How to Cut Stair Stringers With a Framing Square

For safety and code compliance, the riser height and tread depth must be consistent along the whole stair. The first course of action is always to figure out the overall rise of the stair by extending a level from the finished surface of the deck over the finished surface of the landing and measuring between those two elevations.

Steps for Cutting Stair Stringers

Doing the math

Say the overall rise is 25 inches. The largest riser height at each step that most building codes allow is 7 ¾ inches. Dividing 25 by 7 ¾ and rounding up will determine the number of risers in the stair:

25 ÷ 7.75 = 3.23 = 4 risers

To find the individual riser height, divide the overall rise of 25 by the number of risers:

25 ÷ 4 = 6.25

The other number you need to lay out stringers is the tread depth, also called the run. The minimum allowed by most codes is 10 inches, and that's a good size to use. Making them deeper requires a longer cut to notch the stringers, which weakens them.

Selecting stringer stock

Stringers should always be cut from 2x12s. Notching smaller stock doesn't leave enough wood to safely carry the loads of a stair.

When ordering stringers, allow about 14 inches of stringer length for each step. Look for straight stock with as few knots as possible. You'll need at least one stringer for every 16 inches of stair width. If you're using synthetic decking for the treads, some manufacturers require stringers to be spaced no more than 12 inches apart.

Laying out the stringer

Lay a straight piece of 2x12 on sawhorses.
Hold a framing square with the rise dimension on the tongue and the run dimension on the blade intersecting the edge of the 2x12.
Clamp a scrap of wood to the underside of the of the square to serve as a guide, then mark out the first cut with a pencil. The longer line is the tread cut or run, and the shorter line is the riser.
Move the square up the stringer, aligning the next tread cut with the top of the riser below, and mark the next tread and riser.
Continue until you've marked out all the tread cuts and riser cuts.

Adjust the Stringer for the Tread Thickness

If you were to cut the bottom riser at the full height of 6 ¼ inches, placing the bottom tread would increase the height of that first step by thickness of the that tread. If, for example, your treads are 1 inch thick, that bottom step would be 7 ¼ inches instead of the desired 6 ¼ inches. That's unsafe, and a code violation.

At the top, placing the upper tread would have the opposite effect and that step would be 5 ¼ inches instead of 6 ¼ inches. The solution is simple – shorten the bottom riser cut on the stringer by the tread thickness. So, that cut where the stringer meets the bottom landing would be 5 ¼ inches from the tread above, while all the other risers would remain at 6 ¼ inches.
Lay out this bottom cut by holding the square on the riser line and drawing a line parallel to the bottom tread.
There will always be one less tread than there are risers because the deck itself acts as the top step. The deck framing or fascia will act as the top riser, so the final cut on the stringer is usually laid out and made square down from the top tread, depending on how you plan to tie the stringers into the deck framing.
With a circular saw, cut out the stringer. Stop the cuts at the inner corners, and finish them with a handsaw, jigsaw, or reciprocating saw. Use this first stringer as a pattern to lay out all the others.

Tools and Materials

Pressure treated 2x12 for stringers
Either decking material or pressure treated 2x12 for treads
Decking material for risers
Screws for attaching treads and risers
Stringer connectors such as Simpson LSC Adjustable Stringer Connectors

Tools

Tape measure
Framing square
Two small clamps
Cordless drill
Pencil
Circular saw
Handsaw, jigsaw, or reciprocating saw

How to Cut Stair Stringers With a Framing Square

Source: https://www.thisoldhouse.com/stairs/21591423/how-to-cut-stair-stringers

Chomley Alearright