Drastic slopes in hardwood floors can result in bending, heaving and wood plank ends that do not remain nailed down. Besides being visually unappealing and uncomfortable to stand on, sloping floors present safety concerns. Most remedies for a drastically sloping hardwood floor involve the subfloor. A subfloor should not vary more than 3/16" over a ten-foot span. If not the subfloor, then the foundation and structure of the house is the probable cause.
A good way to level an uneven hardwood floor and provide support to the subfloor is to add wooden shims. Place the shim between the subfloor and the joist; check to ensure the subfloor is now level; then nail down through the subfloor and shim straight into the joist.
It is also possible to install shims between the subfloor and hardwood floor. Stack asphalt roofing shingles to even out larger high and low spots and taper as necessary to achieve a level surface. Swing a six-foot-long straightedge in a 360-degree circle at various points on the floor to identify these spots. The shingles do not require nailing down, but tacking into place will keep them from shifting.
Self-leveling compounds are a good choice for leveling a severely sloping floor. The first step in applying a self-leveling compound is to apply a latex primer to the sub-floor. This will ensure a good bond between the self-leveling compound and the sub-floor. Fill in all low areas first, and then spread a thin coat over the remaining sub-floor.
Floor joists need to remain stiff, solid and level to provide sufficient support for a subfloor. Uneven, or poor-quality floor joists can be a major cause of sloping in hardwood floors. It is possible for a 2x10 floor joist to be off by 3/8" or more. Check existing joists for depth, cracks and knots, bending and warping. Replace with high-quality wood joists of equal size.
Another solution for supporting floor joists is to sister them. Installing a second joist next to the original is a good way to add strength and additional support to a weak or damaged joist.
Examine the subfloor from underneath to determine its condition. Check for signs of warping, sagging and water damage, all signs the subfloor needs replacing. While replacing only damaged sections of a subfloor is possible, a drastically sloping hardwood floor may require total replacement. A good idea, especially if the hardwood floor is in a kitchen or bathroom, is to install a vapor barrier over the subfloor. Moisture can quickly and permanently damage and warp a new subfloor, leaving you once again with a drastically sloping hardwood floor. A common material used as a vapor barrier is 15-pound black felt paper.
If the underlying reason for your sloping hardwood floor is due to foundation issues, no amount of inside work will solve the problem. Solving structural issues could require jacking up the whole house and installing new footers. When done right, this process takes a long time, so plan on several weeks if you want to avoid structural damage.
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