Stair Stringer Layout Guide

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How Stringer Layout Math Works

A straight flight is a right triangle. The vertical leg is total rise (finished floor to finished floor). The horizontal leg is total run (the going of the flight). The hypotenuse is the stringer. Everything else — number of steps, unit rise, unit run, pitch angle — is just dividing those two legs into equal, code-legal slices.

Count risers, not treads, first. You always have one more riser than tread on a typical floor-to-floor stair: the last riser lands you on the upper floor. If total rise is 108" and you want about 7" per step, you take 108 ÷ 7 ≈ 15.4 and round to a whole number of risers that keeps each rise at or under 7.75". Fifteen risers at 108" is 7.200" each. Fourteen treads carry the run.

Unit run is IRC tread depth: nosing to nosing, measured horizontally. The nosing itself (usually 3/4" to 1-1/4") is extra tread stock hanging over the riser face. Do not add nosing onto the stringer's level cut or you will stretch the flight and miss the landing.

Comfort is the old 2R+T rule: two unit rises plus one unit run should land near 24–25". A 7.2" rise and 10.5" run is 24.9" — it walks. A 7.75" rise and 10" run is 25.5" — legal in many houses, steeper than people like. The calculator reports 2R+T next to the code flag so you can see legal-but-steep versus legal-and-comfortable.

That example is the calculator default. Change total rise and the tool re-picks a riser count that tries to keep unit rise ≤ 7.75" and unit run ≥ 10" at the same time. When the well is too short for both, it still gives the best compromise and flags FAIL. Do not “make it work” by fudging the first riser — that is how you fail inspection and trip people.

Open, housed, and basic-cut stringers share this math. The difference is how the tread meets the board, not how many steps you have. Pick the type in the calculator for the throat note; pick it on the job from the open vs closed comparison.

Typical Residential Code Limits (IRC)

Most US houses point at the International Residential Code. The numbers the calculator flags against:

• Riser height — maximum 7.75", minimum 4" (IRC R311.7.5.1).
• Tread depth — minimum 10" nosing to nosing (IRC R311.7.5.2).
• Variation — largest minus smallest riser (and tread) in a flight ≤ 3/8".
• Nosing — typically 3/4" to 1-1/4" where required (R311.7.5.3).
• Headroom — 6' 8" measured plumb from the nosing line (R311.7.2). The calculator does not check this.

Finish floors change the first and last riser. If the lower floor gets 3/4" hardwood after the stringer is set, that first rise grows 3/4" unless you cut the bottom of the stringer down by the same amount. If the upper landing gets a thicker finish than the treads, the last rise shrinks. Measure finished floor to finished floor, then subtract the tread thickness and any finish you have not installed yet. The troubleshooting page is the fork for first-riser mistakes.

How to Mark a Stringer

Do not skip the story pole. Cut a scrap 1x and mark every unit rise from the bottom. Stand it in the well. The last mark should land on the finished upper floor. If it does not, your total rise is wrong — fix the number before you touch a 2x12.

1. Lock the numbers. Run the Stair Stringer Calculator. Write unit rise, unit run, riser count, and stringer length on the stock. PDF the cheat sheet if you want it on the sawhorse.
2. Crown the board up. Sight the 2x12. Crown toward the treads so the throat does not sag. Cut the squarest, straightest piece for the first pattern stringer.
3. Stair gauges on the framing square. Clamp one gauge at unit rise on the tongue, one at unit run on the body. The square now only sits one way on the edge.
4. Walk the square. Start at the bottom: the first level cut is the bottom of the first tread, the first plumb cut is the first riser. Slide the square so the next rise/run lands exactly on the previous crotch. Tick every corner. Count. You should land on the same riser count as the calculator.
5. Drop the bottom. Subtract tread thickness (and lower finish if it is not in yet) from the first riser so the first step matches the others after the tread is on. This is the cut that makes or breaks even risers.
6. Top plumb and level seats. The top of the stringer needs a plumb cut against the header or a level seat on the landing — match how you are hanging it. Dry-fit the pattern before you clone it.

Housed stringers skip the triangular walk. You still use the same unit rise and unit run, but you strike dado centerlines and router or jig the grooves. The pitch of the board is the same hypotenuse. Details: open vs closed (housed) stringers.

How to Cut Without Killing the Throat

On an open stringer the remaining wood behind the notches is the whole beam. Overcutting the crotch with a circular saw is the classic way to split a 2x12 the first year the house moves.

1. Set blade depth so it just exits the far face — no extra.
2. Cut on the waste side of the line. Leave the pencil. You can always sneak up; you cannot put wood back.
3. Stop the circular saw 1/8"–1/4" short of the inside corner. Finish the crotch with a sharp handsaw or a jigsaw. The two kerfs should meet, not cross.
4. Check remaining throat against the calculator. If it is under about 3.5" on a 2x12, you are too steep or on too small a board. Flatten the pitch or step up stock before you hang it.
5. Clone the pattern. Gang-cut only if the boards are the same width and the pattern is proven in the well.

Basic-cut stringers follow the same saw discipline. Housed stringers are a different risk: a dado that is too deep or a router that walks will show as a loose tread, not a split crotch. Keep housed grooves consistent and dry-fit every tread.

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