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To determine the rise and run of the steps, measure the distance from the floor of the deck to the ground and divide that number by 7.5. The result will be the number of steps the staircase will need (e.g., deck is 112" from the ground; 112/7.5 = 14.93. Round up to get 15 steps).
To calculate the actual rise height, divide the height in inches by the number of stairs determined in the last step. In our example, 112/15 = 7.46", which means that the risers will be 7.46" high.
To determine the length your staircase will extend into the yard, multiply the number of steps by the depth of steps. The standard depth for stair treads is 10". In our example, 15 steps x 10" = 150".
Determine the desired width of the staircase, assuming that 36” is a standard width. Once you determine the point where the stairs will meet the ground, mark the area where the concrete footer will go.
Excavate the area to a depth of 6”, removing all grass, dirt and debris from the site. Use a tamper to level and compact soil. Add pea gravel to a height of 2” then level and compact the pad with tamper. Mix fast-drying cement according to manufacturer's directions to create the stairway footer. Once the cement reaches the proper oatmeal consistency, pour on top of pea gravel until the hole is filled to ground level. Smooth and level the cement pad using a trowel and level. Allow cement to cure for 24 to 48 hours, depending on conditions.
The sides of the stairs, or stringers, are cut from 2x12 boards. Using a framing square, mark out the portions to be cut away from the stringer. Starting at the top corner of the board, place the square so that one leg is 7.46" (the rise) and the other is at 10” (the tread) (Image 1). Trace along the inside edge of the square to mark the cut lines. Move the framing square to the point where the last line comes off the board to make the next mark (Image 2). Repeat until all cutouts are marked.
Because the top step will be attached to the deck, you won't need a riser. Instead, create a return by drawing a straight line along the mark you made from the corner of the board (Image 3). For the last stair, create another return (Image 4). This section will be the part of the stringer that sits on the ground.
Use a circular saw to cut along the lines and a hand saw to finish each cut (Image 5). Using the first stringer as a template, transfer the marks to a second stringer. Make the same cuts.
Set the stringers in place by resting the bottom ends on the concrete pad and the other against the face of the deck. Use a level and plumb to check the placement of the stringers. Attach the stringers to the deck using angle brackets and 3” galvanized deck screws.
Cut a 2x6 board to fit snugly between the two stringers at their base. This will be used to fasten the stairs to the concrete pad. Place the board between the stringers and attach to stringers with 3” galvanized deck screws. Attach the 2x6 board to concrete pad with masonry anchors.
Each tread is made of two lengths of 2x6 board (Image 1). Cut stair treads so that they overlap the stringers by about 1" on both sides. If the stair width is 36”, cut treads 38”. Push one tread against the back of the stringer, center, and fasten with two 3” galvanized deck screws per side. Leave 1/4” between the first and second tread board and attach with deck screws. Repeat until all treads are attached.
Cut two pieces of 4x4 post to a length of 30”. These will serve as the lower railing posts. A small portion of the bottom front tread will need to be notched out so the rail posts abut solidly against stringers. Place posts at base of stringers and mark the portion of the tread to be removed. Cut tread notch with hand saw.
Attach posts to stringers using 4” deck screws, making certain posts are plumb. Cut lengths of 2x4 railings to span the distance from the lower posts to the existing deck railing as shown in the image. Attach the railings to the posts with 3” deck screws, making sure the slope matches that of the stairs. Cut similar pieces that will serve as the lower railings and baluster supports. Install these railings between the posts, making sure they are parallel to the upper railings. Leave less than a 6” gap between steps and this lower railing.
Cut 2x2 boards to create balusters that extend from top railing to lower railing (Image 1). Attach them to railings using 3” deck screws, making sure they are plumb and that the gap between them is no larger than 6”. For a more comfortable handrail, attach a 1x2 cap along the entire top of the railings.