Since putting the roof membrane and turf on it has rained almost continually so when Irene phoned to say the roof was full of water I was a bit worried. The weight of 3 inches of water plus all the turfs etc would be massive. So I went in today to have a look. The water was not running away as the temporary drain off was blocked. It was a good test of the roof structure which had not moved at all despite the huge weight.
I decided that I would change the roof design so I lifted the roof beams along the front of the building a couple of mms at a time and removed the rear facia so the pond liner can lap into a traditional gutter this should prevent all possible blockages and is easy to install.
Lifting the beams was a slow project but it worked a treat so the roof now has an 18mm slope over it's length. And is draining away slowly as you would expect.