Stair stringer installation is fairly simple. On the stringer, notches are drawn using a framing square, and then they are cut off. If you've done the arithmetic (it's elementary) and the layout correctly, the tread cuts and riser cuts will be level. Most stair builders use dimensional lumber for stair framing, but if you want stringers that are extremely sturdy and straight, you should consider using engineered wood.
The International Residential Code (IRC) specifies the maximum rise (height of each step) and minimum run (depth of each step) as 7-3/4 inches and 10 inches, respectively. Numerous jurisdictions, notably in the Northeast, permit an 8-1/4-inch rise and a 9-inch run. The tread depth on steps with a run of fewer than 11 inches must overhang the riser below by at least 3/4 inches and no more than 1-1/4 inches, so the tread depth is actually greater than the run.
Finding Rise and Run
Determine the overall rise by measuring from the finished floor at the bottom of the stairs to the finished floor at the top. If the finished floor is not yet installed, measure from the subfloor by adding the thickness of the higher flooring and subtracting the thickness of the lower flooring. Add the finished floor's thickness to the height of the lowest riser or shim the stringers if the final floor will not be in place when the stringers are installed. Say the total rise is 46 inches.
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Divide the total rise by the IRC's maximum rise of 7.75 in. to determine the minimum number of risers necessary for the stair. This equals 5.93, which is rounded up to six risers. (Always round up; rounding down leads in a higher rise that is not compliant with the building code.) Now divide the total ascent by the number of ascendants: 46 6 = 7.66 inches, or 7-11/16 inches. There are usually one less tread than risers. There would be five treads on this stair. The maximum distance is restricted by material and construction restrictions. The run determines the total projection of the stair, which in turn determines how much floor space the stair takes. Common nominal 12-inch tread material measures 11-1/4 inches, resulting in a code-compliant tread overhang on stairs with rises between 10 inches and 10-and-a-half inches. Using a run of 10 inches, this six-rise, five-tread stair would extend 50 inches from the landing. If you desire a less steep staircase, you might add another riser and tread: This results in a riser height of 69/16 inches. However, the new step would extend the staircase by 60 inches, which could interfere with the 3-foot-deep landing required by the IRC. The IRC also mandates 80 inches of headroom measured vertically above the tread nosing. This headroom can be compromised by staircases with a shallower rise.
Using a larger run can make stairs shallower, but this introduces another issue. After cutting the notches, there should be at least 5 inches of stringer stock remaining at the back. Using 212 stringer material with a 10-inch run leaves just five inches, and a larger run would leave even less. After determining the rise and run, use a framing square to lay out the stringer. Consistency in the layout is essential. The IRC permits no more than a 3/8-inch variance in stair riser height or run depth. Aim for zero. Clamp a wood strip to the square so that its edge lines with the rise and run dimensions of the stair. This generates a precise guide that contributes to the layout's precision.
Step-by-Step
1. Clamp a guide to the square.
Clamp a straight wood strip to the framing square to facilitate constant alignment with the stringer. Align the strip along the outer boundaries of the square with the run dimension on one leg and the rise dimension on the opposite leg.
2. Find the crown.
Examine the stringer's edge to determine if it is crowned. If this is the case, position the stringer such that the crown of the completed step faces upward.
3. Lay out the first tread and the second riser.
Hold the square so that the leg with the tread measurement is closest to the end of the stringer stock, but far enough away to accommodate a riser below. Try to arrange the notches such that cutting them eliminates faults and knots.
4. Lay out the second tread and the third riser.
Slide the square upward and carefully align the tread dimension with the intersection of the riser line and stringer edge. Continue up the stringer until sufficient treads have been set up.
5. Mark the back of the top tread.
You do not need to sketch out the top riser; only put a mark so you are aware of the precise location where the tread comes to an end.
6. Mark the plumb cut at the top of the stringer.
Mark a cutline that extends from the rear of the top tread all the way down to the bottom of the stringer, then turn the square so that it is facing the bottom of the stringer.
7. Lay out the bottom riser.
The first riser should be marked once the riser dimension is aligned with the point where the first tread contacts the stringer while the guide is held firmly against the stringer. Continue drawing this line until it reaches the end.
8. Adjust the first riser height.
If you don't take away one tread thickness from the first riser, that step will be significantly taller than the others. If the flooring has not yet been placed, you will need to add the thickness of the finished floor to the riser.
9. Don’t overcut the notches.
The circular saw should be stopped at the back of the notch, and the cuts should be completed using a handsaw. Cutting with the circular saw beyond the notch saves time, but it causes the stringer to become more brittle.
10. Use the first stringer as a template.
Draw lines to help you match the cut stringer with the edge of the stringer stock as you lay out the succeeding stringers for the project. After you've made the cut, erase the pencil markings.
