Flat Roof / Joist layout

1 Aug 2020
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United Kingdom
Hi guys,

Would anyone be able to suggest the best way to frame this flat roof (cold roof because of height restrictions)? Will be using C24 9” x 3" and all the internal walls are solid block load bearing.

I was thinking about creating a 2 way fall to each side with firring strips. OSB and then 3 layer felt system.

There are 3 fixed sky windows also.


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Probably best running joists left-to-right, with a timber trimmer across the short gap in the middle (assuming it's a detached structure?).
Cross-ventilation will be an issue, particularly with the rooflights.
Why the three layers of felt finish? - so very 1970s. Why not GRP or EPDM?
Yup read my mind - i was thinking left to right.
The firrings will provide a ventilation gap plus im planning on using slightly shorter noggings to allow air to pass over.

The rooflight ventilation is an issue i have thought of but no luck finding a proper solution yet (unless i run small holes from front to back through the joists and have front to back ventilation also?
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Ive heard there are noise issues with grp and epdm? i dont really want it sounding like a tin can.

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