To build exterior deck stairs, determine how many stairs are needed and calculate their dimensions. Create a concrete foundation for the stairs, cut out the sides of the stairs (the stringers) from boards, and set them in place connecting them to the deck and to the concrete foundation. Install the treads onto the stringers using two galvanized deck screws per side. Build the railings, and install them.
To determine how many steps are required, divide the distance from the deck to the ground in inches by 7.5, and round it to the closest integer. To determine the rise height, divide the distance between the deck and the ground by the number of stairs. To determine how far the stairs should extend into the yard, multiply the number of steps by their depth. The most common stair depth is 10 inches. Also decide how wide the stairs should be.
To install the stringers, set their bottom ends onto the concrete footer; the other side should be attached to the face of the deck. Ensure that they are leveled, and use angle brackets and 3-inch galvanized deck screws to secure the stringers to the deck. Install a board to connect the stringers at their base using galvanized deck screws. Use masonry anchors to secure the board to the concrete footer.