Last month’s column focused on layout of steps. With that as a backdrop, here are some suggestions about forming steps. There are four basic types of steps: straight, tiered, curved, and cantilevered. Rises and runs can be patterned, plain, or any combination, so there’s a lot to consider.
Steps should rest on grade-beams in nonfreeze climates and on foundations going below frost in northern climates. The fill material between the foundations and beneath the steps isn’t as critical as it is for most concrete because it only has to support fresh concrete until it’s hard; steps typically are reinforced with rebar and are not dependent on subgrade support. The subgrade should be placed to provide a minimum 6 inches of concrete where the bottom of the risers meet the treads. This is the critical location, both for strength and to provide sufficient cover for reinforcing steel.
Materials and hardware
With the advent of impact drill-drivers, more contractors today are using screws to hold forms together. These tools make it easy to drive screws—both in and out. Using screws makes it easy to make adjustments to forms because they are quicker and forms are not jolted as they are with nails. Many contractors still prefer double-headed nails, but I recommend screws.
Steve VandeWater, manager of decorative concrete sales for Builders Concrete and Supply, Noblesville, Ind., prefers supporting step forms with steel stakes, especially where concrete flows around the stake. Even though contractors typically pull stakes and fill holes while concrete is still fresh, large stake holes segregate aggregate, causing weakened planes that can result in cracks. VandeWater adds that when the subgrade is loose, wood stakes are more stable and may be the best choice.
The material used to form the risers depends on the type of step being formed. For straight risers, 2x4, 2x6, or 2x8 lumber commonly is used while wood or composite siding or plywood is used for curved riser forms. VandeWater suggests cutting a 45-degree angle on the bottom of riser boards so the entire tread below can be floated and finished. He also holds the elevation of the lower riser edge about 1/4 to 3/8 inch higher than the tread so he can strike-off the concrete and hand-float the tread to the correct elevation.
For cantilevered steps, depending on the depth of the cantilevered nosing, the inset can be accomplished by inserting formliners (for patterning), polyfoam board, or expansion joint material in the bottom portion of the riser.
When forming between walls or other flat surfaces (such as plywood) where you will be able to mark riser locations, positioning risers is easy. Set riser boards to the marks and fasten them to the walls or side boards. If steps are longer than 4 feet, install stringers to support risers and keep stair lines straight. Stringers should support both the top and bottom of each riser board (refer to the diagram). Joe Fettig, owner of Fettig Concrete, Newberg, Ore., uses turnbuckles, which are available at construction supply stores. This makes it easy to support and adjust the bottom riser position.
Carpenters build structures to last but concrete contractors who build step forms want solid construction that can be easily and quickly dismantled to facilitate concrete finishing. Regardless of the method you use, it’s wise to take your time building step forms. Check and recheck your elevations. It’s easier and cheaper to do it right the first time than to do it again later.