How to build stair stringers

Published: January 15, 2013 · Updated: September 6, 2017

Asset 2

Constructing outdoor stairs, such as those leading down from a deck, can be a challenge, 
one that some cottage handypeople avoid, thinking it too daunting. In fact, the planning is just simple arithmetic and geometry, math any calculator can overcome. And, although accurate layout and cutting are critical, few tools are required for building.

Yes, precut stringers are sold at building centres, but they’re not customized to your deck’s height, so you’ll likely have an unequal step at the bottom. That’s an Ontario Building Code violation and not very safe: After the first few steps, our brains sense a pattern and program our legs to just clear the rest. Any deviation and we can trip, fall, and die (or, at least, spill a great cocktail).

Code also limits riser height and tread depth. The tread depth, the distance from front to back of the tread, is a bit longer than the run, the horizontal cut in the stringer. That’s because, 
for ergonomics and aesthetics, each tread should overhang the stringer frame. I prefer a 1⁄4″ overhang; the most Code allows is 1″. For comfortable stairs, consider also the relationship between riser and run: A good rough guideline is two times the riser height plus the run should equal close to 25″.

For deck stairs, the tread depth and the run are generally givens, because treads are often built of three 2 x 4s, with a 1⁄4″ gap between each board. That gives you an 11″ tread depth – and a starting point for planning your stringers. Using the formula for comfortable stairs, our optimum riser height will then measure about 7″. (Two 2 x 6s are also commonly used for treads, giving you a tread depth that’s 1⁄4″ wider.)

Here’s how I lay out and cut stringers for outdoor stairs:

1. Determine the total rise: 
On the deck surface, clamp a board on edge, projecting horizontally to where you expect the stairs to end. At that point, measure from the bottom of the board to ground level.

2. Determine the total run: 
If the ground is reasonably level, it’s no big deal to figure out a fairly accurate total run – the distance from the deck out to the leading edge of the bottom step. If the ground slopes, measuring gets a little trickier. Say your total rise measures 51″ at the point you think the stairs will meet the ground. Your optimum stair rise of 7″ divides into that a bit more than seven times, so you need seven steps. Seven treads times 103⁄4″ (the run) equals 75 1⁄4″—that’s the total run.

Now, from a point 75 1⁄4″ out from the deck along that horizontal board, measure down to the ground. If the total rise changes significantly (from the 51″ measure), you might have to include an extra step or two in your calculations, in which case total run will have to be increased by 103⁄4″ for each additional step. This is important, as total run determines where the stairs end. Once you’ve figured that out, you can level this area.

3. Determine riser height: Divide the total rise by the number of steps. In our example, 51″ divided by 7 
is 7.28″; round off to 7 1⁄4″.

4. With total rise and total run, you can calculate the length of the stringers, using 
a formula that’s been around since Pythagoras was puttering at his cottage: Stringer length equals the square root of [rise2 plus run2].

5. Lay the soon-to-be stringer (which starts as a 2 x 10, or wider, board) on a horizontal surface, grab a cold drink, your square, and a pencil. The pros use stair gauges, little clamps that attach to a square so a stair layout can be repeated exactly. You can create your own stair gauge by simply clamping a short board to the arms of the square. In our example, the board would be clamped at 7 1⁄4″ on the short arm, and at 103⁄4″ on the other (use the numbers on the outside edge of the square).

6. Note that the bottom step of the stringer is always cut short, by the thickness of the first tread: In this case, 53⁄4″ (7 1⁄4″ minus 1 1⁄2″).

7. This can be confusing, so once the stringer is laid out, carefully add up the height of each step (plus a tread on top) to make sure that you come up with the total rise. It is not unusual to have to recalculate.

8. You’ll need two stringers – more for stairs wider than 3′ – which must match precisely. Cut the first with a circular saw. (You can’t cut right into the corner of a step without overshooting your pencil lines, so finish cuts with a hand saw or jig saw). Use this stringer as a pattern for the other.

9. Cut the treads to length and assemble the stairs with nails or screws. A facer board screwed to the inside of the stringers attaches to your deck. (If a third, middle stringer is required, make a notch in the top to fit the facer board.)

10. Screw stairs to the structure and add railings and guards as required.