Building Stairs That Pass Inspection

The Rise Is Measured Once, and Everything Else Is Derived From It

A flight of stairs contains exactly one independent measurement. Total rise, finished floor level below to finished floor level above, is taken once, and the riser height, the going, the stringer length, the trimmer position and the location of every fixing are all derived from it. Lift that dimension off the wrong datum and no amount of careful cutting recovers it. The flight is out by a fixed quantity before a saw is switched on, and the only remaining question is which step ends up carrying the discrepancy.

Finished means finished. The 65 mm screed still to be poured, the 18 mm engineered board arriving in week nine, the 12 mm of levelling compound the tiler will lay in the hall below: every one of those is a real number belonging inside the total rise, and every one is routinely absent from the dimension a carpenter lifts off a bare deck. Ask whoever owns the finishes schedule for the build-up in writing, then pencil the constituent thicknesses on the wall beside your datum line with the date against them. When the flooring specification changes in month four, and it usually does, that pencil note separates a ten-minute recalculation from a stringer in the skip.

Shoot both levels with a rotary laser rather than running a tape up a wall that may not be plumb, and take the reading at the actual stair opening rather than at a convenient corner four metres away. Slabs dish. Timber decks crown over a doubled joist. Where the upper and lower readings disagree across the footprint of the opening, record the worst case and set out to it, because the inspector's gauge will find the same spot you chose to avoid.

Dividing the Rise: the Remainder Has Nowhere to Go

Divide 2870 mm by seventeen risers and the answer is 168.82 mm. No tape carries that graduation, no square is set to it, and no pencil line is that thin. The 0.82 mm is nonetheless real: it exists seventeen times over, it totals just under fourteen millimetres, and it lands somewhere in the flight whether you place it deliberately or let the layout place it for you.

Round down to 168 mm, cut seventeen identical risers, and the top step finishes almost fourteen millimetres short of the landing. That single step now differs from its sixteen neighbours by more than most codes allow between any two risers in a flight, and it sits at the worst possible position, where a walker stepping onto the landing has already stopped looking down. Rounding up produces the mirror image at the bottom, where the first step meets a foot arriving at speed off a level floor.

Two approaches survive scrutiny. Work in the true decimal and set every mark from a single cumulative datum at the bottom of the flight, so that no mark inherits the position of the one before it and the rounding error at any step stays inside half a graduation. Or select a riser count whose division lands closer to a whole number and accept the small change in pitch that comes with it. Cutting sixteen at one height and letting the seventeenth mop up the remainder is not a method; it is a defect with a schedule attached.

Riser count also decides things that have nothing to do with comfort underfoot. Add a riser and the flight grows longer by one going, which moves the bottom step further into the room, shifts the newel, and changes where the flight passes under the trimmer. Subtract one and the pitch steepens, the going tightens, and a nosing projection that was comfortable becomes the thing people catch a heel on coming down. Settle the count before anything is cut, because every downstream dimension is hostage to it.

One Pencil Line, Repeated Fourteen Times

Layout on a timber stringer is traditionally done by stepping a framing square with stair gauges along the stock, marking, sliding, marking again. Each transfer inherits the position of the last. A sharp pencil leaves a line around half a millimetre wide, a carpenter's pencil considerably more, and the square has to be registered against that line by eye fourteen or seventeen times in a row.

Half a millimetre of consistent bias, repeated fourteen times, is seven millimetres of drift from the bottom of the flight to the top. Nobody detects it stepping from one tread to the next, and nobody detects it with a tape held across two adjacent risers. It shows up exactly where the inspection method looks for it: as the difference between the largest and the smallest riser in the flight, measured end to end.

The cure is to stop stepping. Set out a storey rod, a single straight batten marked with every riser position measured cumulatively from one end, check the marks once against the arithmetic, and then transfer from the rod to both stringers. Every mark on the rod carries only the error of its own measurement, never the accumulated error of the marks below it. Routed templates and CNC-cut strings do the same job by another route, and for a steel string the drawing office does it for you, which is why the number you send them has to be right.

Verify the layout end to end before cutting. Measure from the first riser mark to the last along the stringer and compare it with the calculated distance for that number of steps. A discrepancy found here is a pencil line and two minutes; the same discrepancy found after both strings are cut, housed and wedged is a flight in the skip and a plasterer back on site.

What the Tolerance Actually Permits

Codes do not ask for perfection. They ask for uniformity, and they measure it in two ways at once: the difference between any two adjacent risers, and the difference between the largest and smallest riser anywhere in the flight. Passing the first and failing the second is a common outcome of stepped layout, because adjacent steps drift apart slowly while the two ends of the flight drift apart steadily.

The permitted figure depends entirely on where the stair is. In North America the International Residential Code and the International Building Code both cap the variation within a flight at a small fraction of an inch, commonly cited as three-eighths, with the adopted edition and any local amendment governing the number the inspector actually applies. In England and Wales, Approved Document K, Protection from falling, collision and impact, requires the risers in a flight to be equal, with BS 5395-1, Stairs, Code of practice for the design of stairs with straight flights and winders, behind it. Australian work is governed by the National Construction Code, with AS 1657, Fixed platforms, walkways, stairways and ladders, applying to industrial access. Machinery access under ISO 14122-3, Safety of machinery, Permanent means of access to machinery, Stairs, stepladders and guard-rails, carries its own geometry. Check the edition your authority having jurisdiction has adopted rather than the one you learned.

