How to Form Concrete Stairs Using Stringers

Cover a 3/4-inch plywood sheet with a sheet of rigid foam insulation that’s the same thickness as the stair stringer. Use a zig-zag bead of glue across the rear of the foam board to hold it in place. Set the stringer onto the plywood sheet over the foam board, so that the top edge of the stringer that will sit against the connecting wall for the top step is against the edge of the sheet and the base of the stringer is 2 inches from the base of the sheet.

Outline the stringer’s position on the foam board with a pencil, and then remove the stringer. Cut through the foam board along the penciled line with a utility knife. Use a straightedge to make certain that your cut is precise. Pull the foam board portion from the top of the plywood board that the stringer overlapped.

Return the stringer to the board, butting it tightly against the top of the cut foam. Nail the stringer to the plywood board, placing a nail every 6 inches along the top and bottom of the stringer.

Use the circular saw to cut the portion of plywood board off that’s above the stringer. Use the lines of the stringer as a cutting guide for the sawblade to make certain that you follow the edge of the stringer precisely. Do the same for any portion of the plywood board extending from the front of the stringer, from what should be the front edge of the bottom step. What’s left should be the completed side of the stair form that’s two inches higher than the base of the stringer and creates a single surface from the base of the stringer to its top for containing the cement pour.

Repeat the form process for the second stringer to create the other side of the step.

Fill the joint between the foam board and the wooden stringer with silicone caulking, smoothing the caulk with a fingertip that you’ve wet with water to avoid leaving a line along the edge of the formed stair.

Cut lengths of 2-by-6-inch planks to the width of your staircase, adding an inch for space to nail them to the front of each rise of the stringer. Set the first board onto the top rise of the stringers, across the front of the two stringer boards. Check that the top of the 2-by-6 is level using a carpenter’s level and then nail it in place through the plank and into the boards to create the topmost riser form. Repeat the process for each riser, making sure that the board lies at the base of the riser and extends to the riser top. If the riser is greater or less than 6 inches, then use a plank that matches the height of the riser, for example, a 2-by-4-inch plank for 4-inch risers. Attach the bottom plank so that it’s flush with the top of the bottom riser, as there will be an excess 2-inches beneath the bottom riser plank to allow for placement of the stair form into the ground.

Use the spade to create a 2-inch deep foundation hole in the location where you’re placing the staircase. Compress the soil at the base of the hole with a tamper and then level it out by running a scrap piece of the planking over the surface. Use a carpenter’s level to aid you in creating a leveled base.

Place the form into the hole with the rear of the form against the wall. Place 2-by-4-inch stakes into the soil around the walls of the form. Drive nails through the stakes and into the form to prevent it from moving as you pour the concrete.

Fill in the dirt surrounding the base of the form to prevent the concrete from running out beneath the form edges. Cover the wall at the rear of the form with asphalt mastic. Use a 1/2-inch layer of the mastic, spreading it in place with a steel trowel. The mastic will create an expansion joint for the concrete stairs against the wall.

Cover the inside of the form with a layer of form oil using a paintbrush.

Pour concrete into the form at each stair level, filling that level to the top of the riser. Tap across the front of each of the riser boards with the hammer softly to knock any air pockets to the surface.

Pass a float over the surface of each of the steps to level them out. Smooth the steps after leveling with the flat of a darby. As you smooth the steps, drops of water will appear on top of each. Run a concrete edging tool along the top of the steps with the front edge of the tool between the concrete and the form to create a rounded edge. Wait for the water to disappear, and then smooth the steps even further with a steel trowel, until you have the look desired.

Wait a few hours for the concrete to set firm, then remove the boards from the front that served as forms for the risers. Smooth out the base of each step in the joint between the stair tread and the riser with a hand float.

Use the trowel edge to cut expansion joints through the surface of each step to prevent cracks from forming. Place a joint 1/4-inch deep every 18-inches along the step width. Wait 28 days for the steps to cure completely. Lightly dampen the concrete with water each of the first seven days to avoid their drying too quickly and growing brittle. Remove the forms from the sides of the steps after the curing period.