Damp Floorboards? Here’s Why

We recently covered what the most common causes of water under your floorboards are. If, rather, you have damp floorboards, there’s another host of issues to address. The worst part is that you do need to move quickly here. Ignoring moisture in your home where it shouldn’t be is a surefire way to have expensive water damage repairs later down the line. Keep reading to see what the most common causes of damp floorboards are, and what to do about it.

The most common causes of damp floorboards are leaking pipes, poor ventilation, penetrating moisture, and rising moisture.

Let’s break that down a bit more, shall we?

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Damp Floorboards: Causes

Let’s start with the most obvious causes, yeah? It only makes sense to start easy and work our way down the list. There’s no sense in wasting time doing complicated troubleshooting when you could have found the issue hours beforehand.

Leaking Plumbing

This is the most likely in terms of severe moisture under or on your floorboards. As we mentioned in the above article, your best bet to determining this as the source of the damp is through your water meter. Turn off all sources of water in your home and mark your water meter’s reading. Once that’s done, take a break for at least three hours.

Return to your water meter and compare its reading to the one you noted earlier. If it’s changed, you have leaking plumbing. If not, the source of your issue is likely elsewhere, and it’s time to move down the list.

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Rising Moisture

This is most likely following heavy rain or in highly humid environments. You may notice the wall just above your floor is starting to show signs of damp. Perhaps the flooring and joists are swelling, but either way – it’s a surefire sign of moisture rising in your home.

This only applies to ground level floors. If your upper floors are showing signs of swelling, it’s more likely that you have a burst pipe somewhere along the line.

This is generally caused by the mortar and brick in the foundation of the building soaking up water and swelling. This, in turn, allows moisture to make its way into your flooring, which will lead to warping and swelling.

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Penetrating Moisture

This is more common on the outside of your home. Look for bits of mortar and brick that are damaged or cracked. You’ll often notice that penetrating damp will appear in patches, rather than in a large swathe.

Look for water stains on your walls, both inside and out, if you suspect this is the cause. It’s especially likely if you’ve experienced heavy rain recently.

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Generally, the space beneath your floorboards should have air bricks beneath them. These are (exactly as the name might imply) bricks that allow air through them. They are essentially masonry vents that are used for foundations and chimneys. If they’re blocked (which you may be able to inspect from outside), this is a likely cause.

Your floorboards need proper airflow – without it, they can harbour all sorts of gross bacteria, mould, and mildew. If your floor’s ventilation is blocked or simply doesn’t exist, this is your most likely source of damp.

Fixing the Issue

Once you’ve found the source of the moisture, you can move on to fixing the issue. Most of the time, this can be done with relatively easy and minimal work, though it depends on the source. Plumbing leaks, for example, will be much harder to fix than ventilation issues. To fix each of these:

  1. Chemical DPC (damp proofing solution) will generally fix rising damp. Simply dry out the problem area and install a new layer of DPC.
  2. Patching bricks and mortar will solve most issues with penetrating damp. You can also use moisture-proof paint as a sort of seal on your masonry when it’s fully patched.
  3. Clean your flooring ventilation will solve issues with ventilation in most cases. If it’s in really bad shape, you may need to fully replace or reinstall ventilation.
  4. Patching leaking pipes is your best bet for leaking plumbing. This is best done by a professional.

Whatever you do, do not just replace the flooring and act like the issue is solved – it’s not.

Replacing Flooring

Most likely, your home’s flooring is backed (or entirely made from) MDF, plywood, or chipboard. There’s a small chance that it’s already moisture-proofed (resistant) but generally, it’s not. If the flooring is solid timber, however, there are some issues. You can’t really dry it out, and waterlogged timber is not great for structural integrity.

You can try to screw the flooring down onto your joists (assuming they’re not damaged, too) to straighten bent or warped timber. Beyond that, if you notice termites (they love moist wood), severe waterlogging, or rot, you’ll need to replace the whole section. This is another situation in which you should leave this job to the professionals.


Once you’ve inspected your flooring, it’s time to move to the joists. If they show signs of any water damage – they are likely dangerous. In other words, if you spot rot, you need to replace or trim (the easier option) them.

To trim a joist, you need to cut them back to healthy wood after testing them with a screwdriver or knife. Push it into the wood – if it goes easily, it’s water damaged. If it puts up resistance, you’re good. Next, you’ll need to cut a new piece of timber to fill the space of the missing rotted wood and attach it to the joist. Secure it on both sides with carriage bolts, and you should be good to go.

If any of this is confusing or seems too complicated, call a professional. A joist and flooring is not the place to try and learn carpentry.

Final Thoughts

If you noticed that you have damp floorboards, you’re likely panicking. It could be a plumbing or sewage leak, rising groundwater – your mind races with potential “what-ifs.” Ultimately, it’s a serious issue that often has easy(ish) fixes. Whether it means replacing your chemical waterproofing treatment, cleaning and painting your airbrick ventilation, or repairing masonry, it can generally be done at home.

If, however, you’re unsure of any of the above terms or steps, I highly recommend you contact a professional. They will be able to pinpoint the problem area quickly and effectively and solve the issue faster than you could.