Newly found damp - not sure what it is?

[hbadami]

Hi

We have a semi detached house and recently we are trying to convert the area under the stair case (which was earlier inaccessible) to a small store room. When the builder opened up the space, we saw a damp mark, which I am not sure if its due to condensation or is rising/penetrating damp. Please help me identify what kind it is and any recommendations on what should be the way forward. I have attached the inside and outside pictures.
Thanks in advance.
Hass.

Struggling to attach pics. Any idea how to do that?

When you type, underneath the box are 3 bits that say ‘choose file’. Click on one and upload your photos. If you’re new, it might be a while before you can actually post pics though.

You mean on the outside wall?
Is it a concrete or a timber floor?
What’s on the outside of the wall?

Thanks for your quick replies. Yes I am new, so will wait for sometime before I can upload pics.

@GenderApostate19 the floor under the stairs is originally concrete. The builder has put a timber sheet, underlay and wooden flooring on the top.

Is this the adjoining side of the house? In which case has your neighbour any sign of trouble? Is the patch actually adjoining the floor or higher up? How big is it? Is it damp to the touch? Until you get your photos up perhaps a description could help towards some advice? Did your builder have any suggestions?

Where are the pipes? A slow leak? Have you checked under the bath?

looking forward to seeing the pics.

Please photograph what's on the other side of the wall(s) as well please, particularly any drains, downpipes, gutters or boiler flues, all the way up to the roof and all the way down to the ground.

How old is the housel

Hi pls find the images.

More pics

Few more

The exact location of damp is in the middle of the black plinth outdoor. Internally its under the stairs. The house was build circa 1930.

When we bought the house, we did DPC injections on all external walls (from inside). Since this space was closed it wasn't done. This is almost 6 ft space and the injections were put in on the either side of this space.

Chemical injections do not cure sources of water.

The wet patch appears go be rising from floor level. I do not see airbricks on the external pics so it is probably a concrete floor. The covering should be removed to allow evaporation.

I see efflorescence, or possibly soap marks, on the wall, suggesting a drain leak.

There is a soil pipe, probably cast iron, against the wall.

The bottom of the wall has been painted black and had a plinth added. This is usually done to hide (it does not cure) damp.

The wall appears to be solid, not cavity.

When the house was built, it would have had a DPC, probably slate. Slate lasts at least 500 million years.

When the house was built the cast-iron soil pipe would have joined to salt-glazed brown clay pipe in the ground. This is pretty sure to have cracked at the first joint or elbow, and to have been leaking for about 80 years.

Look for the DPC. It will have been about 9 inches above ground level when the house was built. It may have been bridged by the plinth or later paving. You may be able to see it at or around a doorway.

Some closer pics of the wall, paving, plinth, doc, and external wall corresponding to the wet patch will help.

I have little doubt that the damp is due to a leak in the underground drain. It needs to be dug up and renewed. This is not a rare or complicated job, a well recommended local builder can do it.

I am told that there is an old house somewhere in the country that does not have broken drains, but I have never seen it.

P.s.

Also look at the pipe above the ground, as it may be leaking.

The soap marks are probably caused by leaking waste from a washing machine.

Damp proofing injections are a total con and often make things worse.
Older houses need to breathe, ordinary gypsum plaster and vinyl paints trap moisture.

Thanks @PigletJohn and @GenderApostate19. I looked at the pipe above the ground doesnt look like there is a leak there, but definitely can be a drain leak. Till few months ago there was a washing machine in the toilet adjacent to this wall (and since this section/wall was hidden) that can definitely be the culprit, if it leaked. I wont know as that is gone now.
To second @PigletJohn I plan to put in a dehumidifier/heater in this section for a few days and see if it dries up. Then follow up after a few weeks to see if I see this wet again. In parallel I will see if I can figure out if the drain leaks too.
I also plan to remove the plinth outdoor and do a water proof plaster with damp proof paint under the DPC. I need to do this as there are many small holes in this section through which I see wood lice (sign of damp) and other insects entering the house. Hopefully this will solve this issue completely.

"water proof plaster with damp proof paint under the DPC"

no point in that. Covering up the damp will not cure it. It will just come out somewhere else. You have to remove the source. If you leave the brickwork bare, some water can at least escape by evaporating off the surface.

by all means fill the gaps in the mortar. If it is lime mortar, it has probably been washed away by long term leak - this is very common. You can wash out the dust, mud and woodlice with a hose shortly before repointing it. Although you can get a pointing gun, for such a small job you can press it in with a dowel or something. As the brickwork is wet, make a stiff, dryish mix and press it in hard. Clean excess off the face of the brickwork with a dry rag as it will set very hard.

People usually say you should repair old houses with lime mortar so they can breathe. For old underground or wet brickwork, where the lime mortar has been washed away, I confess I prefer OPC cement mortar. Cement gets stronger the more it is wet, and is resistant to water, but lime needs exposure to the air, so I think cement in more suitable in this position.

Woodlice eat only rotting wood, so are a bad sign.