How to Build Garage Steps with 3 Stringers (Open Risers)

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Stairs/Handrails Skill Level: Advanced (Level 3)

When You Need Garage Steps with 3 Stringers #

A stringer is the diagonal board that supports the treads (the flat part you step on) and risers (the vertical face between two treads). Residential building codes and field experience point to the same width thresholds for how many stringers you need:

• Stairs up to 36″ wide at the brick mold: 2 stringers are enough (covered in the core guide)
• Stairs 36″ to 48″ wide: Add a center (third) stringer on open risers, which is what this guide covers. Closed risers carry 2 stringers up to 48″ (see the closed-riser guide)
• Stairs wider than 48″: Add more center stringers at 16″ to 24″ intervals

Even below 36″, you might still choose 3 stringers when the stairs will carry heavy loads regularly. If your 2-stringer stairs feel bouncy underfoot, a center stringer is the most effective fix.

Additional Materials #

Beyond everything in the core guide materials list, you need:

• One additional 2×12 for the center stringer (same length as the outer stringers)
• Blocking material: Short lengths of 2×4 or 2×6, enough for one block between each pair of stringers at every tread level. For a stair with 3 risers and 2 treads, that’s 4 blocks (2 per tread level × 2 levels)
• Additional fasteners: 3″ exterior-rated structural screws or 16d nails for attaching the blocking, plus extra tread screws. You’ll add 2 screws per tread at the center stringer

What’s Different: Step-by-Step #

Steps 1 & 2: Measure Total Rise / Lay Out First Stringer #

No changes. Follow the core guide exactly. The garage steps calculator does the Step 1 riser math for you; enter your stair width and it also tells you how many stringers to cut.

Step 3: Cut Stringers #

Cut 3 identical stringers instead of 2. Use your first stringer as a template:

• After cutting and verifying your first stringer (per the core guide), lay it on top of the next 2×12
• Trace the exact profile with a pencil
• Cut and verify - all 3 stringers must match exactly
• Stack all 3 on a flat surface and confirm all tread cuts and riser cuts line up. Even 1/8″ of difference will cause a wobble

Steps 4 & 5: Dry-Fit / Glue the Stringers to the Slab #

No changes to the process, other than doing it for 3 stringers instead of 2. During the dry-fit, temporarily position all 3 stringers to check alignment, then glue all 3 to the slab the same way as a 2-stringer build.

Step 6: Set and Attach Stringers - Modified #

This is the biggest change. You’re installing 3 stringers instead of 2, and adding blocking between them.

Back board: We always join the stringers with a back board before they go up. With 3 stringers, cut two pieces of 2x4 to fit between the stringers at the top, one between each pair, against the house, standing on edge with their tops flush with the top tread cut. Screw through each stringer into the ends of the pieces. The 3 stringers and the back board then go against the house as one frame, and you fasten that frame as below.

Center stringer placement:

• Measure along the header board (the framing at the top of the stairs the stringers attach to), or the top landing, and find the exact center point between your two outer stringers
• Mark plumb on the header, and transfer that line down to the slab - the same line you will use to trace the center stringer’s footprint for gluing in Step 5
• The center stringer sits at this mark, parallel to both outer stringers
• Attach the center stringer to the header board the same way as the outer stringers: drive 3″ exterior-rated structural screws through the stringer’s plumb cut into it
• At the bottom, the center stringer is already glued to the slab from Step 5, along with the outer stringers - no separate bottom attachment needed here

Install blocking:

Blocking is critical. Without it, the stringers can twist under load and the stairs will feel unstable even with 3 stringers.

• At each tread level, cut 2 blocks from 2×4 or 2×6 to fit snugly between adjacent stringers
• Position the blocks at the riser cut (the vertical face) of each step, flush with the top of the tread cut
• Toenail or screw through the stringers into the ends of the blocks. Use 3″ exterior-rated structural screws or 16d nails, 2 fasteners per end
• The blocks should fit tight. If there’s a gap, the block won’t stop the stringer from twisting
• Check that all 3 stringer tops are level with each other after the blocking is installed

Step 7: Install Treads - Modified #

Same process as the core guide, with one addition:

• Each tread now has 3 support points instead of 2
• Drive 2 screws per stringer, so 6 screws per tread total (2 at each outer stringer plus 2 at the center stringer)
• No glue on these open-riser treads, the same as the core guide: the screws hold them
• After setting each tread, check that it sits flat across all 3 stringers with no rocking. If the center stringer is slightly high or low, shim or plane it before you screw the tread down

Step 8: Final Verification #

Follow the core guide verifications, plus:

• Stand on the center of each tread and check for deflection (how much it flexes or bounces under weight). It should feel noticeably stiffer than a 2-stringer build
• Check that the blocking hasn’t shifted during tread installation

Quality Check: Garage Steps with 3 Stringers #

When building garage steps with 3 stringers, verify these items beyond the core guide quality checklist:

• Center stringer is plumb (check with a level on the riser cut face)
• Center stringer is centered: equal distance from both outer stringers at top and bottom
• All blocking is tight between stringers with no gaps
• Treads sit flat across all 3 stringers (no rocking)
• Center tread screws are driven and countersunk (the screw head sits flush with or below the wood)

Troubleshooting Garage Steps with 3 Stringers #

Center stringer doesn’t align with outer stringers: The most common cause is the center 2×12 being a slightly different width, since dimensional lumber sizes vary a bit from board to board. Re-check your layout lines. A stringer that’s 1/16″ off is acceptable. More than that, re-cut it using an outer stringer as the template, not a tape measure.

Blocking splits when fastening: Pre-drill about 3/4 of the way through the block before driving screws. Or switch to toenailing with 16d nails, which is less likely to split short blocks.

Tread rocks on center stringer: The center stringer’s tread cut may be slightly high. Set a straight edge across all 3 tread cuts and check for a gap. Plane or sand the high stringer until the straight edge sits flat across all 3.

External Resources #

• IRC R311.7 - Stairway Code Requirements - residential building code for stairs, handrails, treads, and guardrails

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