How to level an uneven subfloor before new flooring

Before new flooring, a subfloor needs to be flat to the product's tolerance, not necessarily level. Check it with a long straightedge, grind or sand high spots, fill low spots with a patching or self-leveling compound, and fix loose or springy areas first. Sagging framing in older houses is a job for a contractor, not a flooring fix.

What is the difference between flat and level?

Level means horizontal. Flat means no humps or dips over a short distance. Flooring cares most about flatness. A floor that slopes gently and evenly from one side of a room to the other can often take a new floor, as long as the surface is flat. A floor with a dip in the middle cannot, even if it is level overall.

Many older Brooklyn row houses and brownstones have floors that slope after a century of settling. Making a whole room dead level is a major job and often not needed. Making it flat enough for the flooring you choose usually is.

How flat does a subfloor need to be?

Each manufacturer sets its own flatness tolerance in the installation instructions. It is usually written as the largest gap allowed under a straightedge of a given length, such as a fraction of an inch over 6 or 10 feet. Requirements differ by product, and thin glue-down vinyl and long or wide planks tend to be less forgiving. Check the instructions for your exact product, because installing outside the tolerance can affect the warranty.

Here is how common subfloor problems tend to show up in different kinds of floors.

How do you check a subfloor?

1. Clear the room and remove old flooring, tack strips and loose debris, so you are checking the real surface.
2. Lay a long straightedge, such as a 6- or 10-foot level or a straight board, across the floor in several directions.
3. Mark any spot where the gap under the straightedge is more than the product allows, and any high point the straightedge rocks on.
4. Walk the floor and listen for squeaks, and feel for soft or springy areas.
5. On concrete, test for moisture before choosing a leveling product or a floor.

A laser level helps on larger areas. Many installers map high and low points before they quote leveling, because the amount of material and time depends on it.

How do you fix high spots and low spots?

The method depends on whether you are working on concrete or wood, and on how large the problem areas are.

On concrete slabs

• Grind high spots and ridges down with a concrete grinder. Grinding creates fine dust, so dust control matters, and old adhesive on the slab should be tested first.
• Fill small dips, cracks and holes with a cement-based patching compound.
• For wider low areas or a generally rough slab, pour a self-leveling underlayment. It flows out to form a flat surface, but it needs a clean, primed slab and dams at doorways and openings.
• Let any compound dry as its maker specifies before installing flooring, and confirm the flooring manufacturer accepts it as a base.

On wood subfloors

• Refasten loose plywood or planks to the joists with screws to stop movement and squeaks.
• Sand or plane down raised seams and high edges.
• Fill shallow low spots with a patching compound rated for wood subfloors.
• Over old plank subfloors with many uneven boards, a new layer of plywood, shimmed where needed, is often the simplest flat base.
• Self-leveling compound can go over wood only with products and preparation meant for it, often including a primer and reinforcing lath. Check the leveler's instructions and the added weight.

Sagging, bouncing or sloping that comes from the joists is a structural issue. Have a contractor or engineer look at the framing. No underlayment or leveling compound fixes a floor that is moving.

Will leveling change the height of the floor?

It can. A layer of self-leveler or new plywood raises the subfloor, and the new flooring adds more on top. Before work starts, check the finished height at doorways, thresholds, stair nosings, dishwashers and the edges of tile. A small difference can be handled with a transition strip. A larger one may mean trimming doors or choosing a thinner floor, so it is worth measuring early.

Brooklyn houses and NYC apartments: what changes?

• Brownstones and row houses are framed with wood joists, and floors often sag or slope after decades of settling. Flattening may mean sistering joists or building up a new subfloor, which is carpentry rather than floor prep.
• Detached and semi-detached houses in south Brooklyn often have wood subfloors under old carpet or tile, with patched areas from past renovations that sit at different heights.
• Apartment buildings usually have concrete slabs, which can have humps, cracks and leftover adhesive from old floors. Some older dark adhesives contain asbestos, so test before grinding.
• Pouring self-leveler in an apartment needs care. Unsealed gaps at pipes and walls can let liquid leak into the unit below, and many co-ops and condos require approval for this kind of work.
• Basement and ground-floor slabs need moisture testing before leveling, since moisture can affect both the leveler and the floor. See our guide to basement flooring.

In Park Slope brownstones and similar row houses, sloping floors are common. Decide with your installer whether the room needs to be flat, level or both before you pick a product.

Does the flooring choice change how much leveling you need?

Yes. Thicker rigid-core vinyl plank and engineered planks bridge small imperfections better than thin glue-down vinyl, which shows almost everything beneath it. That does not mean a rigid floor can go over a bad subfloor. Its instructions still set a tolerance. Any glue-down installation needs a flat, clean surface for the adhesive to bond.

Leveling is often a separate line on an installation quote, and it can change the overall budget. Ask your installer to check the subfloor before you finalize materials. Our guides on floating, glue-down and nail-down installation and preparing for installation cover the rest of the prep. When you know your room size and product, ask us for a quote and we can work with your contractor.