I have a persistant problem with leakage in a room which is under a flat concrete roof. Removing the top layer of concrete, and putting new concrete in place, didn't solve the problem. Recently a builder put tar premixed with gravel on the roof, but also this didn't help. Later he tried to seal the cracks with epoxy, applying it to the ceiling of the room, but because of the concrete still being humid, the epoxy didn't stick in quite a few places.

Any suggestions?

When I was an engineer, we said that these roofs were leaking or that they weren't leaking yet. If it was my property, seeing that you have already tried the conventional solutions, I should think about covering the roof with conventional, slanted roofing material.

My B- in- law in the UK had a business fiber glassing roofs that were a problem.

BUT,you must make sure water is not ''traveling'' from else where to your leak point.

Covering it with a pitched roof is a sensible suggestion, but may not be possible. Do not know if torch-on felts are available in Thailand, but you would need to get rid of the crap coating that has been applied.

I used TOA Roof seal (looks like thick grey paint) and it has worked well. Obviously this is painted on the top/outside surface.

Also of course make sure all drains are clean and water is not stuck/pooling.

BUT,you must make sure water is not ''traveling'' from else where to your leak point.

A good point, if you are in a townhouse block you'll pretty much need to seal the neighboring roofs areas too.

Keywords:

...leakage in a room which is under a flat concrete roof

Later he tried to seal the cracks with epoxy, applying it to the ceiling of the room

Either the initial waterproof treatment failed or there is not any.

You cannot waterproof a concrete flat roof when the roof covers many rooms, and not just one room.

Epoxy is only for structural repairs. Polyurethane is for repairing leaky cracks. Cracks should only be filled between 6am and 8am, when the temperature of the concrete slab has not yet been heated up by the sun, and cracks are at their widest.

We had a flat roof (well did have until we put a roof over) but still gets some wind driven rain. One of the best products I've found for waterproofing is Sika. They have a huge range of products so just have to check through for what you are using it on and how applied to find the right one.

https://www.google.co.th/search?q=sika&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a&channel=np&um=1&ie=UTF-8&hl=en&tbm=isch&source=og&sa=N&tab=wi&ei=VzZgUPWIIo_yrQfPooHwCQ&biw=1280&bih=639&sei=xDZgUJjkJ8nHrQfF-YH4BA#um=1&hl=en&client=firefox-a&rls=org.mozilla:en-US%3Aofficial&channel=np&tbm=isch&sa=1&q=sika+waterproofing&oq=sika+&gs_l=img.1.5.0l10.86.3495.3.5655.2.1.1.0.0.0.126.126.0j1.1.0...0.0...1c.1.kT6LQ-2geGk&pbx=1&bav=on.2,or.r_gc.r_pw.r_qf.&fp=f09fc9d84b793dcf&biw=1280&bih=639

Pretty much everything comes in a yellow packet/tub!