102I: top floor flat penetrating damp

This is a three bedroom, top two floors of a Victorian terraced house in central London. It was vacant having been tenanted. The buyer needed to understand the root causes, cost of remedial work, whether dampness was ongoing and the risk of rot.

Root causes

The root cause was penetrating damp from a gutter, that had been blocked and partially repaired by the downstair flat owner. The repair was too recent to know if it was effective for the top floor flat.

Surveyor Tips:

  1. Repairs suitable for one flat may not solve the problems in another flat.
  2. It takes time for walls to dry, so you won’t know if the repair is successful without logging changes in dampness.
  3. Past repairs may give a clue as to a defect.
  4. Look at and film rainwater goods during a rainstorm.
  5. Rot needs water to grow, but can be self perpetuating in an humid environment.
  6. Penetrating damp leaves a brown, tea bag stain like discolouration.
  7. Condensation from poor ventilation exacerbates any other dampness.
  8. Make sure to measure damp uo into the corners of top floor ooms and if damp check deep into the eaves for rot.


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There are penetrating damp stains on left-hand flank wall of the back addition, here on the top floor rear room.

A teabag stain like discolouration tell us that water (rain) is penetrating from the outside.

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The stain continues onto the left-hand flank wall of first-floor room below.

Note that all the damp staining it’s on the left-hand wall and not on the rear wall.

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Using a damp meter, I determine that dampness extends to about 1M from the corner of the first-floor room.

I used a Protimeter in radio frequency mode. Water reflects radio waves at a set frequency similar to mobile phone shields. Meters can’t differentiate moisture from other dense matter such as metal and concrete. They help trace damp in a normal, homogeneous wall.

Readings below 300 REL indicate that a wall is dry below the surface, 999 REL is the upper limit. These meters are for scanning, mapping and profiling, see surveyor.tips/profile. Although the meter reading is high, I think it would have been higher before the gutter was cleared. I belief the case of damp here has been fixed and so the al is drying out.

The meter reading was low at the base of the wall close to the area where there was concerned about rot.

I determined that the risk of water being in contact with the timber on the first floor was low. Timber rot needs water to grow, if there is no water then there is no risk of rot.

I inserted a hygrometer probe into the void between the first-floor timber floor and ground floor ceiling below.

Rot releases water vapour as a by-product of respiration (see later in this report). The relative humidity was measured at 82%RH. In my experience if there is rot, then the meter reading on the hygrometer probe exceeds 95%RH.

I removed a section of carpet and I looked carefully at the timber in the corner of the room.

There were no signs of rot in the corner of the room.

I tested the first-floor rear bedroom skirting boards for rot.

There was no give in the skirting board, which there would be if a skirting board was rotten.

I tested through the carpet with a damp meter in conductance mode, the meter reading was low.

I tested the surface on the inside all external walls every metre, chimney breasts and a sample of internal walls with a Protimeter damp meter in conductance mode. These meters measure electrical conductance of salts in water, a proxy for damp. Readings below 20WME are considered dry. See surveyor.tips/dampmeter

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One of the floorboards was loose on the top floor. I therefore was a raised it without damage.

The relative humidity in the void between the top floor floorboards and first floor ceiling was normal, which it would not be if rot was growing.

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Looking between the wall and the second-floor joist, we see no signs of rot.

Note the above joist does not run alongside the left-hand wall. Therefore it cannot be getting wet, and therefore timber cannot be rotten from the left-hand wall. The rear wall is dry, so we conclude that there is no realistic risk of ongoing rot to the second-floor joists.

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There is external algal growth in line with the downpipe and starting about a metre below the gutter.

The centre of the penetrating damp is in line with the downpipe. I understand from speaking to Hannah the owner of the ground floor flat, that the gutter was cleared and possibly the downpipe replaced, and that this was considered sufficient to resolve the problems in her rear bedroom. She paid for the work herself.

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Looking at the rear, we can see some damp bricks in line with the corner of the building.

While these bricks look damp, they a symptom and not a cause. They do not line up with internal dampness, and are therefore no additional cause for concern so long as the gutter continues to functioning correctly.

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Looking up at the gutter we can see that it is clear. We also we see a large amount of moss growing on the roof.

It is my belief that the root cause of the dampness is moss blocking the gutters. The process of removing the moss could damage the roof, with limited long-term benefit, as moss will grow back. So, in my opinion you are better off working with your co-freeholder to regularly monitor the gutter for blockages and leaks, and clearing gutters every six months.

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There are two other areas of penetrating damp in the second-floor middle bedroom.

The areas of damp are either side of the window. They don’t appear to be physically connected.

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The two areas of dampness in the second-floor middle bedroom, each line up with gutter joint.

If and when the gutter is blocked with moss, water will back up and inevitably find its way over, or through a zone of weakness.

