How to lay out a stair stringer
Divide the total rise by the maximum riser height and round up. That’s your riser count. Now divide the total rise by that count and you’ve got the exact riser height. Treads are always one fewer than risers on a flight that lands flush with the floor.
Don’t forget the first cut
Drop the bottom of the stringer by the thickness of one tread. That way the first step finishes the same height as the rest. Miss that and the bottom step comes out a tread taller than every other one. It’s probably the single most common stair-building mistake.
Measure the total rise from finished floor to finished floor
Total rise is the vertical distance between finished surfaces, and the word finished is doing the work. If tile is going down at the bottom and oak at the top after the stair is built, both thicknesses have to be in the number now.
Three-quarters of an inch spread across fourteen risers is a sixteenth each, and nobody will feel it. Three-quarters of an inch at the top, with an inch and a half of tile and mortar at the bottom, is three-quarters of an inch of difference concentrated into the first and last steps. That’s exactly where people trip. Measure with a straight edge run out from the upper floor and a long level, taken where the stair will actually land rather than against the wall.
Deck and exterior stairs give you a second way to get it wrong. The ground moves, and the pad may not exist yet. Measure from the top of the finished decking down to the top of the finished landing pad, not to the soil you happen to be standing on. If the pad is being poured afterwards, set and record its finished height before you cut, and pour to that figure.
A stair dropped onto a pad that ends up an inch low has one oversized bottom riser. That fails the 3/8 in variation rule on its own, and it’s probably the most common single reason deck stairs get pulled up at inspection.
Two limits get checked at the same time as the rise. Headroom must be at least 6 ft 8 in, measured vertically from a line joining the tread nosings. A stairwell opening that was comfortable at one pitch can fail once you steepen it, because the nosing line moves. And a single flight may not rise more than 151 in (12 ft 7 in) between floors or landings. A normal basement or floor-to-floor run sits well inside that. A long open flight in a vaulted space usually doesn’t, and needs an intermediate landing. Those are IRC numbers.
Canada runs different ones. NBC 9.8 allows a 200 mm (7 7/8 in) rise and requires a 255 mm (10 1/16 in) run for a stair serving a single dwelling unit.
Headroom under the NBC is 1,950 mm rather than 6 ft 8 in, and Ontario raises it to 2,050 mm. A flight is capped at 3.7 m, or 145 3/4 in.
Unlike the IRC, the NBC also limits adjacent risers to 5 mm of variation, on top of capping the whole flight at 10 mm.
Fitting the run into the space you have
The building fixes the rise. Run is what you have to find room for, and it’s almost always the constraint. Work the figure out and chalk it on the floor first. Discovering it after the stairwell is framed is how stairs end up steep, cramped and short of headroom at the top.
Take a 108 in rise. Divided by fourteen risers it gives 7.71 in each, thirteen treads, and 11 ft 4 1/2 in of horizontal run at 10 1/2 in treads. That’s before the landing at the bottom, which has to be at least 36 in in the direction of travel. Over 14 ft of floor goes to the stair alone.
There’s a comfort relationship sitting behind the code limits that’s worth knowing. Two risers plus one tread should land somewhere around 24 to 25 in — the old Blondel rule, which comes from the fact that climbing trades stride length for lift at roughly two to one.
Got the floor space? Spend it on tread depth rather than on saving a step. A 7 in riser with an 11 in tread gives 25 and feels easy underfoot. Push to the code maximum of 7 3/4 in with a bare 10 in tread and you get 25 1/2. Perfectly legal, but noticeably steeper, and much harder to come down carrying a laundry basket.
When the run genuinely won’t fit, there are a few legitimate moves and one illegitimate one. The nosing is the cheapest gain. Where tread depth is under 11 in, the IRC requires a projection of 3/4 to 1 1/4 in, and that projection adds walking surface without adding a single inch to the total run.
Beyond that, break the flight with a landing and turn it, or use winders. Winders must still give 10 in of tread depth at the walkline (12 in in from the narrow side) and never less than 6 in anywhere within the clear width. The illegitimate move is quietly making one riser taller to swallow the difference.
Stringers: throat, spacing and support
A cut stringer is a beam with most of its depth sawn away. What remains under the notches (the throat) is the whole of its strength. The working minimum is 5 in of solid material, measured perpendicular from the back edge to the inside corner of the notch.
