Can I Slant a Plywood Subfloor to Put in a Drain?

Gradient

• Purus, a manufacturer of European high-end wet-room floor pans and equipment, offers a working template of what slope is sufficient to effectively prevent water from ponding. The company sells gradient wedges specifically intended for use under subfloors that will drain its equipment; these wedges -- properly called tapers -- have a slant of approximately 3/8-inch every 3 feet.

Leveling Shims

• Leveling shims are available from specialist flooring suppliers and most home improvement warehouses. They can be purchased in a variety of degrees of pitch, or slant, according to each project’s specific requirements. Some shims are designed to be glued to concrete or nailed or screwed to joists under the subfloor, while others have special barbed cleats on the side that fasten to the joists.

Installation Tips

• Ensure the surface to which the subfloor is to be fitted is both flat and level; this is necessary so the desired pitch -- or degree of slant -- in the new subfloor can be made uniform across the entire floor. If a concrete floor is canted, trowel a Portland cement-based compound or use a self-leveling mix to bring it true. If laying the subfloor on off-level joists, plane them or use shims. It is industry practice to leave a 1/4-inch space between the plywood subfloor and all perimeter walls and obstacles such as stairs; this allows for seasonal expansion. (REF 1) If the plywood has a “tongue and groove” pattern on its edges to promote good, strong joins, ensure all the panels are oriented so as to allow proper mating before screwing down.

Laminate-Specific Subflooring

• A relatively recent innovation in the construction industry is the subfloor panel specifically designed for use under engineered hardwood and laminate flooring. These panels resist moisture penetration far better than plywood, and therefore make a much better substrate for materials that react adversely to damp conditions. Such material is worth considering in rooms where running water is present, likely where a drain is thought to be necessary. That said, such subfloor panels are not waterproof, nor are they intended as a replacement for good foundation maintenance or the rectification of preexisting problems. Mold and cracks can be symptoms of hydrostatic pressure -- water being forced up through the floor by sub-structural pressure -- particularly in basement rooms with floors below ground level.