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 subfloor you have.
Uneven subfloors fall into two broad categories: general sloping that affects the perimeter as a whole and irregularities within the floor perimeter. 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 applying a 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. It can also make home resale difficult because most home buyers rightfully balk at such major problems.
The best way to check a floor for general sloping is with a laser level. You can do this with an inexpensive line laser, but you'll get more accurate results with a rotary laser level, which you can rent from a rental center. A laser level projects a perfectly level line across an entire room. To see if your floor is out of level, all you have to do is measure between the level laser line and the floor at various locations along the room's perimeter.
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 may have shifted. If the foundation is not sinking, the sill plate may be the cause. This is a layer of wood lumber sandwiched between the exterior walls and the foundation walls. Sill plates can be affected by rot or can be eaten away by carpenter ants, termites, or other wood-boring insects.
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).
While a handy homeowner may be able to sister a joist or two, widespread repair of joists and support columns is a job for an experienced contractor.
Installing a Thick Underlayment
If the floor is level along its perimeter but there are interior dips or waves that span about 4 inches or less, then putting down a thick plywood underlayment should span and effectively bridge the depressions. A new layer of 5/8-inch or 3/4-inch tongue-and-groove plywood suflooring is like adding a new subfloor over the old to create a stiffer, flatter surface for any type of finished flooring.
- Plan the layout of the plywood sheets so the long dimension of the plywood runs perpendicular to the floor joists. Also make sure the joints between sheets in the underlayment are offset ("staggered") from the joints in the original subflooring by about 1/2 sheet. The short edges of the plywood must be centered over joists.
- Trim the first sheet, as needed, to stagger the joints and ensure the ends of the sheet fall over a joist. If you're cutting along the sheet's length (making a rip cut), remove material from the tongue-side of the sheet, to leave the groove-side intact.
- Place the first sheet into position along one of the walls with the groove edge facing into the room. Fasten the sheet to the floor joists with 2 1/2-inch screws driven through the old subflooring and into the joists below. Drive a screw every 6 to 8 inches along all of the joists.
- Trim (as needed) and place the next sheet in the first row. Leave a 1/8-inch gap between the ends of the sheets. Fasten the second sheet as with the first.
- Install the remaining sheets in the first row, trimming the last sheet to fit, as needed.
- Complete the second row with full-width sheets, but trim the first sheet, as needed, so the end joints between sheets are offset from those in the first row by about 4 feet. Set each sheet in place and tap it along the groove edge with a wood block and sledgehammer to lock the tongue-and-groove joint with the first row, then fasten the sheet.
- Install the remaining sheets of subflooring to complete the underlayment layer, trimming the last row as needed to fit against the wall.
Filling Large Waves in Subflooring
If the waves are wider than about 4 inches, then plywood alone may not provide a sufficient base for your finish flooring. 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.
- Mark the areas of the floor that need filling in to bring them level with the surrounding areas.
- Apply a primer product to the areas where you will add leveler, if recommended by the leveler manufacturer. Some surfaces and leveling products call for primer; others do not.
- Mix the floor leveling compound with water in a bucket, following the manufacturer's directions.
- Pour a generous amount of mixed leveler onto the subfloor, focusing on one or more wavy areas.
- Spread the leveler over the floor with a trowel or a gauge rake so it covers each low area completely. The product will seek its own level (much like water), but it must be spread initially to cover the affected area and not pool in the center.
- Allow the leveler to dry as directed by the manufacturer.