So a nominal 2×12 at 11 1/4 in actual is the smallest stock that works. A 7 3/4 in rise with a 10 1/2 in tread cuts a triangle whose corner sits about 6 1/4 in in from the nosing line, leaving just over 5 in behind it.
Step down to a 2×10 and the throat drops to around 3 in. The stringer then fails in shear at a notch corner long before it ever reaches its bending limit.
Stringer spacing follows the tread material, not the code minimum. AWC’s DCA 6 (a US guide, with no Canadian equivalent) caps cut stringers at 18 in on center with 2× or 5/4 treads, and many builders tighten that to 16 in for stiffness underfoot.
Composite decking won’t go that far. Most lines call for 9 to 12 in on center depending on the product, because the boards are far less stiff than lumber and a stair tread carries a concentrated load in the middle of an unsupported span. Work it out by dividing the stair width by the span your tread material allows, then adding one for the end stringer. A 36 in wide composite stair typically needs four or five stringers, where a wood-tread stair needs only the three DCA 6 sets as its minimum.
Support matters as much as size. DCA 6 caps the unsupported horizontal span of a cut stringer at 6 ft, or 7 ft for a southern pine 2×12. A long flight therefore wants intermediate support. That could be a notched 4×4 post, an intermediate landing, or a solid uncut stringer instead, which is allowed 13 ft 3 in.
Hang the top on a ledger with proper sloped hangers rather than toe-nailing into end grain. Bear the bottom on a footing carried below the frost line, which is roughly 36 to 48 in across the northern states and most of Ontario and the Prairies, and deeper again farther north. DCA 6 won’t let a stringer bear on a shallow pad or patio.
Outdoors, use treated stock throughout and hot-dip galvanized or stainless fasteners. Corroded hardware brings down more stairs than rotten lumber does.
How many steps for a 4 foot rise
Seven risers at 6 7/8 in each, giving six treads you walk on. Forty-eight inches divided by the 7 3/4 in code maximum is 6.19, which rounds up to seven. Then divide 48 back by seven for the exact riser height. The deck or floor surface is the seventh riser.
Six risers would be 8 in each, which fails the 7 3/4 in limit outright. So seven is the answer for almost every 48 in deck. Six treads at 10 1/2 in gives 63 in of run (5 ft 3 in). The cut diagonal through the stringer is the square root of 48 squared plus 63 squared, or 79 in, so an 8 ft 2×12 covers it with enough left for the plumb cuts.
Switch to two 2×6 deck boards per tread at 11 1/4 in and the run grows to 67 1/2 in, and the diagonal to about 83 in. Still inside an 8 ft board. Leave 36 in of clear landing at the bottom.
How long does a stringer need to be for 3 steps
Budget about 3 to 4 ft of cut diagonal, so buy a 6 ft 2×12. Three treads sitting under four 7 1/2 in risers is 30 in of rise and 33 in of run at 11 in treads, a diagonal of 44 1/2 in. Add roughly a foot for the plumb cuts and layout waste.
Three steps can mean two different stairs, depending on where the top tread lands. If the deck or floor is the top surface, you have three risers and two treads. That comes to 22 1/2 in of rise, 22 in of run and a 31 1/2 in diagonal, which a 4 ft offcut covers. If you walk on three treads and then step up onto the deck, it’s four risers and the numbers above apply.
The calculator reports the diagonal through the cuts, not the board you buy. Add about 12 in for the bottom foot and the top plumb cut.
Pre-cut 3-step stringers assume the second reading, not the first. Pylex’s steel 13903 at 7 1/2 in rise by 10 1/4 in run works out at roughly 30 3/4 in by 30 in overall, which is three treads of run against four risers of rise. ProWood’s pressure-treated 3-step at 6 3/4 in and 10 in gives three treads over 27 in of total rise the same way.
Stair rise and run for a deck
The rise on a deck runs from the finished deck surface down to the finished landing pad. Divide it by 7 3/4 in and round up for the riser count, then divide the rise back by that count. Treads must be at least 10 in deep. Most deck stairs use two 2×6 boards for 11 1/4 in.
A 36 in deck works out at five risers of 7 3/16 in and four treads. That gives 45 in of run at 11 1/4 in, with a 57 5/8 in stringer diagonal.
Deck boards set the run for you. Two 2×6 with a 1/4 in gap give 11 1/4 in, three give 17 in for a wide, gentle flight, and 5/4 decking runs to whatever combination you cut.
