Extension Roof Help

[padstar]

I am looking to achieve a similar set up as the images below i.e a flat roof with a tiled pitch to the external perimeter.

I have researched the standard options for flat roofs and it would appear that rubber and GRP are the preferred options now a days with GRP seemingly a slightly more robust option. Is that the case and if so are there any real disadvantages in going down this route. The flat roof is below the main bedroom - Will it be excessively noisy?

I understand a warm deck construction is also common now days and looks like a method i would be happy with. Is the sketch below correct and again is there any issues with using this for my extension roof.

Any help would be much appreciated.

 

[oldbutnotdead]

Interesting setup. What is holding the top layer of OSB up- is it just an insulation sandwich? You may get problems with thermal noise (expansion/contraction creaking and groaning) if that deck isn't firmly anchored, you also usually have a 10mm gap from deck to walls again to allow for expansion/contraction.
The gulley through the wall is a bit of a concern, major scope for that to get blocked with carp as well as needing a bit of fall from sides to centre to eliminate ponding.

 

[padstar]

Top deck screwed through insulation and lower osb to joists to form a "warm deck". Am I understanding how this should be constructed correctly? I had read to leave a 10mm expansion joint at the upstands.

The gulley through the pitch is confusing me. I want a similar detail as shown in the photos but can't find anything like it online. Any ideas would be much appreciated. It must be a fairly common detail I would have thought.

 

[datarebal]

VCL

 

[padstar]

VCL to where on top of the lower OSB board?

 

[oldbutnotdead]

Search on here for noisy grp rooves- there are several and they all seem to feature warm roof. Using 150mm screws through compressible insulation gives a firm deck but not a solid one.
Think Roofing Superstore sell the flanged adaptors you need for the gulley, slight problem is the near horizontal outlet means you have to seal to the pipe rather than just overlapping it.

 

[padstar]

Thanks for pointing this out i had not heard of these issues before but clearly they are a problem. Do you think that is linked to the warm roof construction and if i insulated between joists rather than sandwiching the insulation between 2 layers of board it gets rid of the problem or should i avoid GRP completely?

 

[oldbutnotdead]

I do think the noise is linked to warm roof- I've made several grp rooves with deck fixed direct to joists and had no noise problems. My theory is that the insulation hard against the board traps heat from the sun in the boards whereas with a cold roof the ventilation helps the boards shed heat. My garage roof is fibreglass sheets, cold roof- never had any noise off that either (though it does get a bit warm in the summer)

 

[Notch7]

It’s also possible to have the flat roof flush to the tiles so water runs over and down the tiles.

that allows the lantern to be higher, although it may need cranked steels to keep the lantern upstands not too big.