midway between the two outer stringers on wider stairs (typically 36″ and up). It supports the middle of each tread so it doesn’t flex or bounce when you step on it.
• Closed riser: A vertical board fastened to the front face of each step, covering the open gap between treads. Makes the stairway look finished and prevents debris from falling through.
• Riser cut: The vertical cut on each stringer notch. This is the face the riser board attaches to. On an open-riser stair, this cut is left exposed.
• Riser plane: The imaginary flat surface that spans across all stringer riser cuts at a given step. All riser cuts at the same step should be in the same plane so the riser board sits flat against all of them.

What’s Different in 3 Stringer Garage Steps #

A 3SC stairway combines two modifications on top of the standard build from the core guide. Here’s what changes:

• Risers span a wider opening: On a 36–48″ stairway, riser boards cover the full width between the outer stringers. The center stringer sits behind the riser, hidden from view, and provides a mid-span support point that prevents the riser from bowing
• Blocking is required: Short horizontal pieces are installed between stringers at each tread level to keep everything rigid. With 3 stringers, you need blocking between each pair (left-to-center and center-to-right) at every step
• Build sequence matters: The order is: stringers → blocking → risers → treads. Getting this wrong — for example, installing treads before risers — means ripping things apart and starting over
• More fasteners per step: Each tread gets screwed into 3 stringers instead of 2, plus additional screws into the riser top edge. Budget about 8–9 screws per tread

Additional Materials for 3 Stringer Garage Steps #

You’ll need everything from the core guide materials list, plus the following:

• One additional 2×12 (same length as your other stringers) — for the center stringer
• Blocking material: 2×4 or 2×6 lumber, cut to fit between stringers at each tread level. You need 2 blocks per step (left-to-center and center-to-right), so for a 3-step stairway that’s 6 blocks total
• Riser boards: ¾″ plywood or solid 1× lumber. Each riser board is cut to the full width of the stairway and the exact height of the riser cut. You need one per step
• Construction adhesive: One extra tube — used behind each riser board and on blocking joints
• Additional fasteners: Extra 3″ construction screws for the center stringer and blocking. Brad nails or 1¼″ trim-head screws for riser boards. Plan on roughly 6–9 screws per riser and 8–9 screws per tread

Already covered in the core guide: Prerequisites, complete tool list, building code requirements, and rise/run calculations. Review those sections before gathering materials.

3 Stringer Garage Steps: Step-by-Step Build Process #

Steps 1 through 5: No Changes for 3 Stringer Garage Steps #

Follow the core guide exactly for these steps:

1. Step 1 — Measure rise and run
2. Step 2 — Lay out the stringer
3. Step 3 — Cut the stringers (cut 3 identical stringers instead of 2)
4. Step 4 — Dry-fit the stringers
5. Step 5 — Install the kicker plate

The only difference in these steps is quantity — you’re cutting and fitting 3 stringers instead of 2. The layout, measurements, and cuts are all identical.

Step 6: Set and Attach All 3 Stringers + Blocking #

This step expands on the core guide. You’re installing 3 stringers instead of 2, then adding blocking between them.

6a. Install the Outer Stringers #

Attach the two outer stringers exactly as described in the core guide — fastened at the top to the header or ledger board, and at the bottom to the kicker plate or concrete pad. Before moving on, make sure they are:

• Plumb (vertical) when viewed from the side
• Level across matching tread cuts when you set a level from one to the other
• At the correct spacing for your stair width

6b. Install the Center Stringer #

Position the center stringer exactly halfway between the outer stringers. Before you fasten it, check three things:

1. Tread cuts are level: Set a straight edge or level across all 3 tread cuts at the same step. They should all be in the same horizontal plane
2. Riser cuts are aligned: Hold a straight edge across all 3 riser cuts at the same step. The straight edge should sit flat — if it rocks, the center stringer is forward or back
3. Stringer is plumb: Check with a level on the riser cut face

Fasten the center stringer the same way as the outers — secured at the top and bottom. If it sits on a concrete pad, anchor it with a kicker plate, construction adhesive, and/or tapcon screws.

6c. Install Blocking Between Stringers #

Cut blocking pieces to fit snugly between each pair of stringers at every tread level. With 3 stringers, you need 2 blocks per step:

• One block from the left outer stringer to the center stringer
• One block from the center stringer to the right outer stringer

Position each block at the tread level, in the space between the tread cut above and the riser cut below. Secure with 3″ screws — toe-screw through the blocking into the stringer, 2 screws on each end.

Critical — watch the riser plane: Keep every piece of blocking flush with or slightly behind the riser cut face. In the next step, the riser boards will sit flat against the stringer riser cuts. If any blocking sticks out past the riser cut face, the riser board won’t sit flat and you’ll have to pull the blocking and trim it.

Step 7: Install Riser Boards on 3 Stringer Garage Steps #

With all 3 stringers and blocking locked in, install the riser boards starting from the bottom step and working up.

7a. Cut the Riser Boards #

Measure the full width between the outside faces of the two outer stringers. Cut each riser board to this full width. The height of each board should match the riser cut height on your stringers (the vertical distance between one tread cut and the next).

Material: Use ¾″ plywood or solid 1× lumber. Avoid ½″ plywood on stairs wider than 36″ — it’s too thin and will bow between support points over time.

