Your main enemy is the wind not so much drainage,the drainage is sort of like the icing on the cake not only is the wind driven rain blowing under but all the water on the slow flowing roof just gets blown backward,then once its found a way through it is like a siphon . . . . .Shingles are a wasteof time and money,you could have 10 layers and it will still leak,then you'll pay someone to have to remove all the layers,then install a flat torchdown. NEVER instally anything over another shingle!! Why not? It is simple. Condensation and delamination,the more layers the more overall water . . . Strip it to the wood,repair any bad stuff,install your drip edge ,install your flat deck protection ,flashings and what have you then torch it down,I like to run the end up a wall @ least 8" with some type of. Flashing covering over the end or roll the ends over to close the deck . . . . Good luck buddy
|