Rain whilst Flat Roof being laid - Problem?


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In the process of having a new flat roof laid. The first cement screed/base (using fine sand) was down and then 4 coats of the waterproofing product applied all over it. Then some small ventilation holes drilled to the outside walls to put small pipes (about the size of 5p coin) in for air-circulation about 10 around two sides of the roof. All completely dry and no leaks. Good so far.

However, 5 Hours after having the second cement screed/base on top (this time using coarser sand). It started to rain.

Will this cause a problem. Will all the new cement have to come up because there is nowhere for the water to go except into it. Will it penetrate through the roofmate & just sit there and never dry out. Which will cause a problem later on when the tiles are laid as they will start to lift after time.

PS The roof of the house is in Spain, so it maybe slightly different than normal. Any advice please

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i would suggest you have a close look at it. after 5 hours it should have been fairly firm and if it didn't rain hard it shouldn't have done much damage. if the rain did penetrate the screed i think the damage will be obvious - if it looks ok, then it probably is ok.

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Indeed, it depends how much rain fell and how dry the top layer was when it raiined because Water can penetrate down underneath the top layer and sit between the layers without you even knowing its there.

The top layer could "Look dry" and feel dry to the touch, but depending on its thickness, it could be very wet underneath as water travels downwards.

Out of interest, why would you have a layers of cement on your roof anyway?

This is pretty unheard of in the UK and definately never something I ever came across in my years of roofing when i was an apprentice and/or worked out on-site, nor in all the roofing I did in Florida too, I've never seen or heard of putting layers of cement on a flat roof and would think its probably the worst thing you can do due to the weight of it and its inability to flex and move without cracking and the effort required to lay it all...

There are far better roofing alternatives, cheaper and quicker and far more better suitable that can breathe, are flexible, and give guaranteed lifespans and seal qualities and weight much less than any cement will...

Worse case is, if the top layer was not dry enough or treated, it may absorb moisture and that can then sit between the layers, especially as the first layer had been waterproofed.

Saying that, if your treating the first layer and giving that 4 coats of waterproofing, what exactly does the second layer have to bond to so it doesnt become a seperate layer that just sitting on top of something it cant bond to? (I hope that makes sense?)

You got any pictures of this setup?

Also, when you say your going to put tiles on it, what kind of tiles will you be laying on this flat roof? Has this roof been reinforced to take all this weight?

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  • 1 year later...

In the process of having a new flat roof laid. The first cement screed/base (using fine sand) was down and then 4 coats of the waterproofing product applied all over it. Then some small ventilation holes drilled to the outside walls to put small pipes (about the size of 5p coin) in for air-circulation about 10 around two sides of the roof. All completely dry and no leaks. Good so far.

However, 5 Hours after having the second cement screed/base on top (this time using coarser sand). It started to rain.

Will this cause a problem. Will all the new cement have to come up because there is nowhere for the water to go except into it. Will it penetrate through the roofmate & just sit there and never dry out. Which will cause a problem later on when the tiles are laid as they will start to lift after time.

PS The roof of the house is in Spain, so it maybe slightly different than normal. Any advice please

If you go to site called Permavent low pitch roofing systems and I am pretty sure it will help you. They do supply to France so spain should not be a problem

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