Things You'll Need
Long board or string
3/8-inch thick sheets of plywood
2- to 2 1/2-inch subfloor screws
If you've got an uneven subfloor on which you want to apply flooring, you can make the floor level by using asphalt roofing shingles. Asphalt shingles are durable, stable and relatively thin, making them a good choice to level out small or large dips in a floor. If you have a long dip in a floor, you can apply a row of asphalt shingles over a long space. If the shingles create a small hump, the length of the hump in the floor will make it unnoticeable.
Place a long board at the highest point in the floor, and hold it straight across to discover if your floor is level. If it's not, you can use shingles to build up any dips. If a long board is to heavy, hold a taut string across the room at floor level.
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Use a measuring tape to measure the distance between where the floor dips and the board or string. Estimate the number of shingles you'll need to fill the dip.
Place the first shingle in the dip. Lay two shingles flush against the first shingle on either side. The shingles should not overlap where they meet. Create a row. Continue to layer the shingles in this fashion until you think the floor is level.
Place your long board or string across the floor to discover whether the floor is level.
Measure the size of your room, using a measuring tape. Cut sheets of 3/8-inch plywood to size with a circular saw to cover the entire floor.
Screw the sheets of 3/8-inch thick plywood over the shingles, using a drill and 2- to 2 1/2-inch subfloor screws to attach them to the entire floor .
Apply your flooring over the plywood per the manufacturer's instructions.
Check how level your floor is as you work, to make sure you're not creating new dips and high points.
If the level of your floor is still slightly off after you've applied the plywood, you can use a layer of roofing felt to even out the floor before you install your flooring.