Pitched Roofs

The roof is the top covering of a building intended to shed water and to prevent it from accumulating on the structure. Roofs range from highly pitched (sloped) to flat. Low sloped roofs are commonly found on industrial/commercial type structures. Pitched roofs are the primary design found on residential homes. The most common type of low sloped roof for industrial/commercial structures are the conventional built-up-roof (BUR), modified bitumen, or single ply(EPDM, PVC, TPO). Metal standing seam roofs are becoming increasingly common on low sloped roof assemblies.

Residential roof designs in mild climates or where there is little precipitation tend to have a lower pitch and drainage at the roof perimeter. An example of residential flat roof is that of the adobe construction in the American Southwest. In northern climates, where temperatures drop below freezing, or in southern climates with high temperatures, the need for greater thermal resistance will dictate the type of roof design as well as the water shedding material. In areas that have a high accumulation of snow, such as Buffalo USA or Montreal Canada, a minimum slope of 6 inches in 12 inches (6/12) is required.

Cool roofs and green roofs are becoming increasingly popular, and in some cases are mandated by local codes. Cool roofs are defined as roofs with both high reflectivity and high emissivity. Green roofs have soil placed over the waterproofing membrane in which plants and even trees are planted.

Some building styles, for example, geodesic and A-frame, blur the distinction between wall and roofs.

Pitched roofs with 3/12 or greater slope are covered with asphalt shingles, wood shake, corrugated steel, slate or tile.
 

Costs of roofing materials

Typical composition shingle roofs can last anywhere from a thin 20-year shingle to the thickest which are limited lifetime shingles. The 20 year shingles are relatively inexpensive but the lifetime shingles can cost up to $2 per square foot. When a layer of shingles wears out, they are usually stripped, along with the underlay and roofing nails, allowing a new layer to be installed. An alternative method is to install another layer directly over the worn layer. While this method is faster, It does not allow the roof sheathing to be inspected and water damage, often associated with worn shingles, to be repaired. Having multiple layers of old shingles under a new layer causes roofing nails to be located further from the sheathing, weakening their hold. The greatest concern with this method is that the weight of the extra material could exceed the dead load capacity of the roof structure and cause collapse.

Slate roofs, when properly installed, are often considered the best. A slate roof may last 75 to 150 years, and in extreme cases even longer, although the durability depends on the quality of slate used and the levels of skill and care employed by the roofer. However, slate roofs are often very expensive to install, particularly in areas where slate roofing is not common. In the USA, for example, a slate roof may have the same cost as the rest of the house. Often, the first part of a slate roof to fail is the fixing nails; they corrode, allowing the slates to slip. In the UK, this condition is known as "nail sickness". Because of this problem, fixing nails made of stainless steel or copper are usually recommended, and even these must be protected from the weather.

Steel shingle or standing-seam roofs last about 50 years or more depending on both the method of installation and the moisture barrier (underlayment) used and are between the cost of shingle roofs and slate roofs. Areas prone to heavy snow benefit from a steel roof because their smooth surfaces shed the weight of snow more easily and resist the force of wind better than a wood shingle or a concrete tile roof.

 

Roofing Materials

The weather proofing material is the topmost or outermost layer, exposed to the weather. Many different kinds of materials have been used as weather proofing material:

Shingles & faux shingles.
Slate. Long life, high cost.
Ceramic tile. Long life, high cost.
Imbrex and tegula, style dating back to ancient Greece and Rome.
Snap Loc metal. Long life. High cost, should be on roofs of 3/12 pitch or greater.
Mechanically seamed metal. Long life. High cost, should be on roofs of 0.5/12 to 3/12 pitch.
Concrete (usually reinforced with fibres of some sort).
Asphalt shingle (bitumen embedded in an organic or fiberglass mat, usually covered with colored, man-made ceramic grit). Cost effective. Various life span expectancies.
Redcedar. Long life. However, young growth redcedar has a short life expectancy. High cost. Should be allowed to breathe.
Asbestos shingles. Very long lifespan, fireproof, low cost but now rarely used because of health concerns.
Membrane.
Thermosetting plastic (e.g. EPDM rubber). Synthetic rubber sheets adhered together with contact adhesive or tape. Primary application is big box store with large open areas and little vertical protrusions. Relies heavily on the skill of the applicator. Detail critical.
Thermoplastic (e.g. PVC, TPO, CSPE). Plastic sheets welded together with hot air. Can be rewelded with the exception of CSPE. Lends itself well to both big box and small roof application because of its hot air weldability. Relies heavily on the skill of the applicator. Detail critical.
Modified bitumen - heat welded, asphalt adhered or installed with adhesive. Asphalt is mixed with polymers such as APP or SBS, then applied to fiberglass and/or polyester mat, seams sealed by locally melting the asphalt with heat, hot mopping of asphalt, or adhesive. Lends itself well to all applications. Requires some training.
Built-Up Roof - Multiple plies of asphalt saturated organic felt or coated fiberglass felts. Plies of felt are adhered with hot asphalt, coal tar pitch or adhesive.
Sprayed-in-Place Polyurethane Foam (SPUF) - Foam sprayed in-place on the roof, then coated with a wide variety of coatings, or in some instances, covered with gravel.
Metal roofing.
Corrugated iron sheeting a house under construction in Northern Australia. Shows two Dutch gables, external cyclonic fasteningsCorrugated metal with exposed fasteners.
Standing-seam metal with concealed fasteners.
Mechanically seamed metal with concealed fasteners contains sealant in seams for use on very low sloped roofs.
Flat-seam metal with soldered seams.
Fabric
Dacron/polyester.
Teflon embedded in fibreglass.
Straw, or reed thatch.
Foam roofing is a common material coating for flat roofs.
Tar and gravel roofing.