Help with finishing off ridge tile with EPDM to maximise internal loft height - L shaped dormer

[Matt1972]

Hello,

 

I am going through some detailed design of my L shaped dormer.
 

I can make it work with a cold roof, however I have been informed that even with the best ventilation in the world cold roofs will sweat.
The chap I am speaking to has been doing flat roofs for many years, it's his opinion and I am sure others have their own experience and opinions of cold roofs, any thoughts on their efficacy are more than welcome.

My plan is as follows, if I lower the first floor ceiling and use 100mm deep C16/24 roof rafters and some flitch beams to break up the span in main dormer, I can just about achieve a warm roof. With 100mm PIR above 18mm OSB and 50mm beneath between the rafters.

 

I then plan to wrap the EPDM over the ridge beam and fix it to the tiles on the eaves side to form a watertight ridge, I can then dry fix some tiles above, I could use third round tiles and half rounds at the end cut to shape and with some careful placement keep the ridge level just within permitted development rights.

 

This arrangement would leave me a nice 2300mm head height in the main dormer and 2200 in the outrigger section.

 

So my question is, does that ridge finish sound feasible to you, would building control have an issue with this finish?

 

Also I have used a fall of 1:40 on the dormer and perpendicular 1:40 fall for the outrigger, which means the outrigger has to be lower in height, does anyone have a better suggestion for drainage, perhaps one where I don't lose head height in the dormer.

 

Thanks.

 

Matt.

 

 

Edited December 22, 2022 by Matt1972
change pics

[ETC]

If you are only tied to the ridge height at the ridge why don’t you raise the flat roof up say 600mm beyond the ridge?

[Matt1972]

So you mean, slope the roof back to the ridge?

 

Doesn't permitted development mean any part of the new roof to be below the ridge height?

 

Out of curiosity how would the roof drainage work sloping back?