Every deck stair also needs 36 in of clear width, plus a bottom landing at least as wide as the stair and 36 in in the direction of travel. Add a graspable handrail as soon as the flight has four or more risers.
How much is 7 3/4 riser in a flight of stairs
Seven and three-quarter inches is the tallest single step the IRC allows. Every riser in the flight has to be within 3/8 in of every other. Ten risers at that height climb 77 1/2 in. The tallest flight code permits between landings, 151 in, works out at 19 risers.
In practice almost no one builds at 7 3/4 on the nose. You divide the total rise by the riser count and land wherever the arithmetic puts you. A 100 in floor-to-floor gives 13 risers at 7.69 in. A 120 in one gives 16 at 7.5 in.
Risers are measured from the top of one tread to the top of the next, so any flooring going down afterwards has to be in the number now. And the 3/8 in tolerance applies across the whole flight (largest minus smallest), not between neighboring steps. A single short bottom riser fails even when every other one matches perfectly.
Common questions
What is the maximum riser height?
The IRC allows 7 3/4 in for residential stairs, with a minimum 10 in tread. Local codes can be stricter, so check with your local code office or supplier before cutting.
Why must risers be equal?
Uneven risers are a trip hazard and a code violation. Your feet learn the first step and expect the rest to match. The allowed variation across a flight is 3/8 in.
What lumber for stringers?
Minimum 2×12, because the notches remove a lot of depth. Anything thinner leaves too little solid wood behind the cuts. Support them at mid-span on wide flights.
How many steps do I need for a 9 foot rise?
Work it in inches. Nine feet is 108 in, and dividing by the 7 3/4 in code maximum gives 13.9, which rounds up to 14 risers at 7.71 in each, giving thirteen treads. Thirteen risers would work out at 8.31 in each and fail. Always divide the full rise by the riser count rather than rounding any single step.
Can I use 7 3/4 inch risers in a shop or office?
No. The 7 3/4 in riser and 10 in tread are IRC residential limits. Commercial stairs fall under the IBC, which caps risers at 7 in and requires a minimum 11 in tread. A stair built to residential geometry in a commercial space won’t pass, and the deeper tread also lengthens the total run considerably, so plan the space for it.
How far apart should stair stringers be?
It depends on the tread material. Eighteen inches on center is the prescriptive maximum for 2× or 5/4 wood treads under AWC’s DCA 6, and many builders tighten it to 16 in. Composite treads want much closer, commonly 9 to 12 in on center depending on brand and product line, so read the manufacturer’s instructions rather than assuming. Divide the stair width by the allowable tread span, then add one stringer for the outer edge.
Do the treads have to be equal as well as the risers?
Yes. The IRC applies the same 3/8 in rule to tread depth as it does to riser height, so the largest within a flight may not exceed the smallest by more than 3/8 in. Nosing projections must also be consistent within 3/8 in. That matters if you mix a bullnosed tread with a square-edged landing at the top.
What handrail height does the stair need?
Under the IRC, 34 to 38 in, measured vertically off the sloped line joining the tread nosings. Canada allows a wider 865 to 1,070 mm (34 to 42 in). A round graspable handrail must be 1 1/4 to 2 in in diameter, and a non-circular one needs a 4 to 6 1/4 in perimeter. The guard on an open side is separate. On stairs it’s 34 in minimum from the nosing line, passing no 4 3/8 in sphere, with the triangular opening at tread and riser limited to a 6 in sphere.
How many steps do I need for a 5 foot rise?
Eight risers at a neat 7 1/2 in, giving seven treads. Sixty inches divided by 7 3/4 is 7.74, which rounds up to eight. Seven treads at 10 1/2 in is 73 1/2 in of run and a 95 in stringer diagonal, so cut those from 10 ft 2×12s. A 96 in board leaves nothing at all for the plumb cuts.
Does a short flight of deck steps need a handrail?
It comes down to the riser count. Four or more risers triggers it, so a 4 ft rise at seven risers definitely does. Three risers or fewer needs no handrail, which is why so many low decks are built to three steps and no more. Guards are a separate requirement with a separate trigger. You need one wherever the walking surface sits more than 30 in above the grade below.
What length 2×12 do I need for a 4-step stringer?
Buy a 6 ft board. Four treads under five 7 1/2 in risers is 37 1/2 in of rise and 44 in of run at 11 in treads, a 57 3/4 in diagonal. Once you add the bottom foot and the top plumb cut you’re past a 5 ft board. Cut one as a pattern and trace the rest from it.