You've undoubtedly heard your builder and several of your contracting team members mention the term sub floor. But what exactly is a sub floor and why is there talk of changing sub floors from room to room, from flooring transition to flooring transition?

And what lies beneath this layer?

The sub floor is a base floor, a structural layer of the home's skeleton that is designed to provide the floor support. Basically, it is a layer of floor that lies beneath the finished floor of your new home and on top of the foundation in the basement of a bungalow (although not always required in a slab foundation) and on top of floor joists in a multi-level home such as a two-storey house.

At what stage is the sub floor completed? Once your builder completes the foundation of your home he will turn to constructing the sub floor as this will give all the trades people something to stand on when working in the house. A home designed with a slab foundation may not need a sub floor on that level, as the slab of concrete can possibly serve double duty as the sub floor. You will need a sub floor if your home is raised or pier-grade beam foundation, for upper floors of multi-level homes and as a base underneath some types of hardwood flooring. (Some can lie directly over the concrete slab.)

The key components of a sub floor are, of course, the sub floor itself, which is a layer of plywood that lies flat over the joists. The plywood is generally laid perpendicular to the joist angle to increase stability. The objective is to restrict all movement in the sub floor in order to avoid squeaks and potential cracks caused by motion in ceramic and stone floors.

Builders avoid the dreaded squeaky floor by gluing the plywood sub floor to the joists and by using screws or ring shank nails which are screwed into a thick sub floor before the glue or construction adhesive dries. Nails will work loose in time, but screws will hold tight.

The combination of the construction adhesive and screw nails ensures that the plywood is held tight to the foundation or floor joists thereby eliminating squeaks. Wood movement is undoubtedly the cause of floor noise but, as our building techniques and products improve, there is little excuse for poor floor performance. These annoying squeaks are easy to catch during the initial construction phase and should be dealt with immediately, as they are much more difficult to correct once the home is completed. The biggest cause of floor noise is gaps between the sub floor and joists. The builder will fill any gaps with construction adhesive or use engineered joists. As consistent as modern lumber is, there can still be a substantial difference in joist width after drying has occurred. The consistency of engineered joists eliminates this potential source of trouble, but they don't guarantee a silent floor alone.

Beneath the sub floor are floor joists, long boards that rest on sills (pressure-treated wood that is anchored to the foundation) and girders (long heavy beams of wood that span the length of the floor and provide key support). Of course, all of these will need to be sealed or caulked to protect the wood from the outside world, moisture and bugs. Once the main-level sub floor is in place, the second-storey walls will be constructed and then the sub floor, then the top-storey walls and then the sub floor.

Before any finished flooring material is installed your builder may need to sand, smooth or fill the sub floor to provide a level, even surface for the finished flooring product.