After talking to many deck builders over the years, we have found that stair construction is considered the most difficult part of building a deck. In order to lay out the stairs, you need to measure the exact height of the deck from the top of the decking to the location where the stairs are going to land on the ground. You can input this number into our Stairs Calculator on Decks.com. The calculator will provide you with the number of stairs, riser height and tread depth to cut your stringers. You will need to remove the thickness of the riser board from the top stair and remove the thickness of the tread board from the bottom stair. You can now draw the notches for the stairs onto your 2x12 stair stringers. The difference between the stair rises and tread depths cannot vary by more than 3/8" from the largest to the smallest.
Deck Stair Stringers Spacing
The load supporting requirements for stair treads are more severe than for the rest of the deck because the force exerted by a person walking downstairs is a more intense blow to the surface than a standard stride across a level surface. Because of this, you should never space your stair stringers more than 16” on center. This means you will need to install a minimum of four stringers on any stairway wider than 36”. Always check the maximum span for your decking material as it applies to stair treads.
Some composite materials and lower-grade 5/4 wood decking may require you to reduce the stringer spacing to 10” on center. Always read the composite decking installation instructions for stair treads before building your stairs. It is best policy to overbuild your stairs because of the increased wear and tear they often receive. Pay particular attention to the stringer placement if you are attaching stairs around a corner, as in the case of a cascading staircase.
Determine the total rise for your stairs using a level and a tape measure. Measure the distance from the top of the deck surface to the top of the stair landing. Input the total rise number into the Decks.com Stairs Calculator to determine the number of stairs, each stair rise and run for laying out your stair stringers. The above example shows outside drop-mounted stairs.
Using the information from the Decks.com Stairs Calculator, mark your rise and run on the framing square. The above image shows drawing the top step run at 10" and the rise at 6-3/8".
After laying out your rise and run measurements on your 2x12 stair stringer, remove 1" from the back of the top run if you plan on installing a 1" thick riser board.
The above image is an example using 5/4 x 6 decking which is 1" thick, a stair rise that is 6-3/8" and a bottom stair stringer rise that is 5-3/8". The cut line is in red.
This example uses 5/4 x 6 decking that is 1" thick, a stair rise that is 6-3/8" and a bottom stair stringer rise that is 5-3/8". The cut line is in red.
Notice that the stair stringer top step has a 9" run and the next step has a 10" run. The back tread for the top step had to be ripped down to fit the opening.