Making Calculations

• 1). Select the best 2 x 12 or 2 x 14 lumber you can find. Pass over those pieces that have many knots, especially loose knots. Badly warped or twisted pieces are poor choices as well. Choose pressure treated lumber for outdoor construction.
  
  
• 2). Measure the total rise of the staircase in inches. You must be precise while measuring as building codes specify that the rise of the steps must not vary more than 1/4 inch. The total rise is the distance from the ground or floor to the very top of the stairs. Remember that outdoors, the ground may not be level, so you should measure at the point the stairs meet the ground.
  
  
• 3). Determine the number of risers by dividing the total rise by 7 inches and rounding the result up. Never round it down. Rounding up will ensure your riser height is less than seven inches, the maximum allowed by some local building codes. Universal Building Code states the maximum is 7-3/4 inches, but many people find this height uncomfortable to climb.
  
  
• 4
  Calculating stair stringers
  
  Determine the actual riser height by dividing the total rise by the number of risers. Look at the example calculation in the illustration.
  
  
• 5). Determine the number of treads by subtracting 1 from the number of risers. This places the first tread one down from the top of the stairs, which makes it easier to install a railing later. In our example, we have eleven risers and ten treads.
  
  
• 6). Find the tread run and overhang. For outside stairs, where the actual length of the staircase is not so important, a good tread run is 10 inches. This is ideal since two 6 inch boards laid next to each other total 11 inches. This leaves a 1 inch overhang. For inside stairs, measure the horizontal total run of the stairs, from where they begin to where they end and divide by the number of treads.
  
  
• 7). Adjust for the top riser thickness and the bottom tread thickness. This means that you remove the thickness of the treads from the bottom step's rise and the thickness of the risers from the top step's run. Now you're ready to layout the cuts on the stringers.
  
  

Marking the stringers

• 1). Use the framing square to mark the stringers for cutting. On the outside of the square, locate the measurement along the long leg for the tread run. Place a stair gauge on the square at that mark. Do the same on the short leg for the riser height.
  
  
• 2
  Marking stringers for cutting.
  
  Place the square on the stringer near one end with the short leg closest the end. See the illustration. Use the pencil to trace the square for one step.
  
  
• 3
  Finishing the cut marks
  
  Move the square over and line up the short leg with the previous tracing. See the illustration. Trace the second and subsequent steps in this manner until all the steps (number of risers) have been marked.
  
  
• 4). Adjust the bottom riser height by moving the line back the tread thickness. The bottom step is now the height found in step 7 of the previous section. Adjust the top tread length for the riser thickness. See the illustration.
  
  
• 5). Extend the lines for the top riser and the bottom tread across the stringer. See illustration. These mark the top and bottom cuts for the stringers.
  
  

Cutting the stringers

• 1
  The first cuts are easy
  
  Cut the bottom riser mark all the way across the stringer using the circular saw. Do the same with the top tread mark. See the illustration.
  
  
• 2
  The finished stringer can be used as a template for the rest.
  
  Cut out the tread and riser marks using the circular saw and the hand saw or jig saw. Use the circular saw to cut right up to the junction of the tread and riser lines along both lines. Because the saw blade is circular, the cut should be finished with a straight blade.
  
  
• 3). Measure the tread and risers to be sure they are all equal except the top and bottom which should differ by the amounts calculated in section 1.
  
  
• 4). Mark the other stringers using the first one as a template. Only use the first stringer as a template to avoid magnifying errors.
  
  

Finishing the job

• 1
  The final stages
  
  Install a 2 x 10 stair header below the rim joist to attach the stringers to. Attach it to the deck support posts with 1/2 bolts. If support posts are not available, install the posts for the stair railing and extend the posts below the joists to attach the header to. See the illustration
  
  
• 2). Attach the stringers to the header using stringer hangers and 10D nails. Use the torpedo level on the tread cutouts to insure the stringers are level. Measure to be sure the rise height at the top of the staircase and at the bottom are the same, taking into account the tread thickness. Getting this right is critical. If necessary, add a piece of tread to the bottom step so you can measure easily.
  
  
• 3). Cut all the risers to length. Measure across the stringers at the top and add three inches. Cut the risers to this length. Install all the risers by screwing them to the stringers. The risers overhang the stringers on the sides by 1-1/2 inches on each side.
  
  
• 4). Cut all the treads to length using the same measurement as was used for the strings. Install with deck screws or nails, leaving a 1-1/2 inch overhang on each side.
  
  
• 5). Measure the rise of each step, including from the floor to the top of the bottom tread and from the top of the uppermost tread to the top of the deck or floor. The measurements should all be within a quarter inch of each other. If necessary, make minute adjustments with shims no more than 1/32 of an inch thick under the treads.