Damp in new extension on old building

[Anonymous]

We have a 1860's cottage which has a 1970's dining room extension attached. Recently we have started renovating this room, and when removing the skirting and wallpaper I noticed a couple of damp looking patches in the external walls, in the bottom external corner of the room. There is no damp on the wall between the two ages of building, just in the new extension. It looks from the rougher plaster finish where the damp is, that someone may have tried to treat this before, or the damp has caused the plaster to look rougher. I'm pretty certain that the level of the flower bed and dirt path at the side of the house are to blame, as I think they are over the damp course level. I've dug some back already, but was wondering how far below the damp course I need to move the earth level to, or should I dig a smallish ditch (about 8 inches wide by X inches below the DC) all along the affected walls. Would this cause more damp as pools of water could form? Is making sure the earth level is below the DC enough? Not sure what to do next apart from start digging for victory with my trowel!

 

[Anonymous]

As with any such problems, the best way forward is to try a bit at a time, then assess the level of success.
I would start by ensuring that the outside flowerbed soil is at least 6 inches below dpc level. I would also hack off the plaster inside that you describe as rough - this is probably just blown plaster due to the damp. Make sure you hack off enough to expose all the damp masonry.
Then give the bare masonry a good while to dry out. It's unfortunate that we're now heading into winter because this will lengthen the drying out time, but I would give it at least 2-3 months. Then - depending on how successful you feel the drying out has been - replaster the bare bit of inside wall. If you can use lime plaster without it looking too different from the rest of the plastering, this will help the wall to stay damp-free.
If the above doesn't have the desired effect, then the next stage would be to dig a french drain outside and fill it gravel up to 6" below the dpc. This will allow the air to get to a larger area of damp masonry, and help to dry it out.
Whether you get to this second stage or not, don't replaster inside until you're sure that the wall has dried out. You'll only end up with the same problem again in a few months time.

 

[Anonymous]

Thankyou for this advice. I know it's a cop out, but do you think it's possible to dry it out without hacking off the blown plaster? You see, before the damp started to show (most didn't appear until after I'd stripped, sanded and painted the walls, plus built a whole wall of bookshelves, fixed firmly by horizontal beams and verticals, to the walls). It would mean taking all of that down again. I have a nasty feeling I know what your answer will be and that it will be the sensible thing to do. I just wondered if I did all the outside stuff whether a dehumidifier in the room would dry the walls and plaster sufficiently. We have a portable one I could use. The damp patches are at present about 1 - 2 feet long by about 8 inches high at their highest, and are from floor level upwards. There are two of them. The plaster was blown in places, so I had to remove a little bit, but I'm afraid, not being plasterer, I stabilised the rest, then filled the cracks and pits with filler, then sanded smooth and primed and painted, plus then painted fungicide over the top. Then built the bookshelves on top to make it much harder to sort out.... In all fairness, it has only become apparent in the last couple of weeks that the problem is more serious than a tiny bit of pale mould bloom caused by a cold room and humidity. I guess I'm on a giant learning curve!

 

[Anonymous]

Sorry to be the bearer of bad tidings, but your problem is this: gypsum plaster doesn't breathe. Therefore it won't allow the damp in the masonry underneath to migrate through and dissipate in the air. Add to that the fact that you've painted on all these impermeable coatings, and you've got the damp well and truly trapped.
You could try leaving things as they are for a while after you've lowered the soil level outside, to see what happens. But - sorry - I'm not optimistic that this will be a solution.

 

[Anonymous]

Fair enough. I had an 'orrible feeling you might say that! Will do the flowerbeds over the weekend and then I guess it's time to get the screwdriver out and start taking the bookshelves down again. Doh!
Thanks very much for the advice, David.