Introduction #

Building garage steps with 3 stringers is the right call when your stairway is wider than 36 inches. This is a supplemental guide for building garage or patio steps with 3 stringers and open risers (3SO). The complete step-by-step process is covered in our core guide: Build Garage Steps. Read that guide first. Everything below assumes you know the standard 2-stringer open riser process and focuses only on what changes when you add a center stringer for wider stairways.

When You Need Garage Steps with 3 Stringers #

Residential building codes and practical experience both point to the same thresholds:

• Stairs up to 36″ wide: 2 stringers are sufficient (covered in the core guide)
• Stairs 36″ to 48″ wide: Add a center (third) stringer — this is what this guide covers
• Stairs wider than 48″: Add additional center stringers at 16″ to 24″ intervals

Even below 36″, you might choose 3 stringers if you’re using thinner tread material (3/4″ plywood instead of 2x lumber) or if 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 3 steps, that’s 6 blocks (2 per tread level × 3 levels)
• Additional fasteners: 3″ structural screws or 16d nails for blocking attachment, 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.

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 align. Even 1/8″ difference will cause a wobble

Steps 4 & 5: Dry-Fit / Kicker Plate #

No changes to the process. During dry-fit, temporarily position all 3 stringers to verify alignment.

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: Center stringer placement:

• Measure along the header board (or 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 kicker plate
• 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 (joist hanger or toenail + structural screws)
• At the bottom, the center stringer sits on the kicker plate — toenail or use an angle bracket to secure it

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 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″ structural screws or 16d nails, 2 fasteners per end
• The blocks should be tight. If there’s a gap, the block won’t prevent twisting
• Check that all 3 stringer tops are level with each other after blocking is installed

Step 7: Install Risers #

Not applicable for open riser builds. Skip to Step 8. If you want closed risers with 3 stringers, see the 3-stringer closed riser supplemental.

Step 8: 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 = 6 screws per tread (2 at each outer stringer + 2 at center stringer)
• Apply construction adhesive at all 3 stringer contact points
• 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 as needed before the adhesive sets

Step 9: Final Verification #

Follow the core guide verifications, plus:

• Stand on the center of each tread and check for deflection — it should feel noticeably stiffer than a 2-stringer build
• Check that 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 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

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 (dimensional lumber varies). Re-check your layout lines. If one stringer is 1/16″ off, that’s acceptable. More than that, re-cut 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 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
• Simpson Strong-Tie LSC Stair Stringer Connector — connector hardware for attaching stringers to headers and ledger boards