Leaking flat concrete roof

5 Nov 2009
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West Midlands
United Kingdom
My first post here, so hello!

We've got a small outbuilding out the back with a flat concrete roof. I've just decorated part of it, to use as a laundry room, by painting the largely bare brick walls with breathable damp seal paint and masonry emulsion on top. The damp seal because it looked like it had suffered from a little damp in the past and the old paint was flaking.

Since then I've noticed a few trickles of water coming from pin-prick holes in the paint in the corner between wall and ceiling. I now presume there was damp spreading through the brickwork and evaporating out and not condensing properly thus I didn't notice anything too bad, but now it's perhaps concentrated through the holes. Either way, the roof is leaking. It's flat concrete about 4 inches thick, topped with grit and a lot of moss.

What's the best way to stop the leak? Replacing the roof is not an option and so far I'm thinking of applying Flexacryl or perhaps acrylic waterproofer from Wickes.

My next door neighbour told me not to remove all the moss for some reason!? I don't understand why not, surely the moss holds water. If I paint on flexacryl would this be bad by sealing water into the concrete? Do I need to wait until the summer and allow the roof to dry out?

Any advice appreciated. Thanks muchly.
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get it felted properly or it will likely be a problem in the near future. make sure a good flashing is fitted where the roof abutts the wal and you could also need cavity trays fitted here.

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