Stair Stringer Layout and Cutting

You have measured your total rise. Now mark and cut stair stringers that will pass inspection and not break under load. The single hardest part of stair stringer layout is getting the riser height exactly the same on every step, because the IRC allows no more than 3/8 inch variation across the entire flight. The numbers, the marking process with a framing square and stair gauges, dropping the stringer for tread thickness, checking throat depth, spacing the stringers, and attaching them at top and bottom are covered.

Before you pick up a saw, know your total rise. Measure from the finished floor of the lower level to the finished floor of the upper level. Do not measure from subfloor. The 2021 IRC R311.7.5 limits each riser to between 4 inches and 7 3/4 inches, with a maximum tread depth of 12 1/2 inches and a minimum of 10 inches. Divide the total rise by a target riser height around 7 inches to get the number of risers. Round up to the next whole number, then divide the total rise by that number to get the actual rise per step. That number must be between 4 and 7 3/4 inches. If it lands above 7 3/4, you need one more riser. If it lands below 4, you need one fewer.

You need three numbers before you touch a square: rise per step, run per step, and the number of treads. For a straight stair, the number of treads is always one less than the number of risers because the top landing acts as the last tread. If you have 13 risers, you cut 12 tread notches.

Choose Tread Depth and Check Blondel

For a deck stair, use a 10-inch tread depth as your run. If the stair feels comfortable, check Blondel's formula: 2 times the riser height plus the tread depth should land between 24 and 25 inches. A 7-inch riser with a 10-inch tread gives 24, which is at the low end. A 7-inch riser with an 11-inch tread gives 25, which feels better. The IRC allows up to 12 1/2 inches for tread depth, but going over 11 inches can make the total run too long for the available space.

Clamp stair gauges to your framing square at the rise on the tongue and the run on the body. Place the square on the stringer board so the gauge on the tongue rests against the top edge of the board and the gauge on the body rests against the same edge. Draw the notch outline. Slide the square down so the tongue gauge aligns with the previous run mark, then draw the next notch. Repeat until you have marked every tread.

Do not cut on these lines yet. First, add the drop for tread thickness. If you cut the stringer as marked, the top tread will sit higher than the finished floor by the thickness of the tread material. That throws off the first riser height. The fix is called dropping the stringer.

Drop the stringer by subtracting the tread thickness from the bottom of the stringer. If you are using 5/4 (1-inch) tread stock, cut 1 inch off the bottom of the stringer. If using 2-inch tread stock, cut 1 1/2 inches off. This lowers the entire stringer so that the top of the tread aligns with the top of the finished floor. Without this drop, the first riser ends up taller than the others by the tread thickness, which fails the uniformity rule.

The drop also affects the notch geometry at the top. On the top notch, you need to cut a notch that matches the run minus the nosing projection. The IRC limits nosing projection to between 3/4 and 1 1/4 inches. If you use a 1-inch nosing, the top notch run is the tread depth minus 1 inch.

After cutting the notches, measure the remaining depth of the stringer at the throat, the narrowest part between the bottom of a notch and the bottom edge of the board. The American Wood Council DCA 6 (2021 edition) states that the notch depth cannot exceed 1/3 of the board depth, and the remaining depth after the notch must be at least 3 1/2 inches.That leaves a throat depth of 7 11/16 inches, well above the 3 1/2 inch minimum. A 2x10, with a true depth of 9 1/4 inches, can only be notched 3 1/12 inches deep, leaving a throat of 6 1/6 inches, which is still legal but weaker.

Do not use a 2x10 for a stair stringer unless you add intermediate support. The DCA 6 stringer span tables show that a 2x12 Southern Pine No. 2 at 16 inches on center spans a maximum of 5 feet 6 inches. A 2x10 at the same spacing spans less. If your total run exceeds that, you need more stringers or intermediate supports.

The DCA 6 prescribes stringer spacing based on tread material and load. For a 36-inch-wide stair, three stringers at 16 inches on center are standard. For a 42-inch-wide stair, four stringers. Never space stringers more than 24 inches on center for 5/4 treads. For 2-inch treads, 24 inches on center is acceptable.

