Repairing a damaged subfloor for hardwood installation

[Downaski]

Hi everyone!

What’s the best approach for replacing or repairing a damaged subfloor beneath hardwood floors to ensure it’s properly secured, level, and ready for installation? Any specific steps or recommendations for this process?

Thank you in advance!

[FatimaandAhmed]

Hi Downaski!

If you're dealing with a damaged subfloor beneath hardwood flooring, the key is to repair or replace it in a way that ensures it's solid, level, and stable before putting the new flooring down. Here's a general approach that works well in most situations:

First, remove the hardwood flooring above the damaged area and inspect the subfloor to figure out the extent of the damage. If it's localized, you may be able to cut out just that section and replace it with new plywood or OSB of the same thickness. Make sure the joists underneath are in good shape and not rotted or loose. If needed, reinforce them by sistering new joists alongside the damaged ones.

Once the damaged section is removed, cut your replacement subfloor panel to fit snugly. Use construction adhesive along the joists and secure the panel with screws to minimize movement and squeaks. Make sure the new panel is flush with the surrounding subfloor to avoid any height difference that could affect your hardwood installation.

After the patch is in, go over the whole area to check for dips or high spots. You can sand down high areas or use a leveling compound to fill in low ones. A flat, even surface is essential to avoid problems like gaps or squeaks in your finished floor.

Hope this helps!

[Downaski]

Thanks for the helpful tips! Once the subfloor is repaired and level, are there any specific recommendations for prepping the surface before installing hardwood floors, like cleaning or using a particular underlayment?

[FatimaandAhmed]

Once the subfloor is repaired and level, the next step is making sure it’s clean, dry, and ready for the new hardwood. It’s a good idea to vacuum thoroughly to remove all dust and debris, since even small bits left behind can cause problems with how the flooring lays. Also, check for moisture, especially if you're working in a humid area or installing over concrete. A moisture meter is helpful here, and for wood subfloors, you're typically aiming for a moisture level under 12 percent.

It’s also worth walking the area to listen for any squeaks. If you find any, adding screws into the joists usually takes care of them and helps tighten things up. When it comes to underlayment, the choice depends on how you're installing the hardwood. For nail-down floors, felt paper or rosin paper is commonly used to help with minor moisture protection and reduce squeaks. If you're doing a floating or glue-down installation, you'll want to use the specific underlayment or vapor barrier that suits that method.

Before starting the installation, it's important to check the surface again with a long level or straightedge to make sure it's flat. Most manufacturers recommend no more than one-eighth of an inch variation over six feet. Low spots can be filled with a leveling compound, and high spots can be sanded down. Getting the prep right at this stage can really help avoid future issues like gaps, noise, or uneven boards.

[Downaski]

Thanks again for the detailed response, it’s really helpful! Just curious, when it comes to underlayment for a floating hardwood floor, is there a specific type that works best? There seem to be a few options, but not sure which one would be the most effective. Also, is it necessary to leave space around the edges for expansion, or is that only something to worry about with certain types of installs? Would appreciate any thoughts on this!

[FatimaandAhmed]

For a floating hardwood floor, yes, underlayment does make a difference, and it’s good you're looking into the options. The best choice really depends on your specific needs. If you're installing over a concrete subfloor, you’ll definitely want an underlayment with a built-in moisture barrier. Something like a 2-in-1 or 3-in-1 underlayment is great since it provides cushioning, sound reduction, and vapor protection all in one layer. If moisture isn’t a big concern, like when you're installing over plywood in a dry area, you can go with a basic foam underlayment that offers sound absorption and a bit of insulation. Some of the premium ones even have self-sealing edges or adhesive strips, which can make the installation quicker and tidier.

And yes, definitely leave space around the perimeter for expansion. That’s one of those things that often gets overlooked but can make a big difference over time. Wood expands and contracts with changes in humidity, so leaving about a 1/2-inch gap around the edges, including walls, doorways, and fixed objects, gives the floor room to breathe. You can easily hide those gaps later with baseboards or quarter-round molding. It’s one of those small details that helps your floor stay looking and performing great for years.

[Downaski]

Awesome, thanks again for the clear breakdown. One thing I’m still a bit unsure about, how important is it to acclimate the hardwood flooring before installation? And is there a standard amount of time it should sit in the room, or does that vary based on the type of wood or conditions?

[FatimaandAhmed]

Absolutely, acclimating hardwood flooring is really important and definitely something you don’t want to skip. Since wood is a natural material, it reacts to changes in temperature and humidity by expanding or contracting. If you install it right away without giving it time to adjust to the room conditions, you might run into problems later like gaps between boards or even buckling. As a general rule, solid hardwood usually needs about 5 to 7 days to acclimate, while engineered hardwood may only need 2 to 3 days, but it’s always best to follow the manufacturer’s recommendation since some products are more sensitive than others. Just make sure to store the flooring in the room where it’ll be installed, with the boxes laid flat and some air circulation around them. The space should be at a normal living temperature and humidity, around 60 to 80 degrees Fahrenheit and 30 to 50% humidity, to match how the room will be used long term. If you want to be extra safe, using a moisture meter to check that the subfloor and the flooring are within a few percentage points of each other is a good step before starting the install. It’s a bit of extra time upfront, but it really helps the flooring settle in and perform well over the years.

[Downaski]

Thanks so much for all the helpful info!