Buying the Materials
Measure from the deck rim to the landing spot and add 2 ft. Buy three treated 2x10s, two 2x12 skirts and two 2x4s sized to the next larger length and you'll have plenty of material to work with (the worst mistake is buying material that's too short!). Get a 6-ft. 2x6 for securing the stairs to the deck (Photo 8). You'll also need two 2x6s for each tread and a 1x8 for each riser. Use 3-in. deck screws to fasten the skirts and treads to the stringers and the skirts to the deck. Fasten the risers to the stringers with 8d galvanized nails.
For extra-strong stairs, reinforce the middle 2x10 stringer with 2x4s nailed to both sides (Photo 7). There are a million ways to fasten the stringers solidly to the deck. Photo 8 shows a simple, foolproof, extra-strong method that works especially well even for open-sided stairs built without skirts.
There you go—a pretty, rock solid set of stairs ready for balusters and railings.