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  • Writer's pictureSam Costigan

How to Fix Damp - Part 2 - How Damp Can Cause Structural Problems

Updated: Oct 14, 2021

Earlier in the week we posted part 1 of our blog on how we investigated damp as part of a recent building survey. This blog is following on from part 1 to explain some other contributing factors to damp at low level.

In part 1, we explained how dampness at low level can look to be rising damp but that other causes can show the same symptoms.

In part 2, we are going to discuss how consistent low level damp can lead to structural problems.

Our Investigation

As part of the wider building survey, we asked permission to raise some floor boards to inspect the sub floor in more detail.

We inspected the sub floor in the dining room, living room and hallway and inspected the ground floor joists for signs of damp, wood worm, rot and other defects.

Top Tip: Look close to radiators or pipe runs as there is usually already a small floor board that’s already been cut! Makes the inspection easier.

Inspection Findings

During our inspection we identified the following:

  • High moisture readings to all accessible ground floor timbers

  • Visible damp to accessible joist ends

  • Evidence of wood worm to joists in the dining room and underside of floor boards

Damp joist ends in the hallway

Evidence of wood worm (photo doesnt show it clearly) to ground floor joist in dining room

Top Tip: Lifting floor boards to inspect the sub floor gives a good indication of the location of the damp course where ground levels are too high to see it from outside.

From this inspection we could establish that the joists were built into the masonry wall and sat on top of the damp proof course.

Now that we have established that the joists are sat on the damp proof course, we are in a better position to confirm our findings from part 1 whereby, we have identified that the damp proof course has been bridged.

Older properties were constructed with the timber joists built into the masonry wall. Usually, this is not a problem as the damp proof course isolates the timber from moisture in the brickwork below however, where the damp proof course is bridged, moisture is bypassing it and making contact with the timber. We established that some ground floor joists had moisture readings of 18-19% and some joists were as high as 34%.

Moisture readings in timber of 18-20% put the timber at risk of decay and wood boring insects. Moisture readings over 20% make these defects inevitable, if the moisture levels remain this high for a prolonged period.

Other Contributions to Sub Floor Moisture

If you live in an older property with timber floors, go take a look outside, you’ve probably never paid attention to this but you should have small air bricks at the base of the walls.

Air Brick

These bricks are designed to allow airflow to cross under the ground floor to remove any stale, moisture laden air.

Commonly, especially where ground levels have been raised, these air bricks can get partially, or completely blocked. Furthermore, a common issue we find is that new paths are poorly installed and essentially draining rainwater towards the building and dumping it below the ground floor.

Blocked or partially blocked air bricks cause a rise in humidity below ground, eventually this moisture laden air condenses on cold surfaces when the surface reaches the dew point temperature (stay tuned for a follow up blog on condensation and dew point).

Ultimately, the reduced air flow can cause havoc with a ground floor.

At this property, a large timber board and rubble had been placed over the only air brick to the rear wall, this not only bridging the damp proof course, but also significantly restricting air flow.

Sub floor ventilation restricted. Air bricks blocked.

Both of these issues explained have combined to cause deterioration in the ground floor timbers.


As per Part 1, we recommend that the ground levels are reduced to prevent bridging and that all air bricks are unblocked/uncovered. We have also advised for additional sub floor ventilation.

At this moment in time, we have not specified for any treatment or replacement of the joists as we have recommended for a further inspection to be carried out to all joists before making the most necessary repair recommendations.

Possible Timber Repairs Following Further Inspection

Different circumstances call for different recommendations where timber has suffered from deterioration due to moisture and wood worm.

The first thing to understand is that timber decay and woodworm should not occur under 18% moisture content therefore reducing moisture content should stop any further decay. It won’t repair it, but it will stop it deteriorating further.

Where timber has suffered from damp but the timber remains hard and strong, often, no repairs are actually required apart from slowly drying it out to a suitable moisture content. This may include additional works to isolate the timber from the masonry.

If timber has suffered from decay, it can sometimes be salvaged with treatment however, we are not specialists in chemical treatments and so we will abstain from advising on this. We are strong believers though that chemical treatment is rarely necessary, if at all. Reducing moisture levels will make it impossible for wood worm and decay to continue developing and doing the basic maintenance right, will stop anything from getting worse.

In severe circumstances, joists may need to be replaced and in this case, we suspect some joist replacement may be required which is why we specified a further inspection of all joists.

As usual, we can be contacted at: mail mail:

Tel: 0113 887 1941

Sam Costigan MCIOB

Chartered Builder | Managing Director

My Property Surveyors ltd

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