102H: dry-lined neighbour drain suspected leak

This is a a Edwardian house on a hill. It was dry lined many years ago, presumably because of dampness. The neighbours gulley and underground drain appears to be leaking.

Surveyor Tips:

  1. Be prepared to recommend further action, such as drain CCTV.
  2. Make sure buyer is aware of the potential risk of rot to sub-floor joists.
  3. Here there was an overlaid floor, and no under-stair cupboard access.
  4. It would have caused almost as much damage to investigate, as replacing the rotten timber.


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Damp was detected with a damp meter deep within the party wall on the left-hand side of the reception.

I tested walls with 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 limit. These meters are for scanning, mapping and profiling, see surveyor.tips/profile. 

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There was a stain on the back wall, inside the chimney breast and high damp meter reading.

Chimneys tend to absorb more moisture than walls because more bricks are in contact with moist air or moist soil.

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The damp meter reading to the front wall was low i.e. dry.

The front wall has been dry lined, where plasterboard is separated from the wall with batons. This affectively hides the damp and suggests that there has been a history of dampness to the front wall. I spoke to the vendor about it, but she seemed unaware of what changes her husband had made.

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The relative humidity was marginally high 1M in from the front airbrick, near to the rot.

Normally when there is active rot the relative humidity exceeds 95%RH. However, any reading above 85%RH is considered high, and a sign of water entering the subfloor void and insufficient sub-floor ventilation.

Dry rot, produces a lot of moisture and therefore is associated with high relative humidity over 95% – see later in this report. The risk of Dry rot is remote.

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There are signs of “Blue stains in service” on the timber overlay, a form of white rot.

There is a risk of cellar rot affecting joists in the subfloor void. I have considered the risks. In my opinion, significant decay of structural timber is unlikely. The cost and disruption caused to checking for timber decay, would be high (a few hundred pounds) compared to the cost of replacing damaged timber (maybe five hundred pounds), so not worth doing. So long as the cause of dampness is confirmed and stopped, the risk of rot will disappear.

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Looking to the side we can see an Aryan of alcohol growth, coming from the neighbour’s rainwater gully.

I spoke to the tenant of the left-hand property, who told me that they had no dampness in the front room. However their property is approximately 1 m above your property, and therefore any dampness and bricks will be more evident on your side of the party wall.

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I tested the drain in the left-hand side neighbour’s gulley with 2 L of water. It flowed effectively.

I suspect but don’t know that there is damage to the neighbour’s drain running under their property. The crack may or may not be linked to this drain.

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I understand your RICS homebuyer surveyor was concerned that there may be rising damp.

Rising damp is caused by bricks being in contact with the groundwater. The property is built on a hill, at the back of the building there is a roughly 2M drop. Groundwater is highly unlikely to be present on a hill and even it if were, it could not possibly rise by 2M.

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I tested for a mains water leak by closing off the mains at the street and testing with a pressure gauge.

Mains water pressure held for 30 minutes. Therefore I conclude there is no significant mains water leak.

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There is a depression in the front drive, caused by the vendor’s husband parking in the same place every day.

There are signs of ponding, this could result in water flowing back in under the house. But I doubt that this is the root cause of internal dampness.

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The dampness continues to the rear of the property on the left-hand party wall.

A high damp meter reading was found in the rear reception chimney breast.

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I also tested all the walls with a Protimeter in conductance mode.

I tested the surface on the inside at the base of 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 d Readings below 20WME are considered dry. The range is 8WME to 99WME. See surveyor.tips/dampmeter. Walls measured were largely dry on the surface except where mentioned in this report.

There are some cracks around the mastic surrounding the window frames.

Seal up around the window frames to reduce the chance of penetrating damp.

Other matters

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The flat roof to the rear is made of bitumen felt.

The roof looks “tired” although there are no signs of internal dampness. I would expect you to have to replace the roof within about 10 years. I would recommend replacing it with EPDM, a seamless rubber covering. Make sure that the EPDM is not in direct contact with the bitumen felt. The cost should be around £1,000 for replacement with EPDM.

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The chimney pots are uncovered.

Consider installing cowls over chimney pots, to reduce the risk of rainwater penetrating through to the rooms below. The cost would be around £200, but could increase demanding on access requirements.

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