The New Floor That Left a Tall Top Step

Why this matters

People climb stairs by rhythm. After two or three steps the foot stops measuring and lifts the same height each time, so a single riser that is taller than the rest catches a toe, and a single riser that is shorter makes a foot land hard. A flooring job that raises a landing changes exactly one riser, the top one, and it is easy to finish the job without anyone measuring it. This case starts from a vague complaint, works through the candidates, and ends with a stair whose rise had to be redistributed rather than patched.

The complaint

Two months after the shop replaced the carpet in an upstairs hall and landing with 3/4 inch solid oak, the homeowner called. Visiting family kept catching their toe on the top step going up. Nobody in the household had fallen, but her father had stumbled twice, and she wanted the shop to look at "whatever is wrong with that last step."

The stair was a straight flight of 14 risers, carpeted, from the first floor up to the landing. The shop's job had stopped at the landing edge: carpet and pad came off the hall and landing, the existing 1/2 inch plywood subfloor was built up with 3/8 inch plywood underlayment because the hardwood maker's instructions required a thicker subfloor than 1/2 inch, and the oak went down over it with a stair nose at the landing edge. The stairs themselves were not touched.

Candidates, measured one at a time

The tech brought a 24-inch level, a tape and a straightedge. He did not start with the riser he suspected; he started with what a homeowner's description of "catching the top step" could mean.

Lighting. A dark top of stairs makes people misjudge the last step even when it is correct. The hall had a switched fixture at the top and a three-way at the bottom, both working, and the complaints came in daylight as well. Not the cause.

The new stair nose. A nose that projects farther than the treads below, or stands proud of the landing surface, can catch a toe on its own. He laid the straightedge across the landing onto the nose: flush. Projection past the riser below measured 1 inch, matching the carpeted nosings down the flight to within 1/8 inch. Not the cause.

Loose carpet on the top tread. Carpet that has pulled away from the riser below the top step bunches at the nosing and catches a shoe. The carpet on tread 13 was tight to the tackless and stapled under the nose. Not the cause.

The top tread's depth. A shallow top tread makes a foot land short. Tread 13 measured 10 1/4 inches from riser face to riser face, the same as the treads below it. Not the cause.

The riser heights. He measured each riser vertically from the top of one nosing to the top of the next, with the straightedge pressed down hard onto the carpet so the reading reflected where a foot actually lands. Risers 1 through 13 measured 7 1/2 inches each, within 1/16 inch of each other. Riser 14, from tread 13 up to the new landing surface, measured 8 1/4 inches.

That was the cause, and it was the only thing on the stair that had changed.

How 3/4 inch appeared

BEFORE: carpet and pad on landing and treads

            landing surface
           +=======================
           |
 riser 14  |  7-1/2 in
           |
+==========+  tread 13 surface
|
|  riser 13, 7-1/2 in

AFTER: landing rebuilt, treads unchanged

            landing surface, 3/4 in higher
           +=======================
           |  + 3/8 in plywood underlayment
 riser 14  |  + 3/4 in solid oak
           |  - 3/8 in carpet and pad removed
           |  = 8-1/4 in
+==========+  tread 13 surface, unchanged
|
|  riser 13, 7-1/2 in

Carpet and pad on these treads compressed to about 3/8 inch under the straightedge, and the landing had carried the same carpet, so before the job every riser was 7 1/2 inches surface to surface. The shop removed 3/8 inch of carpet from the landing and added 3/8 inch of plywood and 3/4 inch of oak. The landing rose by 3/8 plus 3/4 minus 3/8, which is 3/4 inch, and all of that landed on riser 14: 7 1/2 plus 3/4 is 8 1/4 inches.

What the code and the foot both say

The International Residential Code, in the 2021 edition as adopted by your jurisdiction, sets a maximum riser height of 7 3/4 inches and, in section R311.7.5.1, says in substance that the greatest riser height within a flight may not exceed the smallest by more than 3/8 inch. Check your adopted edition and any local amendments. See related: Stair Riser and Tread Rules When Flooring Changes Height, which owns these rules.

Against those numbers, riser 14 at 8 1/4 inches was over the 7 3/4 maximum by 1/2 inch and over the smallest riser in the flight, 7 1/2, by 3/4 inch, double the 3/8 inch allowed variation. Whether a building official would require correction on a flooring replacement depends on the jurisdiction and its existing-building rules. The shop did not wait to find out, because the 3/8 inch rule exists for the reason the family was stumbling: a variation larger than that is enough to break the rhythm a climbing foot runs on.

