Roof
- For other uses, see Sunroof.
The roof, the top covering of a building, is one of the universal structures found on all buildings. Its purpose is primarily to shed water off the building and to prevent it from accumulating on top. To achieve this goal, Roofs may be highly pitched (sloped) or low sloped in form. 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 plies (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 exhibit lower pitched roofs and drainage at the exterior wall; 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 waterproofing membrane specification. 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) of roof surface is required. Roof pitches may exceed 45 or 50 degrees.
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 are often covered with asphalt shingles (in the US and most of eastern Canada) although thatch, wood shake, steel, corrugated galvanized iron, slate and tile roofs are used elsewhere.
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Weather proofing 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, ]] 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 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.
Qualities and costs of sloped 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.
Solar roofs
Newer systems include solar shingles which generate electricity as well as cover the roof. There are also solar systems available that generate hot water or hot air and which can also act as a roof covering. More complex systems may carry out all of these functions: generate electricity, recover thermal energy, and also act as a roof covering.
There are different ways that solar systems can be integrated with roofs:
- integrated in the covering of pitched roofs, e.g. solar shingles.
- mounted on an existing roof, e.g. solar panel on a tile roof.
- integrated in a flat roof, e.g. PVC.
- mounted on a flat roof with a construction and additional weight to prevent uplift from wind.
Methods of joining roof to walls
Several systems of construction transmit the weight of the roof to the walls of the building and tie the roof into the structure. These include: ashlar-piece, brace (can be arched or wind), collar-beam, crown-post, hammer-beam and -post, king (or queen) post, purlin, rafter (common or principal), ridge beam, ridge-board, strut, tie-beam (tie rod), truss, and wall-plate.
Non-building roofing
Tents, automobiles and other structures also have roofs. A convertible is an automobile built with a folding, retracting, or removable roof. In a car, a moon roof is a see-through opening in the roof of the car, whereas a sun roof typically opens up.
Terminology
- An attic is the story under a non-flat roof.
- Galvanised roofs are normally used on sheds.
- "Pavilion" is a term used by architects to refer to any autonomous, self-contained structure, regardless of size or roof configuration.
- A simple ridged roof consists of inclined rafters that rest on horizontal wall plates on top of each wall. The top ends of the rafters meet at the horizontal ridge plate or ridge beam. Horizontal purlins are fixed to the rafters to support the roof covering. Tie beams are connected between the lower ends of opposite rafters to prevent them from spreading and forcing the walls apart. Collar beams may be fixed higher up between opposite rafters for extra strength. [1]
Roof shapes
Roofs can be shaped to shed water easily. These include:
- lean-to
- single-sloped or shed roof
- ridged
- pitched or gabled
- shaped gable
- Dutch gable - a hybrid of hipped and gambrel
- crow-stepped gable (also called corbie step) gable
- salt-box
- saddleback
- hip roof
- half-hipped
- mansard
- gambrel
- pavilion
- pitched or gabled
- conical
- domical
- pyramidal
- saw-tooth
See also
- Roof garden
- Tented roof
- Building construction
- Green roof
- Roofer
- Roofing felt
- Tensile architecture
- Thin-shell structure
- Tile
- Shingles for a Conic Roof
Further reading
- Francis Ching; Building Construction Illustrated, Visual Dictionary of Architecture, Architecture: Form, Space, and Order."
Reference
External links
Categories
Roofs | Building engineering | Structural system | Tensile architecture | Real estate