Any advice on a damp wall?

[lalalonglegs]

We have an Edwardian maisonette with a flat roof at the back that covers the kitchen, bathroom and utility room. We had the flat roof converted into a roof terrace by extending the wall upwards around it and had a deck built over the top. A new roof covering was very badly laid at the time and last year it started to leak quite badly in the bathroom.

Earlier this year we had a new roof put on, replastered the ceiling and replastered the wall using sand and cement under the plaster finish but the ceiling and wall in the bathroom are still really damp - better than they were, but still very noticeable. I blamed the new roof but the roofer said it wasn't - in the end I tipped a dozen buckets of dyed water over it to "prove" the roof was leaking but, er, none of the dyed water came through.

So what is causing the damp? I don't think it is condensation - it definitely looks more like the penetrative type of damp - it's two storeys up and is worst in the bathroom (the flat roof is laid so that any water on it should run off to the corner above the bathroom), doesn't seem to have effected the utility at all (the middle room) and does occasionally show in the kitchen. Any ideas/diagnoses gratefully received.

With it being a bathroom and kitchen, it does suggest it might be condensation - what's the ventilation like in these rooms? If you securely tape a square of transparent film to the damp wall, you will see if moisture is collecting on the inside or outside which will show if it's condensation or not.

I'm pretty sure it is penetrative damp as it only appears when it rains

ooo I like the transparent film test idea.

Check the state of the exterior wall relating to the bathroom and kitchen. What is the condition of the pointing there? Also check there isn't a gutter leaking onto that bit of the wall. Is there a chimney column involved or is it just a flat wall? Sometimes rain coming down an uncapped chimney can find it's way inside.

HTH

There's no chimney and the pointing looks ok. There is a gutter taking water off the flat roof - a sort of lip that the water spills down from into a hopper at the top of a downpipe - but I've been out there in the rain and it seems to work. I'm wondering if the bricks themselves - solid Edwardian walls - are the problem. Some of the faces look a bit gnarled. Anyone any thoughts?

is the roof covering, or a lead flashing, dressed up the wall 150mm or more above the roof height? if not, rainwater will splash up onto the wall.

Also look for water penetrating round e.g. gaps round a windowframe, or dripping frrom a gutter or leaking from a downpipe. You may have to stand out there again in heavy rain with an umbrella to see what's happening.

I know outside DS2's room the gutter works fine in normal rain but in heavy rain it just spills over and splashes the wall.

You might need to get closer and check out condition / if they feel damp to the touch.

There is that stuff you can paint onto bricks which makes them weatherproof can't remember what it's called but tagline was something like 'does what it says on the tin!'

Annoying isn't it? I sympathise because we've had to investigate similar mystery patches in our house.

Standing out in heavy rain staring at the back wall is something I'm getting increasingly used to . There's an external staircase leading down to the garden so I can get quite close to the problem areas - I'll have another look tomorrow but I'm pretty sure the gutter is working reasonably well.

We've painted on tons of that damp-proofer solution to no avail. I'm wondering if we should render it?

brass - what was the outcome of your investigations? Was it a dodgy gutter?

PigletJohn - there is lead flashing from the edge of the flat roof up the dwarf wall that encloses the terrace of about 150mm but it is only about 100mm above the top of the deck, iyswim. The deck boards are laid parallel to the damp wall and, imo, there is a big enough gap for the water to drain away down the side of the flashing and run under the boards to the gutter but do you think this might be the problem. There are also lots of potted plants lined up against this wall. Could these be interfering with it in any way?

one bit was wrong kind of render which needed to be dug out and redone.

the other bit turned out to be an uncapped chimney which was sending water to a particular bit of the wall which was then rising capillary style so looking at it you would think it was coming from the floor and not the chimney if that makes sense.

the only other thing I can think of is rogue dodgy plumbing somewhere...

the potted plants could be stopping the wall from drying out if they are pushed up against it

Could it still be condensation caused by the roof becoming much colder when it rains? I know our flat-roofed utility room becomes noticeably colder when it rains. I haven't noticed any condensation to be fair but the room is very well ventilated the cat flap has fallen off the door