How To Level An Uneven Subfloor

The causes of an uneven plywood floor

Moisture Abundance

One of the most prominent causes of uneven flooring is the presence of excessive dampness. Water damage from a leaking pipe or subterranean water can cause the sub-flooring to deform, affecting the flooring. Another consequence of excessive wetness is poor drainage. An excessive amount of water can sometimes cause harm to the building’s foundation.

Movement of the Soil


Another reason why flooring might become uneven is because of soil movement. Soils do shift over time, and water may cause them to expand and contract as they dry. The foundation of the floor may be harmed as a result of the soil movement. This might happen if the dirt wasn’t compacted properly throughout the building process.A solid foundation is essential for a decent and durable floor. Furthermore, if your home’s foundation is weak or improperly installed, it might lead to future problems such as uneven floors.

Beam Drop Beam

A joist is a horizontal structural member used in framing to connect an open area, usually between beams that transfer loads to vertical members. When joists are used in a floor construction system, they offer rigidity to the subfloor sheathing.
If there is a joist that is damaged, the floor will become uneven. 

How to diagnose an uneven floor

Check room slope available

The best way to check a floor for general sloping is with a rotary laser level. Positioned in the center of the room, the laser level projects a perfectly horizontal line on all the walls. To determine if there is a steep slope, measure between the laser level line and the floor at various points along the perimeter of the room. 
You can also get a rough sense of the overall pitch of the floors by laying a long straightedge across the center of the floor, with one end toward the foundation side of the room and the other toward the center of the house. Place a carpenter’s level above the ruler and observe the bubble in the center vial to determine if the room is roughly level or is noticeably sloped. 

Look for small problems.

Place a long straightedge across the floor, parallel to one side wall and approximately a foot from the wall (a long, straight two-by-four works nicely). Look for spaces between the straightedge’s bottom and the floor. The number of gaps and their size will suggest which method to apply for correcting the subfloor as you move the straightedge across the floor. Move the straightedge across the floor in 1-foot increments, measuring the distance between the straightedge’s bottom and the floor at different spots. If the floor sags in one spot across the room, for example, it might be a sign that a floor joist has sagged or is damaged. This suggests that in order to rectify the problem, you may need to strengthen that joist.

ways to level an uneven plywood floor.

Plywood Underlayment

The best solution for shallow waves or dips larger than 4 inches is to use a liquid self-leveling agent to level them out. The floor will be level and smooth once the compound has dried, ready to accept any new floor covering. You can read more about self-leveling. 
If the gaps between the bottom of the straightedge and the floor are irregular dips or sags that span no more than 4 inches and are no deeper than 1/2 inch, covering the subfloor with plywood sheets will likely level the floor enough to accommodate practically any new flooring.

Seal the gaps found in the wood.
Inspect the plywood sheets for any flaws or gaps. Check closely to ensure that no holes are missed. Water penetration is most likely to occur along the plywood’s end grain. Seal all holes between boards with painter’s caulk. You may also use the caulk to seal any holes or gaps in the plywood that you think could allow water in.

Self-leveling on the Subfloor

Properly clean the hardwood.

Remove dirt from the floor, including objects protruding from it, such as wood chips or shavings. Use a saw, razor knife, or sander to cut them out. Next, carefully sweep or vacuum the floor to ensure no dust or particles are left behind. Dust and filth might diminish the compound’s ability to stick to the floor, causing issues later on. After removing any dust and debris, wash the floors with a moist sponge or towel and allow them to dry.
Install a sill sealer at the base of each wall that makes up the perimeter of the floor with a stapler. When walls expand or compress, the sill sealer prevents cracking. It also keeps water or moisture from penetrating the concrete composition.

Identify your high and low spots.

Identify the parts of the floor that need to be filled in to bring them up to par with the rest of the room. Use a long straightedge to locate these low places. Apply a primer to these areas if the manufacturer advises it and let it dry completely.

Mix the self-leveling compound

Together with inside the bucket mix the floor leveling product with water according to the manufacturer’s instructions. Spread a liberal amount of leveler over the subfloor using a towel, allowing it to settle into low spots. The substance, like water, will seek its own level, and it must be spread out evenly to cover the afflicted area without pooling.

Allow the applied leveling compound to dry.

Allow the leveler to completely dry before sanding any high spots and vacuuming away the dust. You don’t need a glass-smooth surface, but you should be able to trace a straightedge over it without seeing any large gaps. While checking with a straightedge, shine a work light across the surface to locate any gaps you missed.

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