The performance of a building's roof is key to the integrity of the structure and the comfort and well being of the occupants. Roof failures run the gamete from the catastrophic structural failure from shifting structures, snowstorms, tornadoes, and high winds, to major leaks caused by falling tree limbs and the intrusion of wind driven water under roof shingles or tiles. Damage is also caused by deterioration of roof sheathing and saturation of insulation from ice damming and wind-blown moisture into the attic spaces through soffit, gable-end, and ridge vents. Minor leaks due to poor shingle condition, or improper caulking, flashing, and drip edge installation are widely common and can be costly when the damage total is tabulated. Careful inspection is warranted, and advised because poor roof conditions tend to be erosive in nature to all roofing components.
In the fairly recent past, steep-sloped residential roofing selection was relatively simple: the choice was between three-tab or T-lock asphalt shingles, cedar or shakes, clay and possibly concrete roofing tiles, or, in a smaller number of cases, slate. Today these same basic choices are available, but these materials have evolved considerably, particularly the development of high-profile laminated asphalt shingles, and new treatment techniques for cedar and shakes.
One of the most telling aspect of a roof's condition is the condition of the ridge. Swayback (sagging) roofs can indicate a number of problems including: lack of or insufficiently sized structural ridge beam; insufficiently sized roof rafters, missing or misplaced collar ties; improper rafter heel bearing; lack of solid blocking at the heel; inadequate rafter bracing or rafter bracing that is not transferred to a structural member; or interior column displacement due to settled footings.
Common problems with trusses include trusses bearing on interior partitions; members cut to accommodate ductwork or equipment in the attic space; and a lack of lateral bracing of top and bottom truss cords. Most of the time, repair of these conditions is simple, but can be very costly. If, however, a structure has excessive deformation it is advisable to reframe it.
Roof sheathing serves a number of functions. It is a key element in the barrier system that keeps out moisture; it serves as a nail base and support or roofing materials; it connects and braces the individual roof trusses or rafters; it provides a diaphragm that, in combination with shear wall, stiffens the entire house against lateral forces from wind and shifting.
Roof sheathing in older houses (before the 1950's) consists typically of 1x boards, either tongue-and-groove, butt-edged, or spaced, and laid perpendicular to the rafters. Houses built since the '50s most likely have plywood, particle board, or oriented strand board (OSB) sheathing thinner than 3/4". Exceptions to this are post-and-beam houses with rafters at 4'-0" or greater spacing.
Roof flashing is one of the last lines of defense in the battle against water penetration. Flashing forms the intersections and terminations of roofing systems and surfaces to thwart water entry. The most common locations for roof flashing are at valleys, chimneys, roof penetrations, eaves, rakes, skylights, ridges, and roof-to-wall intersections. Flashing must be configured to resist the three mechanism of water penetration: gravity, surface tension, and wind pressure. To achieve this, flashing can be lapped shingle style, soldered or sealed to function as a continuous surface, or can be configured with a non-continuous profile to combat surface tension elements. Flashing baffles should be installed in open or closed roof valleys where water flow is expected to be unequal. Flashing materials must be durable, low in maintenance requirements, weather resistant, able to accommodate movement and be compatible with adjacent materials. Common modes of failure include exposure to salt air, excessive heat, acid rain, heavy snows, and scouring winds.
The primary purpose of a roofing underlayment is to provide a secondary protective barrier beneath the roofing material. Typically, this material has been 15# asphalt impregnated felt for lightweight roofing materials, while heavier materials require one or more layers or 30# felt. Fortunately, there is an increasing awareness among manufacturers and roofing consultants of the importance of a reinforced underlayment in areas of severe snow or wind-driven rain. In problematic areas, the first barrier, the shingle system is only expected to drain a majority of the water, while the fortified underlayment creates the final barrier from wind-driven or ice-dammed moisture.