DIY Interior Stairs

Measure the distance from the floor of the lower story to the floor of the upper story, with the tape measure. This is the “rise.” Calculate the horizontal “run” of the staircase by dividing the rise by the height of the steps. For instance, a 104-inch rise divided by the typical 8-inch height of each step would produce 13 steps. That would mean 12 stair treads because the last step is always on the upper story’s floor.

Figure the run by multiplying the width of each step by the number of steps. For example, if your steps are 8.5 inches wide, 12 steps equal a run of 102 inches. Figure the length of 2-by-12 inch structural support board or “stringer” you will need by multiplying the rise times itself and the run times itself, adding the two products together and calculating the square root of that sum.. The 104 inch rise and 102 inch run in this example would require buying standard 16-foot-long 2-by-12 stringers to get the 13 feet of stringer you need.

Lay out the steps on a 2-by-12 stringer with the framing square. The long leg of the framing square is the step width and the short leg is the step height. Use the numbers on the outside edge of the framing square for layout. Use masking tape to mark the width of the steps on the long leg of the framing square and to mark the height of the steps on the short leg.

Measure a distance from the end of the 2-by-12 board equal to the height of a step. This is the point where you start laying out the steps. Line up the framing square’s step-width and step-height marks at one edge of the 2-by-12 and trace the outer edge with a pencil. Repeat until all the steps have been marked out.

Draw a line perpendicular to the width line of the first step at the point where the bottom step starts. Measure along that line a distance equal to the height of a step minus the thickness of the treads. At that mark, draw a line perpendicular to the previous line and parallel to the width line of the first step. This outlines the bottom step. At the back of the top step, draw a line perpendicular to the width line of the step. This marks the line to cut the stringer so it fits to the joist of the upper story.

Cut out the bottom and top steps and all the remaining steps with the circular saw. Cut the width and height sides of the steps until the edge of the blade just touches the corner. Finish the cuts with the hand saw held vertically. Check the stringer for correct fit. If it is correct, use it as a template to lay out the other two stringers you need.

Determine the width of your staircase. The width varies according to your individual situation and local building codes. Cut riser boards from ½-inch plywood to fill in the vertical space between steps. The riser boards' length equals the width of the staircase and their height equals the height of the steps.

Cut stair treads from 2-by-10 lumber. The length of each tread equals the width of your staircase. Measure across the width of each tread a distance equal to the width of the steps you cut plus the thickness of the riser board. In the example staircase with 8.5 inch steps and ½-inch-thick riser boards, you want treads 9 inches wide. Cut off the excess with the circular saw.

Install the risers. Mark the joist of the upper story for the width of your staircase. Cut a hangar board from ¾-inch plywood equal to the width of your staircase and 16 inches high. Nail it to the upper-floor joist. Mark the board where the top step will be. Secure each outside riser to the plywood hanger board by nailing through the board into the riser from behind. Center the middle riser and secure it with nails. Cut a temporary bottom spacer from scrap lumber equal to the width of the staircase and nail it into place across the bottoms of the stringers.

Install the riser boards and treads, starting with the vertical riser board and horizontal tread for the first step. Continue up the staircase, first nailing the vertical riser board, then nailing the horizontal tread. Remove the temporary spacer and fit a riser board to the exposed stringers at the bottom step. Finish your staircase with the railing, enclosure or casement appropriate to your need.