June 27, 20179 yr I'm acutely aware that I'm at risk of boring the forum to death over my flat roof dormer detail BUT..... Another question. Are rear sloping flat roofs now considered bad practice? I.e the fall is back towards the roof slates and the water drains onto the slates. We have exactly that design on our flat roof dormers on the garage but something makes me think it's not exactly best practice. Ive Googled the subject to death and can find so little detail! ...I'll get me coat!
June 27, 20179 yr Author 1 minute ago, Nickfromwales said: If you don't want front draining then how about to one side? What I'm trying to do is two things: Minimise the roof overhang. Not have a visible gutter.
June 27, 20179 yr 7 minutes ago, Triassic said: Right, no visible gutter, then where will the water drain to? A hidden gutter?
June 27, 20179 yr No issue with rearward draining as long as the gutters / transitions are built correctly. The gutter or leadwork will be the same one that catches the flow from the roof above so should be adequately detailed - any reason you are having second thoughts or issues ..? What is the flat roof covering ..??
June 27, 20179 yr Author 37 minutes ago, PeterW said: No issue with rearward draining as long as the gutters / transitions are built correctly. The gutter or leadwork will be the same one that catches the flow from the roof above so should be adequately detailed - any reason you are having second thoughts or issues ..? What is the flat roof covering ..?? GRP. I assume it's just a case of using a valley style former at the rear? Also I assume there needs to be a break in the edging trim towards the rear to allow proper run off to the sides?
June 27, 20179 yr MAke sure you take your waterproofing a significant way up the roof so you are protected when there is standing snow on it. I would suggest 3ft up the slope if you can. That was the distance we did for difficult to access valleys. Ferdinand Edited June 27, 20179 yr by Ferdinand
June 27, 20179 yr A sketch of the proposal would be nice. The main concern I would have is with the sizing of any flashing/gutter where the flat roof meets the sloping. Its possible to have really huge volumes of water come down a roof and I would assume that occasionally this will be enough to overwhelm any standard sized gutters and design accordingly.
June 27, 20179 yr I would ensure there is a deflection built into the back edge (inverted shallow V) to direct the water down to the sides. Have a look at the sheet lead association website for how to do it in lead and you will get the picture
June 27, 20179 yr 41 minutes ago, Temp said: A sketch of the proposal would be nice. The main concern I would have is with the sizing of any flashing/gutter where the flat roof meets the sloping. Its possible to have really huge volumes of water come down a roof and I would assume that occasionally this will be enough to overwhelm any standard sized gutters and design accordingly. Is that a reference to my post, or somebody else's? It should be a straightforward calculation based on the (horizontal plane) roof area it will be draining and the standard extreme rainfall case to be coped with (seem to remember 2 hours of x mm per hour), using 100% of the roof area above the dormer plus 100% of the area of the flat roof area. Can you use the numbers from a trad dormer with a gutter all the way round emptying onto the roof since this is identical to your worst case of 100% of rain ending up at the back, and since the horizontal plan areas receiving the rain will be identical? The roof down the side of the flat dormer will need the same detailing as trad, and should be a standard detail ... with added belt and braces if you choose. The tricky one us the back detail which needs to be deliberately robust. Fibreglass there going far further up the roof than anyone says is sensible should last a long time. Ferdinand Ferdinand Edited June 27, 20179 yr by Ferdinand
June 27, 20179 yr @Barney12 Provided the internal gutter is sized and detailed correctly then the main residual issue is one of maintenance. You'll need to be able to get to the gutter twice a year or so to clear away any leaves etc that have collected that could lead to a blockage.
June 27, 20179 yr Author 1 hour ago, Ferdinand said: Is your build MBC? Do they have a detail? In short no: MBC's detail goes as far as creating a fall for the battens.
June 28, 20179 yr 22 hours ago, Ferdinand said: Is that a reference to my post, or somebody else's? Intended to be a late reply to Barney's OP. Edited June 28, 20179 yr by Temp
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