Porch Stairs, 24 Inch Rise, IRC Riser Check
A short porch flight. Three risers at 8 in would fail, so the equal-riser math matters.
Formula SSC-2026.1
IRC check warning
Risers / treads
3 / 2
Try 4 risers
Exact riser
8 in
203.2 mm
Total run
1 ft 9 in
533 mm
Stringer length
2 ft 7 7/8 in
Order 8 ft 2x12
Incline
48.81°
Square 8 in rise · 10 1/2 in run
Stringers @ 16 in
4
Throat 4.89 in on a 2x12
- Riser is taller than the 7.75 in IRC maximum.
- Estimated 2x12 throat is under 5 in. Confirm with the local inspector.
Rise, run, and the incline are scaled from the live riser and tread. Framing-square gauges match the exact riser and the tread depth.
| Item | Net | Waste | Order | Unit price | Line total |
|---|---|---|---|---|---|
| 8 ft 2x12 | 3.32 ft cut | end cuts included | 4 boards | USD | $67.20 |
| Stringer delivery weight | 90 lb | — | 41 kg | 2.8 lb/ft planning weight | — |
| Target | Risers | Exact riser | Run | Angle | Check |
|---|---|---|---|---|---|
| 6 1/2 in | 4 | 6 in | 2 ft 7 1/2 in | 37.3° | Pass |
| 7 in | 3 | 8 in | 1 ft 9 in | 48.8° | Warning |
| 7 1/2 in | 3 | 8 in | 1 ft 9 in | 48.8° | Warning |
| 7 3/4 in | 3 | 8 in | 1 ft 9 in | 48.8° | Warning |
Show the deterministic formula
Risers = round(total rise ÷ target rise). Exact riser = total rise ÷ risers, reported to the nearest 1/16 in and in millimeters. Treads = risers − 1. Total run = treads × tread depth.
Diagonal = hypotenuse of stringer rise and total run. A dropped header uses total rise minus landing thickness. Stock is the next 8, 10, 12, 14, 16, 18, 20, or 24 ft board.
Stringers = floor(width ÷ 16) + 1, with a minimum of 2. Throat ≈ 11.25 − (riser × tread) ÷ hypotenuse.
Contractor sign-off
Project: Porch Stairs, 24 Inch Rise, IRC Riser Check
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This sheet is an estimate for ordering and layout discussion. It is not a sealed design. Confirm the quantities with a licensed local professional and the authority having jurisdiction before you buy or build.
Engineering notes
Twenty-four inches divided by a 7.5 in target rounds to 3 risers of 8 in, which is over the 7.75 in IRC maximum. The badge warns and the suggested riser count is the smallest count that lands at or under 7.75 in. The width is 48 in, so the stringer count is higher than a 36 in stair. Treads are 10.5 in. Use the warning as a reason to add a riser, not as permission to build the 8 in step.
Code reference
IRC R311.7.5.1 limits risers to 7.75 in and R311.7.5.2 requires at least a 10 in tread. Confirm local amendments, guards, and headroom.
Field questions
- Why does a 24 inch porch fail at a 7.5 inch target?
- 24 ÷ 7.5 is 3.2, which rounds to 3 risers. 24 ÷ 3 is 8 in, and 8 in is over 7.75 in.
- How do I make it pass?
- Use the suggested riser count shown next to the warning, or lower the target until the equal riser is 7.75 in or less. Then recheck the run.
- How many stringers on a 48 inch width?
- Floor of 48 ÷ 16 is 3, plus one, so four stringers.