When installing a floor, it is advisable to lay subflooring first, before laying the finished visible flooring material. Subflooring allows the finished material on top to layer evenly. It also provides a waterproofing layer, to eliminate concerns of rotting or molding due to moisture exposure. One critical step when laying subflooring is to fill the seams, joints and cracks with a sealant, adhesive or filler, to prevent water infiltration.

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To waterproof subflooring, you must fill each crack and joint with a waterproof material.

Caulk

Caulk is one of the most common ways to fill in the wood of subflooring. Apply the caulk by squeezing an even, generous, but low-profile layer of caulk wherever it is needed throughout the subflooring, including the perimeter of the floor where it meets the walls. The caulk should be thick enough to can form a waterproof seal, but should raise no higher than the flooring material around it. If it rises higher than the flooring material, it makes the surface uneven and difficult to lay the finished flooring material. Apply caulk between the slats of wood, around screw and nails, and in any cracks in the wood.

Floor-leveling Compound

Floor leveler is a product designed to fill in chips, cracks or seams in flooring, to maintain or restore an uneven surface. A multitude of floor leveling products with varying compositions are available; some are water-based, while others are oil-based. They are formulated based on the material they will occupy gaps and cracks. Do not try to use concrete floor leveler on wood. To achieve optimum results, use a product formulated to bind with wood. Apply it wherever there are gaps or joints in the wood, or other unevenness.

Wood Putty

Wood putty is a sealant designed to fill unevenness in wood grain. It is similar to floor leveler, but is specifically formulated with not only wood, but wood grain in mind. Follow the manufacturer's instructions for applying it throughout a piece of wood or between pieces of wood to fill the subflooring and seal it. Apply wood putty to any gaps, cracks or joints in the subflooring.

Varnish or Sealant

Varnish or sealant can also fill in the gaps and seams of subflooring. Typically, this type of product makes the subflooring waterproof. It is not the best choice for the task, since it is relatively thin compared to wood putty and may, therefore, require multiple applications to adequately fill a gap in the wood. When applying multiple layers of a liquid sealant or varnish, allow each layer time to dry so it does not seep out from the floor.