Plywood sheets are best used for subfloors in attics, basements, or around the house. When using the material as a structural base beneath carpet, laminate, tiles, or hardwood flooring, you need to use the right plywood thickness and type of plywood to meet uniform building codes. Otherwise, the finished floor won’t be strong and durable for daily use.

Uneven plywood floors are usually caused by four main things: foundation issues (which are rare and you’ll notice other problems around the house if you have these), structural joist issues, water damage, or incorrect material being used for the initial wood subfloor.

When subfloors become uneven, solutions differ depending on the cause of your problem. The best solution is always to replace the flooring entirely, especially if the incorrect material was used to begin with. But this is costly and cheaper workarounds can fix other, more minor problems.

Keep reading to learn how to level a plywood floor and why floor leveling is so important.

Why is a Level Floor Important?

Flooring has three layers: the surface covering, the underlayment, and the structural subfloor. Without a level subfloor, you could experience all kinds of issues on your property. Firstly, an uneven wood subfloor will put stress and pressure on your surface covering, which can cause breakages.

For example, hardwood flooring installation over an uneven subfloor will be difficult to fit and, over time, the planks can eventually bode or snap. Likewise, with ceramic tile installation, an uneven subfloor will put pressure on the material and cause cracks from normal daily use.

Secondly, an uneven subfloor will squeak or bounce when walked on. This will be the case no matter what floor covering you have, including carpet, laminate, or hardwood. 

Lastly, an uneven floor will be a daily inconvenience. When the buckles are noticeable, you’ll feel the slant when walking and your furniture will be visibly wonky or wobbly.

How Can I Tell if a Hardwood Floor is Uneven?

Modern-day subfloors should be made of either OSB or tongue-and-groove plywood sheets that interlock to form a stable, even base. Sheets used should be thick enough to meet the minimum uniform building codes to ensure they can withstand the weight of daily interior use. Learn more about plywood thickness for construction projects.

However, homes built before the 1970s (when subfloors did not have such strict codes) or constructed with incorrect subflooring material can present as unlevel or become unlevel over time. This can make your finished floor covering (like hardwood flooring or carpet) uneven.

It’s usually very easy to determine if a wood subfloor is unlevel. There will be noticeable dips when walking across the flooring and visible slants from wall to wall. To confirm your hardwood floor is uneven, you can:

1. Lay a 4-8ft (or longer) spirit level horizontally across the floor near an edge
2. Check if the bubble is resting between the two lines – if it isn’t, your floor is uneven
3. Repeat the above two steps on every wall in the room and across the center of the room – if the bubble is slanted everywhere around the room, your hardwood floor is entirely uneven, if it’s slanted in patches, only a few plywood sheets are unlevel

Remember, most floors in homes are not perfectly level. Small, insignificant slants across an entire floor is not usually a cause for concern, as this is to be expected in any property, especially if it’s older. However, if your flooring is severely unlevel (with the bubble way outside the two lines) or has severe dips, this is a problem that should be remedied. 

Likewise, if the wood floor used to be level and is now slightly slanted, or has high and low spots, this could be an indicator that there is something wrong with the subfloor, foundation, or floor joist. It could also be a sign that you have a plumbing leak somewhere that is causing the wood subfloor to bode.

How Do You Level a Floor Under Plywood?

Unless you have foundation issues in a property, you can simply lay your plywood subfloor to fix an uneven base. For example, if you’re laying plywood over a concrete floor, you don’t need to level the concrete first, as you can lay the plywood to create a level surface.

Likewise, if you’re installing plywood over a floor joist system, the joists should be level, but it’s not a requirement, as you can make it level yourself during the subfloor installation.

To ensure your plywood floor is level, you can sand down the joists in high spots or replace them with new, level joists. This is an easy way to ensure the base under the plywood subfloor is level, but it has risks. 

Firstly, sanding joists can be difficult, and you may not achieve the desired results. Secondly, replacing joists can be costly and time-consuming. It can also be difficult to remove joists without disrupting the structural integrity of your property, so proceed with caution. 

One of the simplest ways to ensure a floor/base under plywood is level before application is to install shims across the floor joist system, concrete boards, or concrete floors. This will bring high and low spots to an even level, allowing you to lay the plywood floor evenly with screw heads.

How Do You Level an Extremely Uneven Floor?

When your subfloor is extremely uneven (more than L/360), you should investigate why the flooring is so uneven to determine the best course of action. Generally, uneven subfloors are caused by thin plywood (that does not meet uniform building codes), structural problems (like a bowed joist), foundation problems, or water damage caused by leaky plumbing.

Review the table below to determine how to level a plywood subfloor depending on the issue you have.

Cause	How to Repair Uneven Floor
Thin Plywood	Replace the entire subfloor with new, thick plywood sheets
Structural Problems	Temporarily remove the plywood subfloor to investigate the floor joists, replacing any bowed joists or undersized lumber
Foundation Problems	Signs of a foundation problem usually include uneven floors, high and low areas, wall cracks, and doors out of square. If you suspect you have foundation problems, call an expert to assess the damage and advise on the best course of action.
Water-Damaged Plywood	Fix the leaky plumbing and either seal the plywood to prevent wood rot or replace the water-damaged sheet entirely with new plywood, depending on how severe the damage is

Sometimes, uneven areas of finished flooring can also be caused by plywood wearing down over time. This is particularly the case in an old house or basement (where moisture levels are higher than normal) as plywood can sag over time. Learn more about the best type of plywood for subfloors and other construction projects.

The best way to level uneven subfloors is to replace the entire floor with new plywood. However, when a subfloor is only mildly uneven, you can apply a self-leveling compound atop the floor’s surface. This will strengthen the surface and make it level. This solution is only suitable if the plywood is the correct thickness and has merely worn down in a couple of areas over time. It is not advisable if your wood floor is completely uneven.

Can You Put Leveler On Plywood?

Yes, you can apply a self-leveling compound, like concrete or latex primer on top of a plywood subfloor. This is a quicker and cheaper floor leveling option compared with replacing the plywood floor entirely. It can level your flooring, make it more durable, and repair any cracks or worn areas of the original subfloor. Once laid, you can fit a new floor covering atop the concrete/latex, like carpet, ceramic tiles, laminate, or hardwood flooring.

Self-leveling concrete contains a polymer, which makes it easy to spread and smooth across floors once poured. It also ensures your flooring maintains a uniform, level appearance across the room. Once applied, the leveler generally only needs a few hours to cure before you can walk on it or apply a new floor covering atop the surface.

A floor leveler will stick to wood, but you need to prepare the surface properly first. You should also check your subfloor uses the right material as per your local building code requirements. Otherwise, your flooring will become out of plane again over time as the subfloor bends. To meet codes, a plywood subfloor should be a minimum of ¾ inches thick. You should also ensure the bending doesn’t exceed L/360, as the leveling compound won’t fix this.

Before applying leveler to plywood, you need to ensure the wood is clean of all contaminants, including glue, grease, oil, or finishing. Sand the surface thoroughly with an orbital sander to remove any visible contaminants. Any water-damaged sheets should be replaced, and loose boards tightly secured (so they are not showing) with deck screws.

Summary: Things to Remember

• Always ensure plywood subfloors are level before laying laminate, carpet, tiles, or hardwood flooring to avoid breakages or cracks
• Use the right plywood thickness recommended for subfloors
• Thin plywood sheets should be replaced to make subfloors level
• Use shims to raise floor joists before installing plywood
• A self-leveling underlayment can be applied over plywood to level small indentations and worn areas, but shouldn’t be used for extremely uneven floors