Stair Framing

A stair is one of the few things in a house where every part of the math shows. People feel a tall step in their knees, and a small change from one step to the next is a common cause of falls. Inspectors measure stairs with a tape, so a stair that is off by a small amount can fail.

This guide covers straight wood stairs. Numbers are from the International Residential Code (IRC), which most US houses follow. Some states and towns change these limits, and commercial buildings use a different code. Check what applies where you work.

What You Will Be Able to Do

• Measure total rise and calculate riser height and tread depth.
• Lay out a stringer with a framing square.
• Adjust the bottom of a stringer for tread thickness and floor finishes.
• Check that the stair opening provides enough headroom.
• List the main code limits for stairs in houses.

Stair Terms

Key Limits

Most residential stairs under the IRC must meet these:

A nosing is usually required on solid risers when the tread is under 11 inches. A handrail is needed on at least one side of any stair with four or more risers. A landing at the top and bottom must be at least as wide as the stair and at least 36 inches long in the direction of travel.

Calculating Rise and Run

The first number you need is total rise, measured from the top of the finished lower floor to the top of the finished upper floor. Measure it where the stair will go, as floors may not be level.

Worked example. Total rise is 108-1/4 inches.

1. Divide by the maximum riser: 108.25 / 7.75 = 13.97. Round up to 14 risers.
2. Find the riser height: 108.25 / 14 = 7.73 inches, or about 7-3/4 inches. That passes.
3. Find the number of treads: a stair has one fewer tread than risers, because the upper floor acts as the top step. 14 risers = 13 treads.
4. Pick a tread depth: 10 inches is the code minimum. Many builders use 10-1/4 to 11 inches for comfort.
5. Find the total run: 13 x 10-1/4 = 133-1/4 inches, or about 11 feet 1-1/4 inches.

A long-used check is that two risers plus one tread should come to about 24 to 25 inches for a comfortable stair. Here, 7.73 + 7.73 + 10.25 = 25.71 inches, which is a bit long, so a shorter tread would also work.

If the run will not fit in the space, add risers to make each step shorter, or work out a landing. Do not stretch the riser past the limit.

Laying Out a Stringer

Stringers are usually cut from 2x12 lumber. Pick a straight board with few knots, especially along the bottom edge. Set the crown up.

1. Set stair gauges on a framing square: riser height on the tongue, tread depth on the body.
2. Start at the bottom, with the square on the stringer. Trace the first riser and tread.
3. Slide the square along the board so the gauge touches the end of the first tread, and trace the next step.
4. Repeat until all treads and risers are marked. Count them twice.
5. Mark the bottom cut level with the floor, and the top cut plumb where it meets the header.

Dropping the Stringer

If you cut every step the same, the first step will be too tall and the top step too short, because the tread boards add thickness at each step except the bottom.

• Subtract the tread thickness from the bottom of the stringer. If a tread is 1 inch thick, cut 1 inch off the bottom.
• Adjust for finish floors. If the lower floor will get more thickness than the upper floor, change the drop to match, so all steps come out equal.

Be sure to think in terms of finished surfaces. Stair errors usually start when a framer works from subfloor to subfloor and ignores tile or hardwood that comes later.

Cutting and Installing

1. Cut with a circular saw up to the corner of each step, and stop.
2. Finish each cut with a handsaw or jigsaw. Overcutting past the inside corner weakens the stringer.
3. Test the first stringer in place. Check level on each tread and plumb on each riser.
4. Trace it to make the others, so that they match.
5. Install the stringers with hangers or a ledger at the top, as your plan shows. Fasten the bottom so it cannot kick out.
6. Space stringers to suit the tread thickness and width. Most 36-inch stairs have three stringers. Wider stairs need more.
7. Add treads and risers, using glue and screws to prevent squeaks.

Cut stringers should keep enough wood between the inside corner and the bottom edge for strength, and common guidance is at least about 3-1/2 inches. Follow your plan or engineer if it gives a specific value.

Framing the Opening

Floor framing has to leave an opening for the stair, and that opening must allow the required headroom.

1. Find the line of the nosings, then measure up 6 feet 8 inches from it to find the lowest point of the header.
2. Include the depth of the floor system: joist, subfloor, ceiling drywall and any finish.
3. Figure out how many treads have to pass under the opening's edge before people clear it.

Worked example. With a 7-3/4-inch riser and a 10-inch tread, headroom of 80 inches and a floor system 11 inches thick require a clearance of 91 inches from the nosing to the top of the floor above. 91 / 7.75 = 11.7, so the opening must run back past about 12 risers' worth of height. 12 treads x 10 inches = 120 inches, which is about 10 feet of opening measured from the top of the stair.

The header and trimmers around the opening have to carry the cut joists. Use doubled members and hangers, as the plan shows.

Temporary Stairs

During construction, workers use stairs before the finish treads are in. OSHA rules for stairways on construction sites are in 29 CFR 1926 Subpart X. Stair runs of 4 or more risers, or that rise over 30 inches, need at least one handrail, plus a stairrail on each unprotected side. Treads must be installed and secure, and steps may not be left as bare metal pans or skeleton frames unless they are filled or covered. Stairs must slope between 30 and 50 degrees, and riser and tread sizes must be uniform within 1/4 inch in any run. Keep stairs clear of tools and debris.

Guard the open sides of a stair and the floor opening at the top. An open floor edge of 6 feet or more needs fall protection.

Common Mistakes

Check Your Understanding

1. Total rise is 96 inches. How many risers do you need, and how tall is each?

96 / 7.75 = 12.4, so round up to 13 risers. 96 / 13 = 7.38 inches, or about 7-3/8 inches.

2. How many treads does a stair with 13 risers have?

12 treads.

3. Treads will be 1-1/8 inches thick. What do you do at the bottom of the stringer?

Cut 1-1/8 inches off the bottom so the first step does not end up too tall.

4. A stair has a variation of 1/2 inch between its tallest and shortest riser. Will it pass under the IRC?

No. The maximum variation is 3/8 inch.

5. Why do you stop your circular saw short of each inside corner?

An overcut weakens the stringer at its thinnest point. Finish the cut with a handsaw or jigsaw.

Key Takeaways

• Measure total rise from finished floor to finished floor.
• Keep risers at or under 7-3/4 inches, treads at 10 inches or more, under the IRC.
• Risers and treads must be nearly identical, with no more than 3/8 inch of variation.
• Drop the stringer by the tread thickness.
• Plan 6 feet 8 inches of headroom before you frame the opening.
• Never overcut a stringer.