It appears that somebody has tried filling the gaps in the joints, suggesting an owner was aware of the problem.

I tested the gutters by pouring 2 L of water onto them. The gutters didn’t leak. However, there is always going to be a risk of moss blocking again, whether or not these gutters are replaced. And moss will return even if it is removed. It is better to check for leaks during a rainstorm, and if there is no leak, leaving the gutter as it is, but monitor and clear regularly.

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There are white, efflorescent marks under the gutter join.

Efflorescence results from regular wetting and drying. This tells me that water has regularly leaked from the gutter, demonstrating that was the source, of the penetrating damp.

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There are effervescent marks on the corner of the gutter.

The valley appears to be intact, with no obvious additional cause for concern.

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Looking under the corner gutter we can see droplets of water collecting.

The water under the gutter drips onto the timber facias. The facias do not appear to be unduly damaged by rot.

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Looking further up the valley, there are no obvious reasons for internal dampness.

This is not a roofing report, if you’re concerned about the roof then you should seek the roofing survey.

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Looking in the loft underneath the valley, there are signs of mould but no sign of rot.

Mould grows where relative humidity exceeds 85%RH for 6+ hours. Excessive humidity results from insufficient ventilation, poor air circulation and a cold surface. See surveyor.tips/mould. I suspect ventilation is part of the problem, see chimneys below. Of note: 1) there were no signs of a water pipe leak. 2) Insulation could be improved to reduce heat loss.

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The timber beam was measured with a damp meter.

The timber beam is dry which it would not be in if water what is penetrating from the valley above.

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The worst of the rafters were tested with a damp meter.

These damp metres are calibrated for timber. There is a risk of timber rot above 28 WME. These timber did not feel rotten.

The chimney pots don’t have cowls on top and the sarking material is thick, stopping airflow. 

As previously state,d I believe there is excessive humidity in the loft, caused moisture (possibly from the chimney pots) and insufficient ventilation. 

Ideally install cowls to reduce rain dropping down the chimney and evaporating into the loft. Ideally remove a brick from the flue accessible from the loft, so that improve loft ventilation (if fireplaces are not being used). The total cost is minimal, about £150, but access such as from a scaffolding, could be much higher.


In the kitchen immediately below we see disturbance to the ceiling.

The ceiling was measured with a damp meter and found to be damp. There has been an attempt at hiding the dampness by painting with stain block (a normal procedure). It is my opinion that the dampness comes from a combination of the gutter and condensation forming in the loft resulting from the high humidity and poor loft ventilation.

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Dampness was also identified to the Front right of the first-floor reception.

The dampness is deep within the wall, down to window level.

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Looking at the guttering, it appears that water has got through the seal and or is splashing onto the party wall. 

I suspect, but don’t know that the join between sections of gutter has become detached. A guttering or roofing expert should find it easy and quick to repair this section of gutter. Make sure that they test it for leaks, with water before leaving. 

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There is dampness to the front right-hand side of the loft (looking at the front of the house from the roadside).

The timber to the front right-hand side, immediately inside from the gutter join, appears to have the very early stages of rot. It is too early for structural damage. The risk of rot causing structural damage will disappear once the gutter to the front has been repaired.

Other matters

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Cracks sometimes result from the installation of double-glazed windows.

There are no signs that the cracks around the windows are causing internal dampness. It would be wise to rake out and fill these gaps with cement or mastic sealant.

There is some disruption to the timber window frames.

Some timbers have been affected by Phellinus contiguus or window rot. Window rot is very common in old properties. It is a slow growing white rot. None of the timbers are structurally significant. The rot can be cut out, sanded, filled and painted for protection.

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There is some inactive woodworm on exposed timber floorboards on the second floor.

Anobium punctatum or Common furniture beetle, is often found in Victorian properties. It is rarely active and easily remedied with Permethrin, see surveyor.tips/woodworm. The tip is to cover the holes with gloss paint or varnish, as has happened, to see if they ever reappear.

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The bathroom extractor fan does not provide sufficient ventilation.

Ventilation is most effective when air is extracted close to the vapour source; bathroom, kitchen, drying clothes and occupied rooms. The internal ventilation does not meet Building Regulation 2010 Part F requirements. This is best achieved with mechanical extractor fans. 

 See surveyor.tips/vent_regs specifically P39 and P19:

  1. Bathroom 15 l/s with a 30-minute overrun.
  2. Kitchen 30 l/s adjacent to hob; or 60 l/s elsewhere in kitchen.
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Your neighbours’ cost could theoretically be shared under the leasehold terms.

I note a few areas of potential ingress in the flat below. I did not inspect her flat. There is no reason for me to believe that these are causing dampness.

The front gutters are looking clear.

I suspect that the front gutter was also full of moss, and that this was the root cause of dampness to the front right-hand wall above the reception.

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