Damp issue 1930’s semi-detached

[Montana 3827]

My husband and I are renovating a 1930’s house that we recently purchased. After removing the floorboards from the suspended floor we discovered there are damp patches in the ground underneath (please see photos included). There are no visible leaks and it has not rained recently. It appears to be coming up from the rear of the property. We’ve just had the drains checked and it was determined they are working properly. The underfloor is clean, air bricks are clear and well-ventilated. The damp doesn’t seem to go past the DPC as the bricks above it are all dry. My question is, is this something to be concerned about and does anyone have an idea what the cause of this could be? We are planning on insulating between the joists under the finished floor and want to get this issue dealt with (if intervention is necessary) before installing it. Any input is appreciated.

[Super_Paulie]

Is this not just ground moisture? Underneath the sub floor in my 1930s was always a bit damp/patchy around the outskirts, especially after rain. I just added as many air bricks as I could in the new extension and made sure the old ones were clear/replaced as they were just jammed with 100 years of crap.

 

Once I had a howling gale under there I added my insulation and sealed the entire thing up.

[Redbeard]

Even if the drainage is functioning OK is there anything about the 'lie of the land' which is directing water towards those walls? (In one case the damp only seems to come in just inside the wall). Can it be 'encouraged away' by 'softening', re-directing, French drain?

 

You might also consider this as bed-time reading (go to bed very early, though!):

https://assets.publishing.service.gov.uk/media/5f05d211d3bf7f2be6e0217a/suspended-timber-floors-underfloor-insulation-best-practice.pdf

 

If you can tick all the 'boxes' in that guide you are probably a long way towards being OK.

 

[ProDave]

There is unlikely to be any damp proof membrane under the concrete of that sub floor, so it is probably just ground water.  That is why under floor ventilation of that type of floor is so important, so check carefully all the air bricks are free, not blocked, and most importantly, not covered by external ground level that has been raised too high and is blocking them.

[saveasteading]

All the above.

And then it is OK to spread a dpm over the whole area. It will  stop evaporation and dampness into the void.

1 hour ago, ProDave said:

not covered by external ground level that has been raised

Shpuld not  even  be close. Keep 50mm min below the air bricks and dpm.

It is a common issue that the ground level rises over decades, (leaves and muck ) even without paving being added.

[Hughgo12]

21 hours ago, saveasteading said:

All the above.

And then it is OK to spread a dpm over the whole area. It will  stop evaporation and dampness into the void.

Shpuld not  even  be close. Keep 50mm min below the air bricks and dpm.

It is a common issue that the ground level rises over decades, (leaves and muck ) even without paving being added.

Would that not bring the risk that moisture builds up, tracks along the DPM and up the wall?

[saveasteading]

2 hours ago, Hughgo12 said:

moisture builds up, tracks along the DPM and up the wall?

I doubt it. The moisture is already there in the ground. It should permeate downwards and away unless there is another source.

 

It's debatable though. If there is a healthy draught through the void it will clear the air, and chill the floor above it.