Wood Subfloor Moisture Issues: How to Fix Them and How to Prevent Them

When you look at a beautiful floor, you might not realize that in order for that floor to be installed, several layers need to come before it, including the subfloor.

Wood subfloors come after the floor joists and before the underlayment and the final flooring. And even though you don’t see the subfloor once the flooring installation is complete, that doesn’t mean that it’s not important or that it can’t impact the final layer of floorboards.

That’s why this page is all about subfloor issues, and specifically, we’ll be talking about wood subfloor material rather than concrete subfloors.

You’ll learn about:

Let’s jump right in by talking about different wood subfloors and issues.

Wood subfloor issues

A hand measuring a board of wood with a pinless moisture meterIt’s not often that a wood subfloor will show visible signs of moisture damage. Of course, if the subfloor is visibly wet, moldy, or warped, then it’s simply not suited for installation at that time.

The most reliable “sign” of potential moisture damage in a wood subfloor is the moisture content measurement that you get when you use your moisture meter. The only way to really know that there is a moisture issue is to know what the moisture content (MC) of the subfloor is.

That being said, different types of wood subfloor may exhibit moisture damage in different ways so let’s learn a bit about two popular wood subfloor types as well as look at the signs of potential moisture damage in them:

  • Plywood
  • Oriented strand board (OSB)

Plywood subfloor and visible signs of moisture damage

The first kind of wood subfloor that we are going to talk about is plywood subfloor.

This subfloor appears like solid wood but is actually layers of wood arranged in opposite directions. It’s a durable subfloor, and it’s easy to install as it often comes in tongue-and-groove fasteners that interlock with each other.1

It’s also relatively water resistant, but that doesn’t mean that it can’t fall prey to possible moisture issues.

If plywood has moisture damage, then it will likely appear wavy or warped prior to installation.

Oriented strand board subfloor and visible signs of moisture damage

An oriented strand board subfloorOriented strand board (OSB) is a bit like particle board in that it is made up of many small wood pieces, pressed together to form a solid sheet that can be used in a variety of ways.

Unlike particle board, however, OSB is made up of larger flat flakes of wood and arranged in a specific pattern rather than a randomized pattern.

If an OSB subfloor has a moisture problem, then you may begin to see delamination in the board.

But maybe you have already installed the floor over the subfloor and are wondering how you can determine whether or not you have a damaged subfloor on your hands.

If that’s the case, the next section will cover the different ways these wood subfloors with water damage can impact your wood floor.

How damaged wood subfloors impact wood floors

While the visible signs of moisture damage on the wood subfloor itself might not be many, if your subfloor has moisture damage, you will almost certainly be able to see the effect of that damage on your hardwood flooring.

Hardwood floor is like a sponge. If there is moisture around it, it will likely absorb that moisture into itself. This means that if your wood subfloor has excess moisture, it’s going to migrate up through the subfloor and into your beautiful hardwood floors.

And depending on the severity of the subfloor’s moisture damage, you could be looking at a moisture issue that impacts the entire floor.

One of the most obvious signs is that your hardwood floor will begin to cup: It will exhibit a concave shape, like a soup bowl.

If the hardwood floor is sanded down in an effort to fix the cupping, but the subfloor is still wet, then the hardwood floor will begin to crown, or exhibit a convex shape once the boards start drying again.

A more subtle sign of moisture damage in your subfloor is excessive squeaking in your floor . When subfloors have a lot of moisture, they will begin to warp, causing the nails that hold them to the joists to loosen, which results in squeaky steps.2

A wet subfloor could also cause mildew growth, which might cause a musty smell in a given room.

While it’s obvious that moisture is the cause of these issues, it’s still important to know the cause of the excess moisture. If you don’t know where the moisture is coming from, then you can’t fix the problem.

So, to better understand how to solve the problem, let’s first look at some possible causes of excess moisture in wood subfloors.

Possible causes of subfloor moisture issues

Warped wood boardsExcess moisture in a wood subfloor can be caused by a variety of reasons, but let’s talk about a few common ones.

First of all, it’s likely the subfloor wasn’t dried properly to begin with. When it arrived at the installation site, it was already too wet, but because it wasn’t measured with a moisture meter, the subfloor was installed with excess moisture.

Another cause might be that the subfloor was stored in a warehouse without a controlled environment. Remember, wood is like a sponge, so if the wood was exposed to the elements, it will probably have absorbed some moisture.

Or let’s say you are not working on new construction but on a flip or restoration. If there was flooding or any major leaks, then those could also be causes of excess moisture in the subfloor.

Now that we know some causes of excess moisture in a wood subfloor, let’s look at some ways to fix the problem.

How to fix subfloor moisture issues

The methods for fixing subfloor moisture issues are all pretty intensive. This is because, oftentimes, the floor has to be ripped up to remedy the problem. While the methods for ripping up floors vary slightly depending on the type of floor, you can generally:

  • Determine what parts need to be removed
  • Saw the boards into manageable sizes using a circular saw
  • Pry them up with a pry bar3

The first way that you can fix a subfloor moisture problem is by removing the finished floor and exposing the subfloor. Make sure that an HVAC system is installed or use a dehumidifier, and then, let the subfloor dry until the moisture levels have gone down.

The only way to truly make sure that the moisture content is down to the proper level is to use a reliable moisture meter.

Another way to fix a subfloor moisture problem is by adding an underlayment that acts as a vapor barrier to keep moisture from entering the hardwood floor.

If the moisture is coming from the ground beneath the house, this vapor barrier can be applied by accessing the crawl space and putting down some kind of plastic sheet or other mitigating material. If the subfloor never absorbs the moisture, then the hardwood floor won’t either.

Installing a vapor barrier in your crawl space is something that most homeowners can “DIY.” Put simply, installing a vapor barrier entails:

  • Measuring the space
  • Purchasing the vapor barrier
  • Laying it down in your crawl space
  • Securing the vapor barrier with landscape fabric stakes4

The last fix is to simply replace the old subfloor with a new subfloor. If the subfloor has significant moisture damage that has compromised its integrity, like excessive flaking, warping, or mold, then a new subfloor is the answer.

Reinstalling a new subfloor is best accomplished by a professional. Of course, when the new subfloor is being installed, it’s of the utmost importance to measure the moisture content of the subfloor.

Because this time around, it’s always best to prevent the problem than to fix it, so let’s get to it: subfloor issue prevention.

How to prevent subfloor moisture issuesThe Bessemeter DS500, S300, and D300 wood moisture meters

One way to prevent future subfloor moisture issues is to make sure that vapor barriers are installed before any problems arise. Better safe than sorry.

And the best way to save yourself time, money, and a lot of headaches is to make sure that your subfloor is at the proper MC before you install it.

The only way to truly make sure your subfloor is at the proper MC is by measuring the moisture with a moisture meter like the Bessemeter pinless moisture meter. Once the moisture content of the subfloor is the same as the equilibrium moisture content of the installation location, then, you can go ahead and install the wood subfloor as usual.

Moisture is wood’s biggest enemy, and that goes for wooden subfloors as well.

Protect your subfloors and your hardwood floors by being intentional before installation even begins. Measure the moisture content of your subfloors and set yourself up for success.