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,

Slate Roofing Contractors Association of North America. Roofing contractors listed on this page are members, or member applicants, of the Slate Roofing Contractors Association Renaissance Roofing, Inc. Belvidere, Renaissance Roofing, Inc. prescreened Gutter Contractors, Roofing Contractors in Belvidere, IL. Warren County, New Jersey Wikipedia,

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