well i use exterior ply a lot cheeper than marine ply
the only difference between them is marine ply has continuous layers and external ply may have joins or small gaps in the internal ply layers
personaly i would go for 18mm exterior ply for roof and floor
6" drop should be fine for this, remember that felt is often used on flat roofs with less drop than this. You should not need to paint the roof with anything first. If the roof is going to get much sun on it I would use felt adhesive as well as clout nails.
6" drop should be fine for this, remember that felt is often used on flat roofs with less drop than this. You should not need to paint the roof with anything first. If the roof is going to get much sun on it I would use felt adhesive as well as clout nails.
12" overlap is plenty, use 2 rows of nails about 4" apart and water will not blow under very far. I used to sometimes use a bit of adhesive on the joint, as over a period of time the felt can start to curl back to a certain extent, but I would not go to the trouble of adhering the whole roof.
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