A subfloor is meant to be flat and level. If not, your floor covering, which is the solid hardwood, laminate, tile, or other exposed flooring that goes on top of the subfloor, will never be flat and level. This problem can be fixed, but it all depends on exactly what kind of uneven flooring you have.
Uneven subfloor and floor covering fall into two broad categories: general sloping that affects the perimeter as a whole and irregularities within the floor perimeter itself. If the perimeter as a whole slopes to the side or middle, the repair project will usually be a major one requiring the assistance of a contractor. If all four sides of the room's perimeter and middle are level, then the problem lies within the perimeter and likely can be cured with the addition of underlayment or by pouring leveling compound.
Entire Floor Slopes From End to End
A general slope means that one end of the room's perimeter is lower than the other end. This problem affects nearly all basic living operations and is a safety hazard. Home resale can be difficult since most home buyers rightfully balk at such major problems.
Determine if all four sides are level with a laser level. An inexpensive line laser can help you determine just how far out of level your floor is. An even better but more expensive tool for gauging floor level is a rotary laser level. Rotary levels can be borrowed from rental companies for a nominal price on a daily basis.
If your room is out of level from end to end, this means that your problem resides below and cannot be fixed with leveling compound or underlayment. An entire foundation wall or foundation footing may be in bad condition or the ground itself its subsiding. If the foundation is not sinking, the sill plate may be the cause. This wooden footing, sandwiched between the exterior walls and foundation walls, might be rotten or insects such as termites and carpenter ants might be eating it away.
Another telltale sign that the problem is with the foundation or sill plate is when walls, window frames, and door frames show signs of cracking. If this is an older house, such cracks may have been repeatedly patched and painted over.
Unfortunately, this fix is typically beyond the skills of most do-it-yourselfers. Consult a foundation repair contractor or a general contractor.
Floor Slopes Toward the Middle
When all four sides of a room's perimeter slope down toward the middle of the room, this usually means that joists or beams below the subfloor are sagging, termite-ridden, or in some rare cases, broken.
In the crawlspace or basement, the upper floor can be braced by adjustable steel columns, also known as lally columns. Next, the affected structural members can be replaced or sistered with new, stronger beams made of a structural lumber called microlam or laminated veneer lumber (LVL).
Small Waves and Gaps in Subfloor
If the gaps span about 4 inches or less, then putting down a 5/8-inch plywood underlayment alone should span and effectively bridge the depressions.
For ceramic tile, which requires a rock-solid base to avoid cracking grout or tile, fill these small waves with leveling compound. Other types of flooring are more forgiving. Laminate, vinyl (or resilient flooring) solid hardwood, and engineered wood, each in their own sense and to a limited degree, can tolerate small gaps. Laminate and vinyl flooring are especially forgiving due to their pliable nature, especially vinyl. Solid hardwood and engineered wood are both strong enough to bridge small waves and dips in the subfloor.
Large Waves and Gaps in Subfloor
If the waves are wider than about 4 inches, then plywood alone will not provide a sufficient base for your finish flooring of any type. Wider waves can be filled in with self-leveling compound. Self-leveling compound comes in large pre-mixed buckets or in dry form that can be mixed with water. The reason it is called self-leveling is that you merely pour the liquid compound onto the floor and, like water, it seeks its own level. Unlike other wet compounds such as concrete, you do not need to screed the compound or touch it in any way. If the depressions are deep, the leveling compound will fill in only those areas, leaving high areas dry. If the depressions are shallow, you may have to cover the entire floor with leveling compound. After the compound has fully hardened, you can install underlayment.