7b. Attach Each Riser Board (Bottom to Top) #

Start with the bottom riser and work upward. For each riser:

1. Apply adhesive: Run a bead of construction adhesive along the riser cut face on all 3 stringers
2. Position the board: Set the riser board flat against the riser cuts. It spans the full width of the stairway — the center stringer is completely hidden behind the riser board
3. Check plumb: Hold a level against the riser face to make sure it’s vertical. The top edge of the riser should be flush with the tread cuts above
4. Fasten: Drive 2–3 screws through the face of the riser into each stringer behind it. That’s 6–9 screws total per riser — make sure you hit the center stringer, not just the two outers
5. Repeat for each step, working bottom to top

Why bottom to top? Each riser has to be in place before the tread above it goes on. Since treads overlap the riser’s top edge, working upward means you never have to reach under an installed tread to fasten a riser.

Center stringer = mid-span support: The center stringer sitting behind the riser prevents the board from bowing outward — especially important on stairs wider than 40″. Always fasten the riser to the center stringer, not just the outers.

Step 8: Install Treads on 3 Stringer Garage Steps #

With all risers in place, install the treads. Each tread sits on the tread cuts of all 3 stringers and overlaps the top edge of the riser below it.

1. Apply adhesive on the tread cut of each stringer
2. Set the tread: It should sit flat across all 3 stringers with no rocking. If it rocks, check that your stringer tread cuts are all level with each other (shim the low point if needed)
3. Fasten to stringers: 2 screws per stringer = 6 screws from above, driving down through the tread into the stringer
4. Fasten to riser: 2–3 screws through the back edge of the tread down into the top edge of the riser board below. Space these evenly across the full width, with at least one near the center stringer
5. Total per tread: 8–9 screws

Nosing overhang: The tread should overhang the riser face by ¾″–1¼″. This is the same dimension as the core guide — the riser board does not change the overhang.

Step 9: Final Verification of Your 3 Stringer Garage Steps #

Walk the entire stairway and check each of the following:

• No wobble or shifting: Steps should feel completely solid. Stand on each one and shift your weight — there should be zero movement. If anything shifts, check your fastener connections and adhesive bonds
• Treads sit flat: Each tread should be flat across all 3 stringers with no rocking. Set a level across the tread in both directions
• Risers are plumb: Hold a level against each riser face — they should be vertical and flush with the tread cuts
• No riser bowing: Sight down the face of each riser from the side. It should be flat, not bowing outward between stringers. The center stringer prevents this, but verify
• Center stringer is hidden: Looking from the front, the center stringer should be completely behind the riser boards and not visible
• Blocking not visible: No blocking should protrude past the riser plane or be visible from the front
• Concrete attachment is solid: If the stairway sits on a concrete pad, verify it’s properly anchored — construction adhesive, tapcon screws, and/or a kicker plate. The base should not shift at all when you push against the stairway. This is the most common place for wobble

Quality Checklist for 3 Stringer Garage Steps #

Run through this list after completing the build. This covers 3SC-specific items — also review the core guide checklist for general items.

• ☐ All 3 stringers are plumb and level across tread cuts
• ☐ Center stringer is centered between the outer stringers
• ☐ Blocking is installed at every tread level between each pair of stringers (2 per step)
• ☐ Blocking is flush with or behind the riser cut face — nothing protruding
• ☐ Riser boards are fastened to all 3 stringers, not just the outers
• ☐ Riser-to-center-stringer joints are tight — no visible gap from the front
• ☐ Center stringer is completely hidden behind riser boards
• ☐ Each tread has 6 stringer screws + 2–3 riser screws (8–9 total)
• ☐ Tread overhang (nosing) is consistent at ¾″–1¼″
• ☐ Steps don’t wobble — solid and stable underfoot, zero movement
• ☐ Stairway is anchored to concrete pad (adhesive + tapcons or kicker plate)
• ☐ No riser bowing on any step

Troubleshooting 3 Stringer Garage Steps #

Riser doesn’t sit flush against center stringer: The center stringer riser cut may not be in the same plane as the outer stringer riser cuts. Hold a straight edge across all 3 riser cuts at the same step. Plane or sand the high point. If the center stringer is recessed more than ⅛″, shim behind the riser board at that point before fastening.

Riser bows outward between stringers: Even with 3 support points, thin material (½″ plywood) can bow on spans over 18″. The fix is to upgrade to ¾″ material. If the riser is already installed, you can add a horizontal cleat behind the bowed section and pull it flat with screws through the riser face into the cleat.

Blocking interferes with riser installation: If blocking was installed proud of (sticking out past) the riser cut face, the riser board won’t sit flat. Remove the blocking that’s in the way, trim it back about ¼″, and reinstall. Blocking must be flush with or behind the riser cut.

Tread rocks on the stringers: One stringer’s tread cut is higher or lower than the others. Set a long level or straight edge across all 3 tread cuts at that step. Shim the low stringer or plane the high one. Don’t rely on screw pressure alone to pull the tread flat — it will creak and work loose over time.

Entire stairway wobbles or shifts: Almost always a base connection issue. Check that all stringers are securely fastened at the top (ledger/header) and bottom. At the base, the stairway must be anchored to the concrete pad with construction adhesive and tapcon screws, or secured to a kicker plate that’s anchored to the pad. A loose base is the most common cause of wobble in garage steps.

External Resources #

• IRC R311.7 — Stairway Code Requirements — residential building code for stairs, handrails, treads, and guardrails
• Simpson Strong-Tie LSC Stair Stringer Connector — connector hardware for attaching stringers to headers and ledger boards