Yesterday I worked all day on setting out the dormer rafters. This looks simple. But, there’s much to consider. Two problems required hours of staring. First, how does a valley flashing, which is under the tiles, drain on to the top of the tiles? My useful guide to roofing simply doesn’t show this. It all works fine if the valley runs out to the edge and into a gutter. But ours runs onto the main roof.
Checking a nearby house, they seem to have simple bent the copper up? Seems crude. However, this morning I think I found the solution. The flashing is cut and bent to turn down the roof at the end and comes up onto the tiles surfaces.
The second problem was how to set the rafters out. They need to be square. The length of the eave needed to be set also. From the model I hadn’t realised that the valley would mainly discharge above the velux: bad design. The design was an 800mm eave, I settled on 600mm to avoid the valley draining onto the velux flashing. A quick application of Pythagoras and a lot of messing about and all the rafters were square.
With all set, I started cutting. By dark I had a number in place and the others loosely set out forming the dormer roof. I was able to sheet up and make nearly water tight roof.