Flat roof....water tight....yet there is still water coming in!

[neverquirefinished]

Property is a 1950's pre-cast concrete mid-terrace house, with a flat roof.  All walls/floors/ceilings are cast concrete.

The roof covering is around 15 years old, and appears to be in fairly good condition. It is of a torched felt construction, with (I assume) timber cavity underneath, with some form of insulation.

 

First leak was several months ago, I had a look, and noted a few small areas of cracking around a drainaway.   Roofer came out and laid around 1m2 of torchon.

 

Leak vanished over night. Perfect.  Ceiling coverings replaced.

 

Fast forward a few months, and its leaking again.  Yet, and this bit hurts my brain......It only tends to leak into the property when it is DRY/fair/warm outside.  We had a week of rainstorms, and not a single drip came in.  yet it was nice and sunny, around 14 degrees a few days ago, and it was dripping every few minutes.

 

Is it possible that this current ingress of water is simply water that is 'stored' in the insulation, following the first leak?

 

If that IS the case, I'm assuming I am looking at a full strip and re-laying of all wood/insulation/membranes? 

[Mr Punter]

Yes, new covering.  It is normally junctions where the leaks are, so make sure these are included, not let to "others". 

[SteamyTea]

Condensation maybe. Nights have been cold.

Wind direction maybe. Been Easterlies, a relatively rare direction.

 

Get it GRP'd properly. Problem solved for ever.

[neverquirefinished]

I did consider condensation. the initial leak was almost certainly rainwater ingress. It leaked in every time it rained.

 

The leak IS in the bathroom, which has very little in the way of ventilation - although this was never a problem - no damp/mould etc.

 

 

 

With regards to GRP, I doubt that is an option.  Every house i have seen of this build type has a bitumen and felt roof construction. 

 

It's a shame I can't find a build diagram or spec - I'm having to assume that there is a series of waterproof layers.

 

The roof is cast concrete panels, 7 I believe, which span the entire width of the house.  I have to presume that these are bitumened in the joints from the top, then a 1st waterproof layer, before timber/insulation and then the top covering of felt which is visible.

 

I guess I can pay a roofer to come and prove my theory - First a flood test to prove its watertight, then once its drained, have him cut a section of felt out above the leak, and check if the layers below are sodden?

 

 

 

 

 

[scottishjohn]

the way my petrol station was made --to  make it hard for people to break in 

 

concrete panels --then a tar boiler and flooded with tar ,then polystyrene insulation 120mm thick ,then felt on top ,then chips -with a slight slope to one side where it ran to a gutter

villians tried to dig through the roof --gave up when they got to concrete

however I was not aware of this  for a long time

only when i went on roof to get on top of petrol forecourt canopy ,to clean the gutters -once a year  job  did I find thid great big hole in the felt and a big lump of the insulation gone 

 It never leaked one bit ---cos they had sealed all the concrete  roof panels with the hot tar that had been poured  on.

 So my guess, if it is concrete slabs, is that there was never a good sealing coat of hot tar before the insulation and felt was laid .

 If you are going to have to strip it all back then yuo know what to do --good old tar boiler and flood the concrete panels with it 

 

Edited March 30, 2020 by scottishjohn