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Before discussing these issues we'd like to point out that the most important thing is to take into account the thickness of the finished floor material when determining your finished floor-to-floor height. You will also need to know the finished floor thickness to carry out the suggestions below.

There are two ways to install stairs without the finished floor in place: Method 1. install the stair on top of shims that are the same thickness as the floor to be added; or Method 2. install the stair directly on the subfloor, and fit the finished floor around the bottom stair parts.

For spiral stairs the two parts that contact the bottom floor will be the central base plate and the bottom tread foot out at the perimeter of the bottom tread.

Method 1: The best method is usually the first, using shims. Precision Pine suggests cutting shims shaped like the base plate and bottom tread foot, but 3/8" undersized around all edges. This will leave a 3/8" recess around the stair parts in which to hide rough cut edges of the finished floor pieces when they are installed. At the customer’s request Precision Pine will include these shims in the stair kit at no additional charge.

You could temporarily build the stair on shims. This means you would partially fasten the top and bottom of the spiral to the adjacent structure. The top and bottom fasteners could be removed later to allow the bottom of the stair to move slightly so the finished floor can be installed under the final positions of the base plate and bottom tread foot. IMPORTANT: when using this method be sure to keep the stair braced in close to an upright position. Move the bottom of the stair only a few inches at a time before repositioning the braces. Otherwise you could create a dangerous situation if the stair is allowed to tilt past the point of being restrained. The Precision Pine spiral installation manuals discuss methods of bracing the stairs.

Method 2: When it is not possible to use shims the spiral can be installed directly on the subfloor. To assure the top of the completed stair will reach the correct height Precision Pine will need to know the thickness of the finished floor. This thickness will be added to the height of the bottom tread at the factory. Once the finished floor is installed, the height you step up from the bottom floor to the bottom tread (the rise) will be the same as the rises between the other treads, as required by building codes.

This method will require careful cutting of the finished floor around the bottom stair parts for a neat fit. The use of trim stock (say, quarter round) to cover rough flooring cuts will be tricky around the curved stair parts.

Heated floors: Before installing heating equipment in your floor it would be best to determine the approximate position and shape of the spiral bottom parts. Precision Pine can provide this information for its base plate and bottom tread foot once your spiral layout is known. You should then keep the heating tubes or elements well away from where these bottom parts will be when the spiral has been installed.