Calculate the number of stringers by dividing the stair width by the spacing, then rounding up. A 36-inch stair with 16-inch spacing needs three stringers.The outer stringers sit flush with the outside of the treads. The inner stringers support the middle of the treads.

At the top, the stringer must bear on the floor framing for at least 1 1/2 inches per DCA 6. Cut the top of the stringer to match the floor thickness. Nail or screw it through the floor sheathing into the rim joist. At the bottom, the stringer sits on the landing or bottom floor. Use galvanized joist hangers or metal brackets rated for the load. Bolt the stringer to the rim joist at the top and to the landing framing at the bottom.

The most common failure at attachment points is using screws not rated for shear loads. Use 16d nails or structural screws. Do not use drywall screws.

You have a total rise of 60 inches. Divide by 7 inches: 8.57 risers. Round up to 9 risers. 60 divided by 9 is 6.67 inches per riser, which is within the IRC range. You need 8 treads. Use a 10-inch tread depth. The total run is 8 times 10 inches equals 80 inches, or 6 feet 8 inches. Check Blondel: 2 times 6.67 plus 10 equals 23.34, which is below 24. The stair will feel choppy. Increase the tread to 11 inches: 2 times 6.67 plus 11 equals 24.34, which is acceptable. The total run becomes 8 times 11 inches equals 88 inches, or 7 feet 4 inches.

Use three stringers at 16 inches on center. Mark the first stringer with a rise of 6.67 inches and a run of 11 inches. Drop the stringer by the tread thickness, 1 inch for 5/4 stock. Cut the notches. Check throat depth: a 2x12 with a 3 13/16-inch notch leaves a throat of 7 11/16 inches, well above the 3 1/2 inch minimum. Attach at top with 16d nails to the rim joist and at bottom with joist hangers to the landing.

• Measure total rise from finished floor to finished floor.
• Calculate number of risers by dividing total rise by a target of 7 inches and rounding up.
• Divide total rise by number of risers to get actual rise per step. Verify it is between 4 and 7 3/4 inches.
• Calculate number of treads: number of risers minus one.
• Choose a tread depth between 10 and 12 1/2 inches. Check Blondel's formula: 2R + T between 24 and 25 inches.
• Clamp stair gauges to framing square at rise and run.
• Mark notches on stringer board. Measure and mark the drop for tread thickness at the bottom.
• Cut notches with a circular saw; finish corners with a handsaw.
• Measure throat depth: must be at least 3 1/2 inches.
• Check stringer span against DCA 6 tables for your lumber grade and spacing.
• Attach stringer at top and bottom with approved fasteners.
• Verify all risers are within 3/8 inch of each other before installing treads.

Parameter	IRC R311.7 Limit	Recommended Value
Riser height	4 in – 7 3/4 in	6 1/2 in – 7 1/2 in
Tread depth	10 in – 12 1/2 in	10 in – 11 in
Nosing projection	3/4 in – 1 1/4 in	1 in
Riser variation	Max 3/8 in in a flight	0 in
Tread variation	Max 3/8 in in a flight	0 in
Blondel's formula (2R+T)	No code limit	24 in – 25 in

The most common mistake is measuring the total rise from subfloor instead of finished floor. If you have subfloor to floor transition, the difference can be 3/4 inch, which makes every riser off by that amount and fails the uniformity check. Measure from the top of the finished floor upstairs to the top of the finished floor downstairs.

The second most common mistake is not dropping the stringer for tread thickness. Without the drop, the first riser is taller than the others by the tread thickness. That is usually a 1-inch or 1 1/2-inch difference, well over the 3/8 inch allowed.

The third most common mistake is cutting notches too deep, especially on a 2x10 stringer, leaving a throat depth under 3 1/2 inches. That weakens the stringer and can cause it to crack under load. Use a 2x12 for any stair over 4 feet of span.