Building Deck Stairs

Question

I am building new deck stairs for our backyard wood deck. I am planning 4 steps, 8 feet across with a total run of about 42 inches. The total rise on one side is 29 1/2 inches and the other side is 33 1/2 inches. I plan the step rise to be about 7 inches and the run about 10 1/2 inches. I am planning on using 6 stair stringers across the 8 foot width. How do I cut the bottom of the stair stringers so the steps stay level to make up for the unevenness of the ground?

Answer

There is a difference in elevation of 4" from one end to the other. Let's split that and fiqure out the rise and run at the center. You need 5 rises of 6.3" and 4 runs of 10.5". Here is a drawing to explain what I'm doing:

Notice the stair stringers are on center, the overhang on each end is 1 1/2" plus half the thickness of the stair stringer (3/4") = 2 1/4". I broke the heights down for each stair stringer and the distances for each stair stringer as follows: Stair stringer 1 bottom rise is 4.3"; 2 is 5.1"; 3 is 5.9"; 4 is 6.7"; 5 is 7.5" and 6 is 8.3".

Your pattern stair stringer will be laid out for the above run and rise—10 1/2 and 6 3/8" respectively. The rise I gave you is before the thickness of the stair tread is taken off. I suspect you will go with a 1 1/2" stair tread but you may go with 1" as well (5/4"). So don't forget to cut the thickness of the stair tread off the bottom of each stair stringer as per a usual stair stringer.

When you layout the pattern stair stringer go ahead and cut off the thickness of the stair tread. Then after marking the other 5 stair stringers we will cut off an additional .4" (7/16") and call it #3. So when you come to stair stringer #1 you cut off 2" from the pattern stair stringer. Stair stringer number 6 has 2" added on to the bottom of the pattern. Stair stringer 2 has 1.2" cut off; #4 has .4" added on and #5 has 1.2" added on.

I suggest a mud sill of treated 2x10 to nail the stair stringers to on the bottom, if the ground they rest on will be dirt or grass, etc. keep the top of the mud sill flush with the top of the dirt, etc. if this is where you originally measured the total rise. If the bottom is concrete like a side walk or whatever, lay the mud sill on top of the concrete and cut off an additional 1 1/2" off the bottom of each stair stringer to compensate.

Dave