Stair Riser and Tread Rules When Flooring Changes Height

Why this matters

People climb stairs by rhythm. After two or three steps the foot stops measuring and repeats the last lift, so one riser that is taller or shorter than the rest catches a toe or a heel exactly where a fall hurts most. New flooring changes riser heights without anyone touching the stair: a thicker floor at the top landing makes the last step up taller, a new floor at the bottom makes the first step shorter, and a new tread overlay moves both ends at once. The code rules that govern this fit on an index card, and this card is the library's owner of them. Other flooring cards cite the figures here rather than restating them.

The rules, as the 2021 IRC states them

The International Residential Code governs one- and two-family dwellings where a state or locality has adopted it, often with local amendments, and the edition in force varies. The figures below are from the 2021 edition, section R311.7.5, as adopted by your jurisdiction; check the edition and amendments your building department enforces.

Risers (R311.7.5.1). Maximum riser height is 7-3/4 in, measured vertically between the leading edges of adjacent treads. Within any flight, the greatest riser height may not exceed the smallest by more than 3/8 in.

Treads (R311.7.5.2). Minimum tread depth is 10 in, measured horizontally between the vertical planes of the foremost projection of adjacent treads, at a right angle to the tread's leading edge. Within any flight, the greatest tread depth may not exceed the smallest by more than 3/8 in.

Nosings (R311.7.5.3). On stairs with solid risers, the nosing projects at least 3/4 in and not more than 1-1/4 in, the radius of curvature at the nosing is not more than 9/16 in, and any bevel is not more than 1/2 in. The greatest nosing projection may not exceed the smallest by more than 3/8 in between two stories, and that comparison includes the nosing at the level of floors and landings. A nosing is not required where the tread depth is at least 11 in.

Headroom (R311.7.2). Minimum headroom is 6 ft 8 in, measured vertically from the sloped plane along the tread nosings, or from the floor of a landing. Raising the floor at the bottom of a stair under a low header takes from it.

Commercial stairs fall under the International Building Code where adopted, with tighter dimensions (in the 2021 IBC, section 1011.5: risers between 4 in and 7 in, treads at least 11 in) and the same 3/8 in uniformity tolerance within a flight.

Who it binds. The IRC applies to new construction as adopted; whether a flooring change on an existing stair triggers review depends on how your jurisdiction handles alterations to existing buildings, which varies. The rule is still the yardstick the trade, the insurers and a court will use for a trip on a nonuniform riser, so treat 7-3/4 in and 3/8 in as the job's limits whether or not a permit is pulled.

How new flooring moves the risers

Riser heights are measured finished surface to finished surface, so every change in finish thickness lands on a riser:

• A thicker floor at the top landing raises the landing surface. The top riser (from the top tread up to the landing) grows by the net change.
• A thicker floor at the bottom raises the floor the first tread is measured from. The bottom riser shrinks by the net change.
• A new overlay on every tread raises each tread by the same amount. Intermediate risers are unchanged; the bottom riser grows by the overlay thickness and the top riser shrinks by it.
• Removing a layer (old carpet and cushion, an old underlayment) does the reverse, which is why the net change is always new thickness minus the thickness removed, at each landing.

Note the direction at the top. A new hardwood landing does not shrink the top step; it makes it taller, which is the complaint in The New Floor That Left a Tall Top Step.

Worked example: a hardwood landing over a site-finished stair

A two-story house. The stair has 14 risers, each measured at 7-1/4 in, for a total rise of 14 times 7-1/4, which is 101-1/2 in. Treads are site-finished oak and are staying. The upstairs landing and hall have 1/8 in sheet vinyl glued to the subfloor; the customer wants 3/4 in solid hardwood on the subfloor after the vinyl comes up. The lower floor is not changing.

                               UPPER LANDING
                  ^    +=====================  new 3/4 in
   top riser      |    |                       hardwood
   now 7-7/8 in   |    | . . . . . . . . . .  old 1/8 in
   (was 7-1/4 in) |    |                       vinyl level
                  |    +---------------------  subfloor
                  v    |
          +============+  top tread, unchanged
          |
          |  risers below: 7-1/4 in, unchanged
          |
 +========+  next tread down

Net change at the landing. New 3/4 in minus the 1/8 in removed: 6/8 minus 1/8 is 5/8 in up.

Top riser. 7-1/4 in plus 5/8 in is 7-7/8 in. Check back: 7-7/8 minus 5/8 is 7-1/4.

