Building Safe Stairs: Codes, Rise and Run, and Getting It Right

Stairs are where people get hurt

I say this to every homeowner and every apprentice who joins my crew: stairs are the single most dangerous part of a house. They don’t look dangerous. They’re just steps. But a riser that’s a half inch too tall, a tread that’s a quarter inch too shallow, or an inconsistent step height buried somewhere in the middle of a flight — that’s how ankles break, how elderly homeowners end up in the ER, and how builders end up in court.

In over twenty years of renovation work — including tearing out and rebuilding the original stairs in my own 1920s bungalow — I’ve seen every stair mistake there is. Risers that varied by more than an inch from top to bottom. Treads so narrow you had to descend sideways. Headroom so tight that anyone over six feet would crack their skull on the header. Every single one of those problems came from the same root cause: somebody skipped the math.

This guide walks through the fundamentals of stair design. We’ll cover the building code requirements you need to hit, the relationship between rise and run, and how to calculate your stair dimensions before you cut a single stringer.

The terminology you need to know

Before we get into numbers, let’s get the vocabulary straight. Stair jargon trips people up, and if you’re reading code books or talking to an inspector, you need to speak the language.

• Total rise — the vertical distance from the finished floor at the bottom to the finished floor at the top. This is the height your staircase needs to climb.
• Total run — the horizontal distance the staircase covers from the face of the first riser to the face of the last riser.
• Riser — the vertical board (or the vertical distance) between one tread and the next. Riser height is what your foot lifts for each step.
• Tread — the horizontal surface you step on. Tread depth (or “going”) is measured from the face of one riser to the face of the next, nose to nose.
• Nosing — the part of the tread that overhangs the riser below. Usually 3/4 inch to 1-1/4 inches.
• Stringer — the sawtooth-cut structural board that supports the treads and risers on each side. Most residential stairs have two or three stringers.
• Headroom — the vertical clearance between the tread nosing and any overhead obstruction (ceiling, header, floor above). Code minimum is typically 6 feet 8 inches.

What the building code actually says

Building codes exist because stairs built on intuition alone tend to be inconsistent and unsafe. The International Residential Code (IRC), which forms the basis for most local codes in the United States, sets clear limits. Your local jurisdiction may amend these slightly, so always verify with your building department, but here are the key numbers.

Riser height: maximum 7-3/4 inches (196mm). Most comfortable range is between 7 and 7-1/2 inches.

Tread depth: minimum 10 inches (254mm). This is measured horizontally from the nosing of one tread to the nosing of the next, not including the overhang.

Nosing projection: between 3/4 inch and 1-1/4 inches beyond the face of the riser below, if the tread depth is less than 11 inches.

Uniformity: this is the one that catches people. The maximum variation between the tallest and shortest riser in any flight is 3/8 inch. Same for treads. That means if your top riser is 7-1/2 inches and your bottom riser is 7-7/8 inches, you’ve already failed inspection. Your body memorizes the rhythm of a staircase after the first two or three steps. When a step breaks that rhythm, your foot lands wrong and you stumble.

Minimum width: 36 inches clear between finished walls, or between a wall and the edge of the handrail.

Headroom: 6 feet 8 inches minimum measured vertically from the tread nosing to the ceiling or header above.

The rise and run relationship

Here’s the practical piece. You’ve got a floor-to-floor height — say, 106 inches from the basement slab to the first floor. That number is fixed. You can’t change it without raising or lowering a floor. Your job is to divide that total rise into equal steps that each fall within the code-allowed riser height.

Divide total rise by your target riser height. If your total rise is 106 inches and you aim for 7.5-inch risers, that’s 106 / 7.5 = 14.13. You can’t build a fraction of a step, so you round to 14 or 15 risers.

• 14 risers: 106 / 14 = 7.571 inches per riser. Under the 7.75-inch max. Works.
• 15 risers: 106 / 15 = 7.067 inches per riser. Also under the max, and a bit more comfortable.

Once you have riser height, figure tread depth. A widely used rule of thumb in the trade is that riser height plus tread depth should equal roughly 17 to 18 inches. So if your riser is 7.07 inches, aim for treads around 10.5 to 11 inches deep. That gives a comfortable, natural walking stride on the stairs.

Now multiply the number of treads (always one fewer than the number of risers, because the top floor acts as the final tread) by the tread depth. With 15 risers and 10.5-inch treads: 14 treads x 10.5 inches = 147 inches, or just over 12 feet of horizontal run. That’s the floor space your staircase needs. If you don’t have that space, you’ll need to adjust — steeper risers, a landing with a turn, or an L-shaped or U-shaped layout.

Use the Stair Calculator to run these numbers quickly. Enter your total rise and it will give you riser height, tread depth, and the number of steps:

Figuring out the space you need

Stairs eat more floor space than most people expect. A straight-run staircase with 14 treads at 10.5 inches each needs about 12 feet of horizontal space, plus clearance at the top and bottom for landings. An L-shaped stair with a mid-flight landing needs less linear space but takes up width in two directions. U-shaped stairs fold back on themselves and fit into a tighter footprint, but they’re more complex to frame.

Before you commit to a stair layout, measure the floor area you have available. If you’re working in a renovation where the stair opening is already cut, measure that opening carefully — length, width, and the header height above the lowest point of the opening.

The Square Footage Calculator is useful here for confirming the total area of your stairwell opening or the landing platform:

If your available footprint is tight, consider whether a landing turn makes sense. A quarter-turn landing (L-shape) typically needs a 3-foot by 3-foot platform at the turn point. That adds material cost — you’re framing a small floor section mid-flight — but it can save significant linear run.

Common mistakes I see on job sites

After two decades of building and inspecting stairs, certain mistakes come up over and over.

Not accounting for finished floor thickness. If you measure total rise from subfloor to subfloor but then install tile on one level and hardwood on another, your bottom riser and top riser will be different heights. Always measure from finished floor to finished floor, or calculate the difference and adjust your stringers.

Cutting stringers with a circular saw alone. A circular saw blade is round, so it overcuts at the inside corners of each step. That overcut weakens the stringer right at the stress point. Cut to the line with your circular saw, then finish the corner with a handsaw or a jigsaw. It takes two extra minutes per stringer and saves you from a cracked board under load.

Ignoring the bottom riser. The bottom riser is measured from the finished floor to the top of the first tread. If you’re setting stringers on a concrete slab and the treads are 1-inch oak, that first riser will be one inch shorter than the others unless you account for the tread material. Some carpenters trim the bottom of the stringer by the thickness of one tread. Others add a riser board at the bottom. Either way, the finished riser heights must be uniform.

Skipping the mockup. On any stair where the layout is unusual — curved, winder treads, open one side — build a rough mockup from scrap plywood before cutting your good lumber. Clamp it in place, walk it a few times, and check headroom at every point. Fifteen dollars in scrap plywood is cheaper than re-cutting three 2x12 stringers.

Get it right the first time

Stairs are permanent. They’re structural. They get used dozens of times a day. The time you spend on layout and calculation before you pick up a saw is the most valuable time in the entire project. Measure your total rise twice. Run the numbers through the Stair Calculator. Check that your floor space can handle the run. Verify headroom at the tightest point. Then cut your stringers with confidence, knowing every step will land where it should.

A good staircase is one nobody thinks about. You walk up, you walk down, your foot hits exactly where your brain expects it to. That invisible precision is what separates a safe set of stairs from a liability — and it all starts with getting the math right.