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Installing spirals on concrete lower floors:

Most building codes do not allow untreated wood against concrete. The reason is that even cured and old concrete allows damaging moisture to reach the wood.

Even if your stair installation will not be inspected to meet building codes, Precision Pine strongly recommends that you prevent moisture from contacting your stair. If your installation will be inspected you might ask your building official which of the three treatments below will be acceptable.

Concrete fasteners are not supplied with the spiral kit. Customers choose the type fasteners that best fit the concrete they are working with (older and likely harder, or newer and softer) and the drilling tools they have. Fasteners such as concrete screws and/or expanding plugs require clean holes precisely located. A sharp concrete bit and sophisticated hammer drill are needed for these holes, particularly on older concrete. Concrete bolts set in epoxy require less precise holes.

1. The simplest solution would be to apply a vapor barrier between the concrete and the lower parts of your stair: the spiral stair base plate and bottom tread perimeter foot, or the lower ends of straight stair stringers. The vapor barrier could be two layers of 6 mill polyethelene sheet cut to the correct shapes of the stair parts, leaving about 1" of plastic all around. Two layers of building felt cut the same way also have been used. This method requires precise holes in the concrete to match the pre-drill holes in the spiral parts.

2. A more certain way to meet the code would be to have Precision Pine manufacture the lower stair parts with treated pine. The bottom layer of the spiral base plate and the bottom tread foot will be as smooth as the rest of the stair. The color would be different but the two parts could be stained. Again, precise holes in the concrete are required.

3. Precision Pine will construct the bottom tread with a rise 3/4" less than required. This allows the customer to fasten 3/4" treated plywood on the concrete and screw the bottom spiral parts to the plywood. The plywood can be cut to be recessed under the nosing of the tread for toe clearance, and can extend beneath and behind the bottom tread where you will not be walking.

An advantage of this third method is that you can use large holes in the plywood for the concrete fasteners so the concrete holes do not need to be precise. Large fender washers on the fasteners can be used to cover the large plywood holes. Precision Pine recommends also gluing the plywood to the concrete with the proper construction adhesive.