The mirror case sits at the bottom of a stair. When a new floor goes in on the lower level and the stair is untouched, the bottom riser shrinks by the build-up instead of growing. A short first step catches a toe less often going up, but going down it makes the last step land hard and early, which is how ankles get rolled. The same two rules apply to it.

The repair options

Remove the underlayment. Taking out the 3/8 inch plywood would drop the landing by 3/8, leaving riser 14 at 7 7/8 inches: 3/8 over the smallest riser, at the variation limit rather than over it, but still 1/8 inch over the 7 3/4 maximum. It would also put the oak back on a subfloor thinner than the hardwood maker allows, trading a stair problem for a floor failure. Rejected.

A thinner floor on the landing only. Replacing the oak on the landing with a thinner product and stepping down to the hall's oak puts a level change at the top of a stair, the worst place for one. Rejected.

Shim the top tread only. Raising tread 13 by 3/8 inch would cut riser 14 to 7 7/8 and make riser 13 grow to 7 7/8 as well; the flight would then hold two risers at 7 7/8 over twelve at 7 1/2, a 3/8 variation, and both still over the 7 3/4 maximum. Rejected.

Redistribute the rise across the whole flight. The stair's total rise grew by 3/4 inch, from 14 times 7 1/2, or 105 inches, to 105 3/4 inches. Spread evenly over 14 risers that is about 7.55 inches each, just over 7 1/2 and inside 7 3/4. Each tread is raised by a share that grows going up the flight: tread 1 by about 1/16 inch, tread 7 by half the 3/4 inch, tread 13 by thirteen-fourteenths of it, a little under 3/4. That cannot be done under carpet. It can be done by converting the stair to hard treads and risers, each tread set at its calculated height.

The homeowner had already asked about hardwood on the stairs, so the fourth option became the job. The work followed 'How to Install Hard-Surface Treads and Risers on a Stair', with one change: the tech laid out each tread's finished height from a story pole marked at 7.55-inch intervals from the first-floor surface to the landing, rather than taking the existing tread heights as given.

The work, in physical order

Measure everything before anything comes off. The floor-to-landing total rise was confirmed with a level and tape at 105 3/4 inches before the carpet was pulled, because once the old treads are cut back there is no reference left to check against.

Pull the carpet and tackless. Tackless pins are sharp and angled; gloves on. Staples were pulled from every tread and riser.

Cut back the nosings and set the treads. Cutting the old bullnose flush with a circular saw and an oscillating tool throws wood dust; dust extraction and respirators, with the stairwell closed off from the rest of the house. The new treads were set in adhesive and fastened per the tread maker, each shimmed to its story-pole mark and checked with a level front to back.

Check the handrail before and after. Raising the treads lowers the handrail relative to them, and the change is largest at the top. The IRC, in section R311.7.8.1 of the 2021 edition as adopted, sets handrail height at 34 to 38 inches measured vertically from the sloped plane through the tread nosings. Before the work the rail measured 35 1/2 inches above the nosings at the top of the flight; raising tread 13 by about 0.7 inch brought it to about 34 3/4 there, still inside the range. A rail that started at 34 inches would have dropped below it, and resetting the rail brackets would have joined the scope.

Keep the stair passable. The family needed the stair overnight, so the crew worked bottom to top in one day and never left a tread unfastened at the end of a shift, with a sign at the top while adhesive cured.

How it was confirmed

The tech re-measured all 14 risers nosing to nosing on the finished stair: the tallest 7 5/8 inches and the shortest 7 1/2, a 1/8 inch variation, every riser under 7 3/4. He then climbed and descended the flight several times at normal pace, watching for any catch at the top. A month later the homeowner reported no more stumbles.

What the shop changed

Every job that changes floor height at the top or bottom of a stair now has two required fields on the estimate: the existing top and bottom riser heights, measured, and the planned build-up of the new floor. If the new top or bottom riser would land more than 3/8 inch from the others or over 7 3/4 inches, the estimate cannot go out until the stair work is in the scope.

References

• International Residential Code, 2021 edition, sections R311.7.5.1, Riser height, and R311.7.8.1, Handrail height, as adopted and amended by your jurisdiction.
• Hardwood and stair tread manufacturers' installation instructions, including minimum subfloor thickness.
• See related: Stair Riser and Tread Rules When Flooring Changes Height; How to Install Hard-Surface Treads and Risers on a Stair.
• See related: Transition Profiles and the Accessible Level Change.