Roofing Structure

Roof Components

• Shingles, which comprise the layer of a roof exposed to the elements, are often thought of as simply the roof. Flashings are key components of the outermost layer. These are pieces of metal placed on top of areas such as vent pipe chimneys, valleys, joints that intersect vertical walls or near any intersection of roof projection where there may be a seam. Roofing structure, however, is much more than the outermost layer. Immediately under shingles is the underlayment, which protects the building from moisture. The next layer is decking or sheathing, the material to which shingles and other outer layers are attached. Fascia is the trim used to cover the rafter end of the eaves, while soffit is the finished underside of the eaves.

Trusses

• Underneath the actual roof is the supporting structure. These are the trusses, which are wooden beams that provide a foundation for all of the aforementioned layers. At their simplest, trusses form a sophisticated set of strong rafters that allow for roofs to be constructed in a variety of shapes. Truss structure is also flexible, ultimately allowing for a variety of shapes to be used in roof construction. Truss parts include the peak, top and bottom chord, panel lengths, braces, truss plates, panel points, bearings and cantilevers, which help control ventilation.

Measurements

• Four measurements define a roof’s structure. First is the span, the horizontal distance between walls. The going is the second, which defines the horizontal distance from the walls to the apex. The rise, or the angle of the rafters, is the vertical distance from the top plate to the ridge and the pitch. Another name for the rise is the slope, the term used by roofers to describe a roof's steepness. The angle of the pitching point on the top plate of the wall to the highest point on the underside of the rafter gives the fourth measurement.

Characteristics

• The rise and the pitch often determine roof shape. Roofs with steeper pitches drain better, a factor that contributes to longevity. Valleys occur at the intersection of two sloping roof planes and often lead to leakage in older structures. Materials that can compose roofing structure include wood, steel, plastics, adobe, straw bale, regular concrete and various types of reinforced and treated concrete.