In choosing the size of your stair stringer, a 2 x 10 should be the smallest board chosen. A rule to follow is: the amount of solid wood remaining on the stair stringer after the stair rise and stair runs are cut out should be a minimum of 3 1/2" to support the weight of your 350 pound neighbours when they come over to inspect your work. (See Figure 1.) In some cases, a vertical post or two can be placed beneath the stair stringer to the floor to help support the stairs. In other cases, the stair stringers can be nailed securely to the sides of the walls for added support. When circumstances demand no posts under the stair stringer or walls for support, use a 2 x 12 stair stringer for added protection.
Laying out the stair stringers is a very precise job. With 14 stair rises, a mistake of 1/16" can multiply to a mistake of 7/8". Get your framing square, stair gauges and a sharp pencil from your tool box and let's get to work. The framing square is made up of two legs joined at the heel to form a right angle. The longer leg, called the body is 2" x 24" and the shorter one, called the tongue is 1 1/2" x 16". If you hold the body of the framing square in your left hand with the tongue pointing to the right you are looking at the face of the framing square, which usually has the brand name on the heel. The back of the framing square is the reverse side. Along the edges of the body and tongue on both sides are the graduations of the framing square. These consist of the following fractions of an inch: eighths, tenths, twelfths, sixteenths and thirty-seconds.
Figure 2 shows the correct method of laying your framing square on the stair stringer. Start at the top end of the stair stringer near the end of the 2 x 10 or 2 x 12. With your stair gauges guiding the framing square in place at the 7 5/8" stair rise and 10 1/2" stair run positions where they meet the edge of the 2 x 10, scribe very sharp lines along the edges of the framing square, so the lines meet at the framing square's vertex. Slide the framing square along the stair stringer until the rise marker on the framing square meets the first run mark you drew on the board. Continue this procedure along the full length of the stair stringer board until you have counted the full number of stair rises and stair runs you calculated earlier, in our example these are 14 stair rises and 13 stair runs. One thing to watch with the framing square is to make sure you take your stair rise and stair run from both the inside or the outside of the framing square. Where your pencil marks meet must be where the framing square starts its measurement.
I've mentioned before that there is one less stair run than stair rise. When laying out the stair stringer the extra stair rise is not on the stair stringer, but is measured down from the top floor or deck. This leaves the stair stringer itself with an equal number of stair rises and stair runs.
The stair stringer is laid out with the correct numbers of stair rises and stair runs. Now we must compensate for the bottom stair riser when we add the stair tread on top of it. The BC Building Code stipulates that the minimun thickness for a stair tread is 25 mm.(1"). Carpenter framers making stairs in a new house rip 1" plywood into strips for the stair treads or use 1 1/2" lumber. In our example let's use 1" plywood as our stair tread. Now take 1" off the bottom step of the stair stringer. The bottom of your finished stair stringer should look similar to Figure 3.
When installing the stair stringer to the stair opening, measure from the top floor down the face of the trim joist to the rise measurement, say 7 5/8" and then add on the thickness of stair tread or 1" that you took off the bottom of the stair stringer. If the stairs are going between 2 walls and the finish, probably drywall, is not on the walls yet, as is the case in new construction, before installing the stair stringers nail a 3" ripping of 5/8" or 3/4" plywood along the bottom outside of the stair stringers. This enables the drywallers to come after the stairs are in and slip the drywall between the wall and the end of the stairs, rather than having to cut out the drywall in the shape of each stair. They will appreciate your consideration. It will save you a few bucks, too. If the walls are finished, of course, just nail the stair stringers to the studs right through the finished walls.
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