There are a few prerequisites to installing a new floor, whether it be hardwood, laminate, vinyl, or tile. One of the more important requirements is that the subfloor be flat, without humps and valleys.
Even in newer homes, plywood subfloors and concrete slabs are rarely flat. High and low spots can affect the look of a finished floor, interfere with the installation, and even lead to material failure, especially in tile floors.
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How to Level a Concrete Floor
How to Level Different Types of Floors
Most flooring manufacturers recommend a variation of no more than 3/16 inch over a 10-foot span. Floor leveling is a relatively easy DIY project, although some subfloors with rot, insect damage, or structural issues may require professional attention.
Finding the high and low spots
Start by locating the high and low spots before leveling a subfloor. To do this, you will need a straightedge (at least six feet long) and measuring tape.
Use a long straightedge, such as a long spirit level or a straight board held on its edge. Starting at one side of the room, swing the straightedge from one reference point across the floor and mark the high and low areas (any deviations greater than 3/16 inch) with a pencil. (You can also use a string line stretched between two points or a laser level to measure the variations in the floor’s surface.) Continue around the room until you have surveyed the entire floor.
Steps for Leveling a Wooden Subfloor
You’ll need:
- A random-orbit sander
- Coarse (60-80 grit) sandpaper
- Electric/cordless driver drill
- Flathead screws long enough to penetrate the subfloor into the floor joists
- Wet/dry vacuum
- On a plywood or wood subfloor, check for loose nails and seams; remove any nails, screws, and staples that stick up above the surface.
- Drive screws through the subfloor into floor joists to replace any loose nails or to secure seams. If possible, connect the vacuum to the sander to reduce airborne dust.
- Use the sander to remove any high spots that you’ve marked, checking with the straightedge as you go. When you’ve sanded down the high spots, sweep and vacuum the entire floor.
- Alternatively, if there isn’t too broad a disparity between the high and low spots, you can improve the flatness by installing a new layer of plywood over the entire old subfloor. This solution isn’t universally applicable because it will raise the level of the finished floor and may complicate transitions to other rooms.
Steps for Leveling a Concrete Floor
You’ll need:
- Angle grinder or floor grinder
- Diamond grinding wheel
- Wet/dry vacuum, ideally equipped with a HEPA filter
- Respirator
- Before you start, tape plastic sheeting over doorways and windows to minimize the migration of concrete dust into the rest of the house. Be sure that your respirator is tight-fitting.
- If you’re using a handheld angle grinder, have a helper hold the vacuum nozzle close to the grinder to capture dust as you work. (Some grinders have vacuum attachments, which eliminate the need for a helper.) Floor grinders are equipped with dust bags but will still raise a fair amount of airborne dust
- Concentrating on each of the high points, sweep the grinder back and forth, checking your progress periodically with a straightedge.
- When you’re finished, thoroughly vacuum and/or sweep the slab to remove all debris.
How to Fill in the Low Spots
In part, the type of finished flooring you’ll be installing can determine how you fill the low spots. Floating floor systems, such as laminate or vinyl, can sometimes be supported by stapling layers of builder’s felt (tarpaper) onto low spots on a wood subfloor.
However, the amount of trimming and rechecking can get tedious. A faster solution that works for all types of flooring on both wood and concrete subfloors is to use a self-leveling underlayment. Available as either pre-mixed or a dry mix, it’s a cement-based material meant to be mixed (if required), poured, then troweled into low spots.
Some underlayments are universal, (they can be used beneath any type of flooring), while others are more material-specific, so be sure to read the fine print before purchasing. Most self-leveling products also require the use of a latex primer that strengthens the bond to the subfloor.
You’ll need:
- Mason’s trowel
- Bucket, electric drill, and mixing paddle (for dry mix underlayments)
- Wide trowel for smoothing
- Underlayment primer
- Apply a coat of primer to the subfloor with a paint roller. Allow the primer to dry. If you’re using dry mix, combine with water according to the manufacturer’s instructions. The consistency should resemble pancake batter—not too stiff and not too soupy.
- Scoop the underlayment into the low areas and use the wide trowel to spread out the puddle so it’s level with the surrounding area.
- After the underlayment is dry, use your straightedge to check the filled areas—you may have to feather the edges or high spots with a sander.
How to Level a Floor for Tile
If you plan to install tile over a plywood subfloor, after flattening the plywood you’ll need to go one step further. Installing a layer of ¼-inch cementitious backer board on top of the plywood will create a stable, bondable substrate.
You’ll need:
- After flattening and cleaning the plywood, mix the thinset according to the instructions. Using the notched trowel, spread it over an area the size of one sheet of backer board.
- Place the backer board onto the thinset and screw it down into the plywood. (Most manufacturers mark recommended screw locations on the backer board.)
- Allow a ¼-inch space between the board and walls, and a 1/8-inch space between panels. When you need to cut a panel, mark the cut, use the backer board knife to score the mark on both sides of the panel, and snap the cut.
- You can also use a circular saw fitted with a masonry blade, but be sure to cut outdoors—it makes lots of dust.
