The main factor that may weaken the stability of the roof structure is dampness. On its negative impact it is particularly exposed rafter framing, which deform under impact of water. Therefore is important roof protection against getting in  water into its internal structure.

Typically, investors during the construction of the roof attach great importance both for building materials, as well as careful execution. Unfortunately, later very often they forget about the survey, and any minor defects  commission outsource cheaper and often non-professional roofing companies. This is a big mistake,

Shingles vs metal roof valleys Sawmill Creek Other than cost for the galvanized or copper valleys on homes today, is there any advantage to using shingles. Very few are even replacing the metal on older homes. Gerard USA Metal Roofing Gerard Roofing Technologies Canada Highway #3

A very important element that must be considered when the roof is projected, is ventilation, allowing drainage of excess dampness from under the roofing.