When it comes to roof installation, depending on the kind of roof, many items come into play: Sub surface layers creating the water barrier between the decking of the house or building and the actual roof. Everything must be properly overlapped and sealed to prevent water entering. Proper ventilation is a must. Too much can cause heating issues and too little will cause the attic to heat up and create moisture which can rot and warp wood. Creating the proper channels and leveling for proper water run-off. Pooling water will cause rot and damage to roof. Everything must be correctly sealed when performing roof installation, even a small exposed nail can cause serious internal damage.