Leaky shed roof

[adzeman]

You are probably correct in your analysis, need to get up there and look. Its leaking in a different area then last time.
Because of time restrictions I only repaired the defective area, should have done the lot.
Where I had the big leak I installed a roof light and to support the flashings boarded it with 19mm waterproof ply. The Roof nail went into this so I think it is the sheer volume of rainwater creeping up the edges.

It is a drip and not a deluge as previous. Have some pictures but cant post from home. Cant get Broadband from our village, but if interested will post from another location.

 

[OPJ]

We had 90mm of rain in Devon over the past 7day, and it was heavy, I expect the rain overflowed the corrugations and crept in that way?

Through the holes made for holding sheets down?

What I've found with my roof is that, because the each sheet is only overlapped by one corrugation (less materials, less work, etc. :roll, this gives rain water an excellent opportunity to find a way in to my workshop! :x I also find it creeps in around the fixings.

What I've been meaning to do for some time is to get up there and just fill all these gaps with sealant. One thing I like about the Onduline system is that you can over sheet without having to remove the old covering (particularly important if it's asbestos). With this though, they do recommend you coat both sides of the old sheets with a thin 50/50 mix of PVA, which sounds like a good idea to help keep the dust down if you did decide you wanted to remove it...

 

[adzeman]

My problems started when we had the snow earlier this year. Its was wet and sticky and laid about 100mm thick. This put weight on the corrugated bitumen sheets which deflected.
When the snow melted the sheets did not go back to their original shape.
Rain as you described entered the work shop. The underside of the roof is insulated for sound which became wet turned into a stogy mess, a nightmare!
The sheets do flex with the wind and move and distort in hot sunlight.
The half of the roof I replaced was built up with 19mm ply, coated with bitumen paint and the original corrugated sheets replaced with new sheet nails. Flashband was used to flash the upstands, any joints in the plywood and as an eaves flashing into the gutter. Any rain getting past the overlap just travels down the ply sheets into the gutter.
Just whish I had done the other half of the roof, which I will do next dry period.