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

Even below 36″, you might still choose 3 stringers. Two cases call for it: you’re using thinner tread material (3/4″ plywood instead of 2x lumber), or 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 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.

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 / Kicker Plate #

No changes to the process. During the dry-fit, temporarily position all 3 stringers to check 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 (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 kicker plate (the board at the bottom that anchors the stringers to the slab or floor)
• 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: a metal joist hanger, or toenail it (drive fasteners in at an angle through the face of one board into the other) with structural screws
• At the bottom, the center stringer sits on the kicker plate. Toenail it 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 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″ 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 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, so 6 screws per tread total (2 at each outer stringer plus 2 at the 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 it 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 (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
• Simpson Strong-Tie LSC Stair Stringer Connector - connector hardware for attaching stringers to headers and ledger boards

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Reference Photos #

The following photos show examples of this type of work completed in the field. These serve as visual references for quality standards, proper installation techniques, and finished results.

All (12) Before (1) During (1)

Showing 12 of 12 photos

Two-tier closet shelving with hanging rod frame, particle board shelves and wood cleats, ready for paint.

Staircase mid-construction with finished skirt boards, raw treads/risers, and protective coverings in place.

Completed horizontal-rod railing with dark wood newel posts and metal balusters around curved loft opening.

Completed curved iron baluster railing with dark wood handrail and newel posts installed on staircase.

Curved staircase with painted risers, horizontal metal balusters, and dark wood newels—complete railing system.

Iron balusters installed between wood handrails and treads with black finish caps and newel posts.

3D stair design rendering showing newel posts, horizontal rod railing, and landing platform.

Finished stair assembly with wood treads, risers, newel posts, and handrails installed—raw wood ready for paint.

Completed wood staircase with finished treads, risers, newel posts, and handrail in raw wood before painting.

Completed stair assembly with finished treads, risers, newel posts, and handrail—raw wood ready for stain or paint.

Garage stairs with 2 stringers and closed risers—raw wood treads, black risers, white riser boards, fasteners visible, ready for paint.

Finished stair skirt boards installed on both sides of staircase with raw wood treads and risers, ready for paint.

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