How to Level a Particle Board Subfloor
A particle board subfloor may have one or more spots where there are detectable dips, wells or undulations. Subfloor is intended to be covered with padding and carpet, or overlaid with finished flooring, like tiles or finished hardwood. Before any product can be installed over a particle board subfloor, it should be leveled. Leveling a particle board subfloor entails getting to the underlying problem: the floor joists. Does this Spark an idea?
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Getting to the Problem
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A particle board subfloor is nailed to the floor joists below. In a finished house with living space below, an uneven subfloor must be corrected from above. The correct way to rectify the problem is to cut out and remove the subfloor, wall to wall, and expose the joists below. Use the highest point on the joists as an elevation reference. Take a straight 2 x 4 piece of lumber long enough to reach from the elevation reference to the other end of the room and place it on the high point. Extend the 2 x 4, on its edge, across the room to the wall. Place a 4-inch level on top of the 2 x 4. This will begin the process of measuring to create a level subfloor surface.
Shim (cedar wood shims work best) under the far end of the 2 x 4 until it is level. Glue or nail shims on every joist under the 2 x 4 to make up the space, keeping the 2 x 4 level. The shims will stay on top of each of the joists. Next, move the whole 2 x 4 12 inches in either direction and repeat the process until you have covered the entire floor.
Check your work. You should now be able to place the 2 x 4 on any of the nailed or glued shims and get a level reading, as the joists with attached shims are now all level with the high point. Crisscross the floor, rechecking all shims with the 2 x 4 to ensure they are leveled. It is important to get this right before reinstalling the subfloor.
Lay particle board flooring--preferably new, although some sections may be salvaged--across the joists, wall to wall. Nail the newly leveled subfloor to the joists. Be careful to nail the subfloor only through the shims into the joists, otherwise you will create "bellys" in the plywood.
You can now install carpet or finish flooring.
Alternative Solutions
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There are a few alternative methods for leveling subflooring. One is to follow the same procedure as above, but instead of installing shims on the floor joists, you can nail a 2 x 6 along the joists. This is known as "sistering," and levels the joists continuously from one end to the other. This method has drawbacks if plumbing or electric lines are fastened to the floor joists.
An expensive but less destructive method of leveling a particle board subfloor is to pour self-leveling concrete directly over the problem area of the subfloor. This works best if the problem is just a low spot in one corner, or a dip that is less than 25 square feet in dimension. A good product is JIF-SET. Pour it in the corner or over the dip just to raise a small portion of flooring to level. Let it set following manufacturer's instructions. Install padding and carpet or other finished flooring as before.
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