garage roof

[swsouthwest]

I am after some information on putting a flat roof on a freestanding garage .It is 3.7 m x 8m i am a tight budget, what size joists are suitable for 3.7m width and how far apart .What sheeting is best to use on top of joists OSB sheets maybe thickness 18mm or can i get away with thinner ,would they have to be waterproofed. I would like to put a very slight slope on the 8m lentgh just so the water can drain a bit easier would i put the joists sraight onto top of blocks or would i use joist hangers.Any info would be greatful
many thanks

 

[Deluks]

4x2 or 4x3 wall plate, strapped down to wall. 8x2s minimum across the 3.7, mark the centre of the joists and use back to back dimishing strips to get a fall to either side (and stick gutters on) Osb or ply min 18mm and felt or plastic/metal roof sheets on top.

 

[Static]

Make sure the rafters stated above are at 400mm centres and you strap the roof down..