Tread Depth and Risers

Learn stair tread depth and riser height code under IRC 2021, including minimum tread depth, maximum riser height, nosing rules, and the 3/8-inch uniformity requirement.

Stair tread depth and riser height code

Residential stair code sets two core dimensions under IRC 2021 Section R311.7.5. The minimum stair tread depth is 10 inches. The maximum stair riser height is 7 3/4 inches.

These rules define the baseline for stair tread depth code, stair riser code, and overall IRC stair dimensions. They also work with the 3/8-inch uniformity rule and the nosing requirements for residential stairs.

Stair tread and riser code at a glance

Requirement	IRC rule
Minimum tread depth	10 in.
Maximum riser height	7 3/4 in.
Maximum tread variation in one flight	3/8 in.
Maximum riser variation in one flight	3/8 in.
Nosing projection	3/4 in. to 1 1/4 in.
Maximum nosing radius	9/16 in.
Maximum bevel	1/2 in.
Nosing not required when tread depth is	11 in. or more
Maximum riser slope	30 degrees from vertical

What is the minimum stair tread depth?

Under IRC R311.7.5.2, the minimum stair tread depth is 10 inches. This is the standard number most people mean when they search for stair tread depth code or residential stair tread depth.

The code measures tread depth horizontally. Measure between the foremost projections of adjacent treads at a right angle to the leading edge.

Within one flight, the deepest and shallowest treads cannot vary by more than 3/8 inch. A stair can fail inspection even when every tread is at least 10 inches if the flight is not uniform.

What is the maximum stair riser height?

Under IRC R311.7.5.1, the maximum stair riser height is 7 3/4 inches. This is the main number behind searches for stair riser height code and maximum riser height for residential stairs.

Riser height is measured vertically between the leading edges of adjacent treads. The IRC does not set a minimum riser height, but it does require consistency.

Within one flight, the tallest and shortest risers cannot vary by more than 3/8 inch. Risers may also be vertical or sloped no more than 30 degrees from vertical.

Why the 3/8-inch stair uniformity rule matters

The 3/8-inch rule applies separately to tread depth, riser height, and nosing projection. It is one of the most common residential stair code failures.

Uniform stairs are safer to use. Even small changes in step geometry increase trip risk because people expect each step to match the previous one.

Stair nosing requirements

Under IRC R311.7.5.3, stairs usually need a nosing. The nosing must:

• project 3/4 inch to 1 1/4 inches
• have a radius no greater than 9/16 inch
• have a bevel no greater than 1/2 inch
• stay uniform within 3/8 inch across the flight

A nosing is not required when the tread depth is 11 inches or more. That exception is important when comparing 10-inch tread code with deeper custom stair layouts.

How to measure stair tread depth and riser height

Use this sequence when checking stair dimensions for code compliance:

1. Measure each riser height vertically between adjacent tread leading edges.
2. Measure each tread depth horizontally from leading edge to leading edge.
3. Compare all risers in the flight for the 3/8-inch maximum variation.
4. Compare all treads in the flight for the same 3/8-inch maximum variation.
5. Confirm the nosing profile meets the required projection and shape limits.

Related stair code conditions

Some stair layouts have added rules beyond standard tread depth and riser height:

• Open riser stairs must meet the 4-inch sphere rule once the stair is more than 30 inches above the floor or grade below. See Open Riser Code Requirements.
• Winder stairs must provide at least 10 inches at the walkline and 6 inches at the narrowest point. See Winder Staircase Code Requirements.
• Stairs with 4 or more risers need a handrail. See When is Handrail Required?.

Frequently asked questions

What is the minimum tread depth for residential stairs?

The minimum tread depth is 10 inches under IRC R311.7.5.2. Measure it horizontally between adjacent tread leading edges.

What is the maximum riser height for stairs?

The maximum riser height is 7 3/4 inches under IRC R311.7.5.1. Risers in the same flight must also stay within 3/8 inch of each other.

Do stairs need a nosing?

Usually yes. Under IRC R311.7.5.3, a nosing is generally required unless the tread depth is 11 inches or more.

What does the 3/8-inch stair rule mean?

The largest and smallest riser heights, tread depths, or nosing projections in one flight cannot differ by more than 3/8 inch. The rule applies to each dimension separately.

Related pages

• Open Riser Code Requirements
• Winder Staircase Code Requirements
• When is Handrail Required?