What causes damp under floorboards?

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what causes damp under floorboards

Damp under floorboards is quite a complex issue. However, this type of damp is treatable. Many methods of treatment for this type of damp can also be done by anyone with some DIY knowledge. The key is to identify what is causing the damp under your floorboards. From there, you choose the right treatment, and your damp issues can usually be fixed.

In this article we will look at the common and most likely causes of damp. We will discuss diagnosing the damp. After this, we will look at some damp treatments. And finally, we will look at repairing damaged floorboards.

The common causes of damp under floorboards

There are many causes of damp under floorboards, the most common, are the following:

  • Rising damp This would only be on the ground floor.
  • Penetrating damp – There may be evidence of this type of damp elsewhere. Peeling paint and wallpaper, for example.
  • Leaking pipes – This includes radiator pipes and drains.
  • Poor ventilation – The space under your floorboards must have good airflow to prevent damp.

Rising damp is a big cause of damp under floorboards. This is most likely due to not having a damp proof course, or the DPC being damaged. Penetrating damp can also be present under flooring. However, there will likely be other signs of damp and condensation in the property too. If flooring on upper levels is damp, a common cause is a leaking pipe. A pipe can have a minor leak without you noticing any loss of water pressure.

Poor ventilation is an interesting cause of damp under floorboards. This usually doesn’t affect the floorboards themselves, but rather the space underneath them. There may be some signs of damp on the floorboards or the joists. However, this depends on how bad the damp and ventilation issues are.

So, now you know about the common causes of damp under floorboards, let’s discuss diagnosing the cause. Without diagnosing the damp that is causing your issues, it is almost impossible to treat it without trying every method in the book.

Diagnosing the damp affecting your floorboards

Some signs can help you identify different types of damp. Here are some common signs of different types of damp:

Signs of rising damp

The space below your flooring, as well as the joists and floorboards could all be wet. Also, the wall just above your floorboards might be wet too. This does depend on how advanced the damp is though. These signs may also get worse during rainfall. However, this does also depend on how bad the rising damp is.

Chances are, if you have rising damp, the floorboards and the joists will be swollen and could be twisted too. This is due to the mortar and bricks in the building soaking up water. This moisture will then get into the woodwork.

Signs of penetrating damp

You may notice signs of this damp due to defects on the outside of your house. Look for any areas of the mortar that are cracked. A badly damaged brick may also be the cause. You will often find that penetrating damp will affect patches of your home. Looking for areas of damp on the walls is a good way of identifying this damp. Also, penetrating damp does get worse with rainfall too.

Leaking pipes

When the area of damp isn’t by a wall, there is a chance it could be caused by a leaking pipes. If the floor affected is upstairs, this also rules out rising damp and points more towards a leaking pipe or penetrating damp. So if the problem is on an upstairs floors, check for any damp marks on the ceiling below. Even the smallest damp patch could mean a leaking pipe.

Poor ventilation

The space under your floorboards should have some air bricks to provide ventilation. Air bricks are simply bricks with holes in that allow air to flow through the area. If these bricks are blocked (which you can check from the outside of the building), this can be the cause of your damp issue.

Fixing damp under floorboards

If you have identified the damp causing your issues, it is time for treatment. You can use the signs of damp above to get an idea of the damp problem you have. In many cases treating damp isn’t too difficult. In fact, many treatments can be done by anyone with basic DIY knowledge. Some treatments of the different types of damp can be seen below:

  • Rising damp – a new chemical DPC will sort the issue out.
  • Penetrating damp – find cracks in the mortar and brickwork and repair them. You can also use damp proof paint to seal the outside of your home.
  • Leaking pipe – identify where the leak is coming from and repair it. This may require a plumber unless you know what you’re doing.
  • Poor ventilation – clearing out any blockages in the air bricks. Once done, ensure the area is drying out. If it isn’t drying out, poor ventilation may not be the only cause of your damp issues.

Sorting out the cause of your damp issues is a must. If you simply repair the flooring, the damp will continue and cause further damage. We’re about to talk about repairing damaged floorboards. However, before you do any work, please ensure your damp issues are solved. Otherwise, you’ll likely have to repair the floor all over again.

Repairing damaged flooring

Once you’ve fixed your damp issues, it’s time to look at your floorboards. Your home may have flooring that is made from MDF, chipboard or plywood. In this case, there is a small chance that the flooring might be moisture resistant already, but in many cases it will not. If water resistant boards were not used and the floor has been damp for some time, you may notice some warping or swelling in these materials.

If your flooring is solid timber, you are likely to run into issues. Solid timber sucks up moisture just like it did when it was a tree. When the boards are left to dry out, this added moisture can cause twisting and warping. Any minor twists can be adjusted by screwing the flooring down to the joists to pull them straight. If the warping is significant, you will need to replace the boards. It is worth looking for mould on your flooring too. If any boards or pieces of chipboard have mould present, it is best to replace them. Mould is dangerous and causes a lot of health problems. It can also spread when left untreated.

Repairing damaged floor joists

When checking the floorboards, you will also need to check the floor joists. If the joists show signs of damage, your flooring could be structurally unsound. In the worst case scenario, this could result in the floor falling through, due to the joists failing.

Check the joists to see if there’s any rot. If there is, they will need to be trimmed or replaced. It is far easier to trim joists than replace them.

To trim the joists you will need to cut them back to the healthy wood. You can test this by prodding the joist with a screwdriver. Anywhere the joist is rotting, the screwdriver with easily penetrate. Cut the affected piece off. Then cut a new piece to make up the gap between the old joist and the wall. Then, sandwich this new joist between two treated timbers. These should be cut oversized, so they can be secured to both pieces of the new joist. Secure these with carriage bolts, and it is ready for new floorboards.


So, there we have it, these are some of the most common causes damp under floorboards. As well as some ways to solve the issue. We hope this helped you identify the damp causing your issues. Most damp problems can be solved by anyone with basic DIY knowledge. Now you know the cause of your damp, solving it should be simple.