regarding the bitumen roofing sheets, I have to disagree. The central section of my main workshop came with it (laid over OSB boards) and to begin with I thought it was great. It's lighter, warmer and quieter (in the rain) than steel, more durable and easier to fit than felt.
However, it has been on there for 9 years and after about 3 years I noticed the problems. It isn't actually waterproof, the stuff absorbs moisture a little , which means that in the winter it freezes and snaps and in the rest of the year when it is raining constantly becomes very heavy and sags over anywhere without support (the ridges have actually flattened in places too!). In the heat of the summer sun, the stuff gets very soft and sags a lot as well as sagging away from the nails that hold it in place, thus opening up holes for water to get in and leading to the OSB getting mould. Oh and because it's a bit porous, moss grows on it like a flipping lawn!
Suffice to say that I will never use it again. The smaller flat packed building came with felt on a flat roof, which is rubbish and has since been recovered with sheet steel.
My preferred roofing material is box profile corrugated steel, with PVC coating but the paint finish is still good. I have a very good manufacturer just down the road, so I can see the stuff up close before ordering. Cladco are the people I get it from. It's durable, not that heavy and you can screw it through insulation board (celotex) and straight to a wooden board roof. So you get waterproof, 25year guarantee, choice of colours, reusable, moss proof, frost proof, sag proof and with the board and celotex underneath quiet too.
On the board front, go for OSB or flooring chipboard. cheap, durable and water resistant. I've found tit stands up much better to constant moisture than exterior ply (the glues are the weak spot). Marine ply is another kettle of very expensive fish though