Against the maximum. 7-7/8 in against 7-3/4 in: 1/8 in over. Fails.

Against uniformity. Largest 7-7/8 in, smallest 7-1/4 in: 7-7/8 minus 7-1/4 is 5/8 in, against a 3/8 in allowance. Fails by 1/4 in.

The job as sold fails both rules, and it would have been installed as sold if nobody measured the stair before quoting.

Option A: a thinner landing floor. For a new finish thickness f, the top riser is 7-1/4 plus f minus 1/8. The maximum needs f no more than 5/8 in (7-1/4 plus 5/8 minus 1/8 is 7-3/4). Uniformity needs f minus 1/8 no more than 3/8, so f no more than 1/2 in. The tighter of the two is 1/2 in. A 3/8 in engineered floor on the subfloor gives a top riser of 7-1/4 plus 3/8 minus 1/8, which is 7-1/2 in: 1/4 in under the maximum, and 1/4 in over the smallest riser against a 3/8 in allowance. Passes both with margin.

Option B: overlay every tread. Keep the 3/4 in landing and cap every tread with an overlay of thickness t. The top riser becomes 7-7/8 minus t, the bottom riser 7-1/4 plus t, and the twelve between stay at 7-1/4. With t at 5/16 in, top and bottom both land at 7-9/16 in: under 7-3/4, and 5/16 in over the 7-1/4 risers against a 3/8 in allowance. It works on paper. In practice it depends on finding a tread product at that thickness, re-checking nosing projection on every tread, and the lower floor never changing. See related: How to Install Hard-Surface Treads and Risers on a Stair.

Option C: re-rise the stair. Spread the new total rise, 101-1/2 plus 5/8 or 102-1/8 in, over 14 risers, about 7-5/16 in each. That is carpentry and often a permit, and it belongs to a carpenter, not the flooring crew.

Nosing at the landing. Whichever option, the landing edge gets a stair nose profile whose projection is checked against the tread nosings; the 3/8 in comparison runs through the landing nosing too.

The customer took Option A. The conversation: "Three-quarter hardwood up here makes the top step five-eighths of an inch taller than the rest and over the code limit. People trip on exactly that step. A 3/8 in engineered floor in the same species keeps the top step a quarter inch off the others, which is inside the rule."

How the example would change

• Carpet comes off the stairs too. Carpet and cushion removed from every tread drop each tread by the same amount, which moves the top riser up and the bottom riser down by that amount before the landing is even considered. Measure after demolition, not before.
• The lower floor changes as well. New tile at the bottom shrinks the bottom riser. Say the same customer also replaces 1/8 in vinyl at the foot of the stair with a tile assembly (backer board, thinset and tile) totaling 3/4 in: net 5/8 in up, so the bottom riser goes from 7-1/4 to 6-5/8 in. With Option A at the top, the flight now runs from 7-1/2 in down to 6-5/8 in, a spread of 7-1/2 minus 6-5/8, or 7/8 in, against 3/8 in. The fix at the top passes and the flight still fails at the bottom. Run both ends through the same arithmetic before either floor is sold, and when both landings rise, an equal tread overlay (Option B's mechanism) is the tool that moves the bottom riser back up.
• The risers were never uniform. Existing stairs often vary. If the measured risers already span 1/4 in, the job has only 1/8 in of the 3/8 in allowance left before any flooring goes down. Measure every riser and work from the actual largest and smallest.
• Headroom. A thicker floor at the foot of a stair under a beam or soffit can take the clearance below 6 ft 8 in. Measure plumb from the nosing line.

Measuring a stair before quoting

• Measure each riser at its center, vertically from the top of one tread's leading edge to the top of the next, and write all of them down, including top and bottom.
• At the top and bottom, measure to the finished floor surface that will exist, and write the net thickness change at each landing.
• Measure tread depth nosing to nosing, and nosing projection on several treads.
• Put the figures on the quote and the work order. A quote with the stair arithmetic on it is a quote the crew cannot install wrong by accident.

References

• International Residential Code, 2021 edition, section R311.7 (R311.7.2, R311.7.5), as adopted and amended by your jurisdiction
• International Building Code, 2021 edition, section 1011.5, for commercial stairs where adopted
• Local building department rules for alterations to existing buildings
• See related: The New Floor That Left a Tall Top Step; How to Install Hard-Surface Treads and Risers on a Stair; Transition Profiles and the Accessible Level Change