How to fix uneven subfloor for vinyl plank

Let’s say you arrive at a jobsite and you see something that no one wants to see: a floor that has peaks and valleys.

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4 Ways to Level an Uneven Subfloor
1) Self-Levelling Compound
3) Grinding & Sanding
4) Using Thick Underlay
What do I do next?

This usually means that either the floor itself or the subfloor underneath is experiencing problems.

To rule out the floor covering, test it with an appropriate moisture meter to determine if it’s the source of the problem. (Click here to read our guide to testing moisture in hardwood floors.)

If the moisture in the floor is within acceptable levels, the next suspect on your list is an uneven subfloor. This is something you definitely want to repair to ensure a smooth and even final product.

So, how do you fix an uneven subfloor? It’s not as simple as just ripping it up and starting again. The process will depend on the type of subfloor and the nature of the problem.

Here’s our guide to fixing an uneven subfloor:

Step 1: Assess the situation. Is the subfloor made of plywood? Concrete? Particleboard? Is the unevenness a result of an incorrectly installed subfloor, a moisture problem, or uneven joists? Does the problem run the length of the entire room, or only certain parts of it? Is the foundation of the house itself uneven? It’s important to determine the nature of the situation before you decide how you’re going to fix it. Note: If you’re working in an older house, remember to test for asbestos before ripping anything up.

Testing the moisture level in a concrete slab

Step 2: If you’re performing this service for a client, be sure to manage their expectations. If you are able to pinpoint the problem and determine that you must rip up the old subfloor and install a new one, or repair the joists that hold it up, let your customer know that the scope of the project includes more than just installing a new floor. Even if you manage to flatten most of the subfloor, you may still end up with some bumps in the finished floor if the problem is severe or originates in the foundation of the house. Let your customer know about this possibility as well.

Step 3: Fix it! The process of fixing an uneven subfloor depends on the type of material you’re dealing with and the cause of the problem. Here’s how to fix a few common types of subfloors:

- For plywood and OSB (oriented strand board) over joists: First, make sure that the joists are even and not the cause of the problem. Then, measure the difference between the high points and low points of the floor. Depending on the situation, you can either sand down areas of the subfloor that are too high, or install shims beneath the subfloor so that the entire floor is brought up to the same level.

- For concrete slabs: If there are only a few problem areas in the concrete, you can mechanically flatten them by grinding, bead-blasting, abrading, or using a patching compound. If the entire slab is uneven, you may have to use a self-leveling concrete compound to smooth it out.

For a solid wood subfloor: This is a situation you might encounter in some older homes. First, measure the difference between the high and low points of the floor. If the difference is significant and runs the length of the room, you will probably need to rip up the old subfloor and install a new one to ensure consistency. Fasten plywood shims to the joists and then dry-fit the new subfloor over them, measuring the difference between the high and low points again to make sure the inconsistency in the height of the subfloor is as minimal as possible. For a more detailed, step-by-step guide to flattening a wood subfloor, see here.

Step 4: Use a flooring underlayment to smooth out any remaining inconsistencies. A flooring underlayment can help to smooth out any remaining peaks and valleys in the subfloor and prevent problems down the line. However, remember that an underlayment can’t work miracles, so don’t count on it as your only solution. Click here to see our selection of underlayments.

Installing a click-lock floor over a flooring underlayment

You’ll want to make your decision for an underlayment based on the location of the room and whether it’s above-grade or below-grade. If the room is located above another floor in the house, your best option is a sound-resistant barrier. If the room is below-grade, a moisture-resistant underlayment is the best choice.

Step 5: Install the hardwood floor to NWFA standards and take some final moisture readings. Making sure that your final product is up to par and then ensuring that there are no moisture problems waiting to rear their ugly heads will help to prevent callbacks, and will leave you with happy customers.

For more information about prepping a subfloor for a hardwood floor installation, consult this helpful Wood Floor Business subfloor guide and feel free to call us at (800) 787-1786 with any questions about your project.

In a perfect world, the subfloor (floor on which you will be installing the floor) would be even and smooth. However, the reality is our subfloors are often slanted, bumpy or uneven. It may not look like the case, but often you might pull away your old carpet to discover the floor is uneven and bumpy.

So why is this a problem? You cannot install a hard floor over an uneven subfloor. Doing so could result in creaking noises and movements between the boards.

4 Ways to Level an Uneven Subfloor

Correcting uneven subfloors is a common practice in the floor installation process. Before you can even consider installing timber, laminate or other flooring, you need an even subfloor.

The approach you ultimately take depends on how uneven and unlevel your subfloors are. Mildly uneven subfloors can easily be worked around, whereas severely uneven floors could require significant work prior to installation. There are 4 main ways to level an uneven subfloor:

1) Self-Levelling Compound

One common option is using self-levelling compounds. This compound can potentially be used on a concrete slab or a wood subfloor, and can be effective with any dips in the floor. These self-levelling compounds are similar to concrete and typically come in large bags of powder. The powder should be mixed with water to create a thick paste, and you can spread it over the floor and it will automatically level out in the effective areas.

For subfloors with large dips, we tend to use self-levelling compounds to level the floor.

2) Plywood

A second option is installing a plywood above the original subfloor to create a new smooth surface. If there are large variations in the level of the floor, you can find plywood of different thicknesses and install them accordingly, so that the different areas meet at the same level.

To install the plywood, nail the plywood into the subfloor below for stability. If you are installing above concrete, you must use bolts that can enter the concrete surface.

For wavy subfloors, we tend to use plywood methods to level the floor.

3) Grinding & Sanding

A third option is to grind down any humps on the floor. You can use a floor grinder to level concrete floors and you can use a floor sander to sand the surfaces of wooden subfloors. They utilise discs which rotate to finish off rough surfaces. Grinding and sanding is most suitable if you have a large hump in the middle of a concrete slab.

For subfloors with humps or bumps, we tend to use sanding methods to level the floor.

4) Using Thick Underlay

The final option is to use thick underlayment to patch up interior dips or waves. Underlay is a simple and efficient method when there are low variations in the floor level. At FloorVenue, we have underlay of different thicknesses and compositions that enable us to level mildly uneven subfloors.

For mildly uneven subfloors, we tend to use thick underlay to level the floor.

What do I do next?

If there are any floor levelling issues, it’s prudent to have them addressed before installing any floor.

It’s also wise to have a professional on-site inspection before any flooring installation. If you’ve asked for a quote over the phone or online, most flooring companies can only provide an estimate. It’s very difficult for any installer to provide an accurate quote until they are able to see and assess the actual sub-floor situation.

At FloorVenue, we provide free no-obligation quotations where we will send a member of our team to your house to conduct an inspection. Fill out the contact form at the bottom of the page and hear back from us within 24 hours!