Stairs/Handrails Skill Level: Advanced (Level 3)

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 the extra terms this guide adds:

• Blocking: Short horizontal pieces of lumber (typically 2×4 or matching stringer material) installed between stringers at each tread level. Blocking locks the stringers in place. It stops them from twisting, racking (shifting out of square), or spreading apart. Think of blocking as a cross-brace inside the stairway frame. This part matters most on 3 stringer garage steps, since there is more to hold in line.
• Center stringer: A third stringer installed midway between the two outer stringers, used on wider stairs (typically 36″ and up). It is the defining feature of 3 stringer garage steps. It supports the middle of each tread so the tread 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. It makes the stairway look finished and keeps 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 #

These 3 stringer, closed-riser steps combine two changes on top of the standard build from the core guide: adding a center stringer, and closing in the risers. 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. It gives the riser a mid-span support point, which keeps it 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 (lumber sized by nominal thickness, not actual). 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 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 (special screws made to bite into concrete).

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.

Quality Checklist for 3 Stringer Garage Steps #

Run through this list after you finish the build. This covers items specific to the 3 stringer, closed-riser design - 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's riser cut may not be in the same plane (the same flat surface) as the outer stringer riser cuts. Hold a straight edge across all 3 riser cuts at the same step. Plane the high point (shave it down with a hand or power planer), or sand it. If the center stringer sits more than ⅛″ back from the others, shim it (fill the gap with a thin wood wedge) 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 (a short strip of wood fastened on as backing) behind the bowed section and pull the riser flat with screws driven 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.

3 stringer garage steps take longer to build than the standard 2-stringer version. But the wider stance and closed risers are worth it on any stairway your household uses multiple times a day. Once the risers are painted to match your trim, most people cannot tell these 3 stringer garage steps apart from a stairway built inside the house. Budget extra time for the alignment checks in this guide. A straight edge across all three stringers is the fastest way to catch problems before the riser boards hide them.

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