How to Fix an Uneven Subfloor

Photo Credit: Sean Gillies on Flickr. Licensed under Flickr Creative Commons

Photo Credit: Sean Gillies on Flickr. Licensed under Flickr Creative Commons.

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.

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

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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.

Measuring moisture content in a concrete slab with the Wagner Rapid RH

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.

Flooring underlayment

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.

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