Why so tight? Because a person climbing calibrates their gait to the first two or three steps and then swings the foot to a remembered height without looking. A step that departs from that remembered height is met either early, jarring the knee, or late, dropping the walker onto a tread that is not where the body expected it. Descending is worse, because the recovery time is shorter and the fall is longer. The tolerance is not draughtsman's fussiness; it is the distance between a stair that walks and one that does not.

The Drop Cut: the One Riser Deliberately Different

Every riser in a flight is cut the same except one, and that one is at the bottom. The stringer sits on the lower floor, but the first tread sits on the stringer, so the bottom riser as cut has to be reduced by the finished thickness of the tread it will carry. That reduction, the drop cut or sink cut, is what makes the first step match the other sixteen once the treads are on.

Forget it and the error appears at both ends of the flight simultaneously. The bottom riser finishes one tread thickness too tall, the top one finishes one tread thickness too short, and the spread between largest and smallest is twice the tread thickness. On a 32 mm tread that is 64 mm of spread on a flight where the permitted figure is under ten. No packer, no plane and no adjustment fixes it; the strings are scrap.

Complications sit either side of the drop cut. A thicker starter tread changes the amount removed. Carpet, tile or resilient flooring finishing against the bottom riser changes the floor level the stringer actually lands on. A landing nosing at the top, or a trimmer face lined out after the stair is fitted, changes the effective top riser. Each of these belongs in the total rise arithmetic, not in an adjustment made later with a chisel.

Cut one string, dry-fit it, and measure the first and last riser in place with offcuts of the actual finishes laid under and over before cutting its pair. Half an hour proving the drop cut on a single string is the cheapest insurance on the job.

Steel: the Millimetre You Cannot Plane Out

Timber forgives. Three millimetres of slab dish disappears under a packer, a proud riser takes a pass with a plane, a tight housing gets eased. A fabricated steel string forgives nothing, because the dimension left site weeks earlier: the plate is profiled, the tread pans are welded on, the bolt holes are drilled, and the flight arrives on a lorry as a finished object that either fits the opening or does not.

What the fabricator needs is not the drawing dimension but the measured one, taken off the poured slab, plus the confirmed finish build-up at both ends, the thickness of the landing plate the top of the string lands against, the depth of the tread pans, the depth of concrete or screed going into those pans, and the shim allowance under the base plate. Every one of those quantities moves a riser.

Pan fill is the trap. Fill every pan to the same depth and the intermediate risers stay uniform, which is why the error survives a casual check: only the bottom riser, measured from the lower floor to the top of the first filled pan, and the top riser, measured from the last filled pan to the upper finished floor, actually move. Specify the fill on the order as a finished level rather than a volume, or the concretor will decide it for you.

Steel fabrication tolerances are generous compared with stair riser tolerances, and a shop working to its normal standard can deliver a stair that is dimensionally correct by its own rules and non-compliant by the building code. State the riser tolerance explicitly on the order, in writing, alongside the length. It is the one dimension on that drawing with a legal limit attached.

The Same Number, Measured Sideways: Headroom and Pitch

Riser height and headroom are one number seen from two directions. The pitch line runs across the nosings, and headroom is measured vertically from that line to whatever sits above it: the trimmer, a bulkhead, the underside of the flight over. Change the riser height and the pitch line rotates, which moves the point where it passes closest to the obstruction.

Adding a riser to bring the height down is the standard fix for a stair that walks badly, and it is also the standard cause of a headroom failure. The flight grows by one going, so it starts further out into the lower room and passes under the trimmer at a different point, usually a worse one. The trimmer does not move. Resolve headroom against the chosen riser count before the strings are marked, not after they are fitted and the plasterboard is up.

Curved and spiral flights carry the same discipline with harder geometry: the going varies with radius and compliance is judged on a defined walking line. The riser still has to be uniform bottom to top, and headroom still has to be checked at the tightest point of the helix rather than at the entry.

Proving the Spread Before the Inspector Does

Prove the flight yourself before anyone else measures it. Lay a straightedge along the nosings and measure each riser off it, or set a pinch rod to the design riser and walk it up the flight step by step; anything that will not take the rod gets recorded. Do this after the dry fit and before glue, wedges or finish nails, while the fix is still a matter of easing a housing rather than dismantling a stair.

Write the results down as a list: the measured height of every riser, the largest, the smallest, and the spread between them. That list takes five minutes and does two jobs. It proves compliance if the number is queried later, and it tells you which end of the flight your layout method is drifting towards.

The Number Outlives You on Site

Floor finishes are the last thing to change and the first thing to break a stair. A tiler who lays 22 mm of bed and tile where 12 mm of vinyl was scheduled has just altered the bottom riser of a completed flight by ten millimetres, and nobody involved will think about the stair until inspection day. Any change to the build-up at either end of the flight goes back through the total rise, every time, without exception.

Leave the arithmetic behind for whoever comes next. Pencil the design riser, the riser count, the total rise and the finish build-up you assumed onto the underside of a string or inside the wall string housing, where it survives decoration and is found by the carpenter who has to alter the flight in fifteen years. Hand the same figures over with the measured spread from the site check and a note of any variance the authority having jurisdiction accepted. One millimetre, placed deliberately and recorded, is a stair that walks; the same millimetre, accumulating quietly through seventeen steps, is a rip-out.