Type of Subfloor
A subfloor is the layer at the very bottom of your shower pan. Most homes will consist of a standard subfloor constructed of running boards laid over floor joists, a plywood layer over the running boards, or if on a first floor, the subfloor can be a concrete slab. Depending on the subfloor material, you will need to ensure it is sturdy enough to support the weight of a built up shower pan.
Add to Subfloor
In order to add support and reinforce the subfloor structure, you can add a secondary layer that the built up shower pan will sit on top of. This secondary layer can be added on top of the running boards or concrete slab. If a plywood subfloor already exists, it may be redundant to add to the subfloor, which would only be increasing the height and add minimal support and structure.
Material For Subfloor
A standard material used for a subfloor is going to be a cut to size plywood, mainly due to its availability. Alternatively, you can choose to install a cement backer board as your subfloor or as an addition for support and structure. The primary objective is to support the weight of the built up shower pan and provide a durable subfloor in the event of the natural processes of home settling and seismic movements. When choosing your subfloor material, take into consideration how the thickness will effect the overall heigh of the built up shower pan in relation to your shower curb. A piece of plywood can be purchased in 1/2″ or 3/4″ thickness, and most cement backer boards will be 1/4″, 1/2″, or 5/8″ thick.
To install the subfloor, whether it is the primary or secondary layer, you will need to accurately measure the length and width of your built up shower pan, also taking into consideration any non-square corners. Ideally you would need to cut single, large pieces and screw them into the running boards or concrete slab. If needed, cut any finishing pieces to size in order to complete the subfloor installation. It is important to remember, if you choose a cement backer board, you will need to use the manufacturer recommended backer board screws. You should end up with material leading to the edge of your framed walls and shower curb.