Damp mystery - all avenues investigated, no solution to be had!

[Pgate]

Hi all,

Our Victorian terraced property in Greenwich has a persistent damp problem all along our internal party wall which is giving us the runaround!

The pattern of water is a large bit of damp by the front door, then another bit in the middle, then another bit at the back of the house under our staircase - all on internal walls. It then crosses the threshold of the kitchen (which was added some time in 20th century). It seems to be worse after rain, but there is a permanent puddle under the floor.

We have already eliminated lots of possibilities (with various professionals coming to investigate!):
- Not a leak from the neighbours' system - no depressurisation, no rushing noises
- Not a leak from our system - not depressurisation, no rushing noises
- Not groundwater - a localised area, and foundations are too shallow for this to be realistic (local environmental guidance says there is a very slim possibility of groundwater but this would only affect houses with basements)
- Not a mains leak - Thames Water have come to test this
- Not sewage - no ammonia in the water

Some pictures and map of affected ares are attached. The first image shows the damp area (became a puddle when we dug down a bit) which we discovered when we replaced the old suspended floors.

I have run out of ideas - my last remaining hypothesis is that there is an old rain overflow drain of some kind (??) underneath our house which is cracked and leaking. This explains the volume of water (puddles rather than damp patches under the suspended floor - see last photo), increased damp feeling after rain and the profile of the water cleanliness? Might this also explain the pattern of the water - running all along the wall in patches and then turning the corner? I am poised to get a CCTV drainage survey but keen to understand if this is a reasonable theory first!

Any ideas, similar experiences or anything else would be gratefully appreciated!! We are going round in circles and at the end of our tethers Thank you!!

 

[MatthewC]

One question: how high is the road/pavement outside the house, relative to the floors and the levels you have dug to inside?

I once had an issue with an odd leak in a bathroom - there was dampness at the non-tap end. I eventually discovered that someone had moved the shower from that end of the bath to the tap end; they had left the old supply pipe in situ hidden in the wall, and that was the item which leaked, allowing water to track down inside the wall. So look for redundant pipes - Sorry I can't help more.

 

[Stevers]

If that's gypsum plaster, then it's very much the sort of thing it can do without benefit of a DPC. Regardless of what the survey says there's clearly a raised water table local to your house, and yes this could be a lack of rainwater drainage. If you emptied the puddle, you could then run some dyed water into the gullys to see if it turns up under the floor.

How are the houses doing one, two or three doors along - do they have the same sort of problems?