Forming a box gutter in the insualtion of a parapet roof?

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Next week I'll be building the warm, parapet roof of our 5 x 3m extension. I've got my head around everything except the gutter.

According to the architects drawings (attached) I'm to form a box gutter in the insulation but there's no reference to a fall across the 5m length, only a 1:80 fall away from the house to the gutter.

With this in mind, I'm assuming a fall is still required and if this is correct how do I build this in?

The problem I'm seeing is the insulation is 160mm, even if I give the gutter 100mm of depth, programming a fall into this is going to be tricky unless I use super thin firring strips. Which I can do, I'm just unsure if its the right thing to do.

The finish is going to be fibreglass. Can I form the gutter from osb vs a preformed tray?
 

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You don't say what the box gutter length is, 3m or 5m but with a gutter of at least 100mm as you mention it will certainly reach the outlet before it overflows even without a fall built in. For peace of mind you could start with 20mm packers the width of the box at one end and fit reduced packers every metre as you go under the box gutter you make, so if 3m for example 20mm at the start then 15mm at 1m, 10mm at 2m and 5mm at 3m along.
 
You don't say what the box gutter length is, 3m or 5m but with a gutter of at least 100mm as you mention it will certainly reach the outlet before it overflows even without a fall built in. For peace of mind you could start with 20mm packers the width of the box at one end and fit reduced packers every metre as you go under the box gutter you make, so if 3m for example 20mm at the start then 15mm at 1m, 10mm at 2m and 5mm at 3m along.
Thanks, it's a 5m run. I've seen some gutters without a fall in them at all but I'm worried about standing areas of water.
 
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A little standing water in the gutter will not be an issue.
 
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I would put a fall in there, latest British Standards state you should be building to a 1:40 fall with the aim of ending up with no less than 1:80 anywhere, including in box gutters, it won't need much to maintain that philosophy. Also be wary of a fibreglass roof and problems with being noisy (widely associated with not enough fixings), there's a massive thread on here somewhere, personally I would go with epdm or other membrane roof.
 

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