warm deck flat roof incorporating a valley gutter

16 Nov 2012
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United Kingdom
Hi folks,

1st post on here, having read numerous post and advice over the years to help me along the way with a thousand different projects.

My latest "project" is to cover a courtyard and convert it a sort of utility room come store room. It will have a EPDM covered flat roof.

The area of the courtyard is 7m x 2.4m. Along both long sides are existing walls. one wall is the 2 storey end of the house, the other is the side wall of the garage. THe garage has a tiled pitched roof half of which slopes towards the courtyard I wish to cover.

Due to the heights and levels involved the fall on the flat roof over the courtyard must go away from the house towards the garage, where there is obviously a current standard gutter. I would therefore need to incorporate a valley gutter into the design of the roof to take water from both the pitched roof (45+ degrees) of the garage, and the new flat roof.

I would plan to line the valley gutter, and cover the roof in a single sheet of epdm, taking it up 200-300mm under the existing tiles. Valley constructed from Arris rail and ply based to suitable dimensions.

Ideally I would like to do a warm deck as I want the room to be as well insulated as possible, however I haven't been able to figure out how I might incorporate the valley gutter into the warm deck without compromising the insulation. A Cold deck however would be much simpler in my mind, as teh insulation would be between the joists, and the battens would raise the final deck high enough to give a drop into a valley

A wide shallow gutter would be easier to incorporate. would there be an issue with this at all assuming dimensions were suitable for volume of water?

Does a valley gutter need a fall like a normal gutter? (i've see various conservatory installation videos that install them level)

Is there a better solution??

What thickness insulation should i be looking at? 100mm celotex?

many thanks for your assistance
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The insulation should sit directly under the valley trough and will appear to be 'cold deck' which essentially it will be. Only the narrow trough needs to have this detail.

All the rest will be warm deck.

Think of the valley gutter as the lowest start point then form your furrings and warm deck upslope from the valley gutter.

The highest point of the gutter trough will end up being level with the top of the warm deck section.

What all of the blather above is pointing to is that the thickness of the warm deck insulation allows you the opportunity to form the valley gutter trough within that 100mm or so insulation thickness.
Is it really that simple? :oops:

So insulate directly underneath the base of the valley between the joists and then bridge up to the main warm deck once clear of the inside of the trough?

Assuming so, life has just been made easier!!

Moving forward from that given that the joists are only going to be spanning 8ft or so, is it worth using firrings to get the desired fall, or would it be easier to create the fall with the joists themselves?

If not, how does one go about cutting firings accurately? Circular saw on spare joist sized timbers? Or can the be purchased?

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