Since no one wants a new floor that slopes or sags, this is reason enough not to install a new floor on top of an uneven subfloor. For the best results, you need to make sure that the surface underneath is even and level. Typically, a floor tends to settle near the centre of the house. But no matter where in the house you have a sagging floor, the goal is the same. You must identify and address the problem in order to have a level finished floor.
Fasten the wood sheathing securely to the top edge of the floor joists. Keep in mind that floors framed with solid wood joists can differ slightly in height, especially if the builders were not selective when choosing the wood for the floor joists.
Use coarse thread screws in addition to the nails already in place, particularly in areas of the room that get a lot of foot traffic. Nails have a tendency to work loose as wood expands and contracts, whereas screws work better when it comes to preventing floorboards from squeaking. Screws should be long enough to go through the wood into the floor joists by at least 1 1/4 inches.
Locate the highest point on the floor using a level and a 6-foot long 2x4 for a straight edge. Swing the straight edge around in a 360-degree circle. This will help you to identify any dips or low spots in the subfloor. Shims or levelling compound may be needed to level the floor.
Apply levelling compound evenly to problem areas. Start by mixing the compound with water. Another easier method is to use asphalt roofing shingles to repair low spots in the subfloor.
Fill in any depressions with the asphalt shingles. The felt paper used for roofing also works well for covering up smaller imperfections. Simply tack shingles in place to keep them from moving as you work on the floor. This technique helps to make any humps in the finished hardwood floor less noticeable.
Hide imperfections like an uneven floor by installing hardwood flooring perpendicular to the direction of the floor joists. If you run the flooring parallel to the floor joists, any dips in the subfloor will be more obvious.
Fix sagging floor joists if you discover that is the problem. Inspect the underside of the floor from the basement or crawl space. Examine the support beams and posts where they meet the floor. You may be able to jack up joists until they are level. This should be done slowly over time, screwing the jacks up just a turn each month. You don't want to level your floors only to end up cracking your walls. If floor joists are the problem, you can bolt the same type of lumber to the side of a joist to reinforce it.
Install a new plywood subfloor over the existing subfloor to level out the room. You will be using ¾-inch plywood, so make certain first that the floor joists can handle the additional weight, especially if you plan to install hardwood flooring over top of that. If weight is a problem, you could put down a ¼-inch subfloor if that is enough to fix any dips in the floor.
Lay tar paper over the wood subfloor before installing hardwood flooring. This acts as a vapour retardant, protecting the underside of hardwood flooring from moisture damage.