Roof
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Categories: Cleanup from August 2005 | Roofs
A roof is the top covering of a building that prevents the ingress of weather into the building interior. Roofing comes in sloped or 'flat' form; however, roofs should never be truly flat. Flat roofs are often covered with tar and gravel and provided with drains to run off rain and snow. Other materials used include b.e. PVC.
Roof shapes and pitches are often the product of material constraints and climactic concerns. Buildings in mild climates or where there is little precipitation tend to exhibit flat roofs; thus the flat roofs of traditional adobe construction in the American Southwest and that of the Middle Eastern deserts. The pitches of most Greek and Roman temples are rather low: most are below 33 degrees. Rain is not a major concern in those areas, and neither were large amounts of snow, for that matter. However, as you go north these concerns grow: in the far north, where snow accumulated on roofs can remain for months and months at a time, roof pitches can exceed 45 or 50 degrees. The sheer weight of accumulated snow can cause major damage to the structure, and the slow melt at the end of winter can cause devastating leaks and water damage (especially when snow melts, then refreezes inside a crack, which then expands) that won't be discovered until too late. High pitches shed snow and rain very efficiently just by the effects of gravity.
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) although thatch, wood shake, steel, corrugated galvanised iron, slate and tile roofs are used elsewhere.
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Sheath materials
The sheath material is the topmost or outermost layer, exposed to the weather. Many different kinds of materials have been used as sheath material:
- Shingles
- Slate
- Ceramic
- Metal
- Cement (usually reinforced with fibers of some sort)
- Composition (heavy bitumen embedded in a fiber mat)
- Wood (cedar is popular)
- Membrane
- Thermoset plastic (e.g. EPDM synthetic rubber). Synthetic rubber strips chemically welded together.
- Thermoplastic plastic (e.g. PVC). Plastic strips welded together with solvents. Can be rewelded.
- Modified bitumen ("torch on"). Asphalt embedded in a fiber mat, seams sealed by locally melting the asphalt with a torch.
- Built-up roof ("tar paper" strips glued together with hot mopped tar, topped with gravel)
- Spray-on foam (isocyanurate foam with an elastic rubber top coat)
- Panel
- Corrugated metal
- Standing-seam metal
- Flat-seam metal
- Fabric
- Dacron/polyester
- Teflon embedded in fiberglass
- Straw thatch
Durability and Cost
Typical composition shingle roofs can last anywhere from a thin 20-year shingle to the thickest of 50-year shingle. They are relatively inexpensive and you can put on up to 2 - 3 layers of shingles before weight becomes a problem and if you under stand that the new (over-layed) roofs will be installed right on top without getting a good look at the structure. Once the 3rd layer is done, the roof has to be stripped off sometimes down to the rafters and new plywood installed for a good nailing surface. Even though older houses may have lap-board installed, roofing companies will take this off too because they will not guarantee that the shingles will become uneven and start leaking. Sometimes new plywood can be installed right over the existing roof boards to help keep the labor cost down.
To have an attractive looking shingle roof some manufacturers have begun to sell a lifetime shingle. This is gaining popularity because people want a roof that looks like a deep rustic wood shake shingle or even a rustic tile roof. These types of shingles are best installed without any roofing layers under it and usually on a steeper roof so one can enjoy the appearance and the surface easer that if the roof was installed on a low sloped roof.
Slate roofs, when properly installed, are considered the best there is. They last around 75 - 150 years or longer depending on the slate used and the careful attention of the roofer, but are extremely expensive to install (sometimes the same price as the rest of the house) and it should be installed by someone that knows what they're doing, or at least soneone with the skills of a roofer and the understanding of basic fluid dynamics. keeping the nail heads covered and protected from the weather properly is the key to a long lasting slate roof. Often, the problem with slate shingles is that the nails used to hold the slate shingles in place rust out before the slate starts to wear out.
Steel roofs last about 50 years or more depending on the moisture barrier (underlayment) used and is in between the shingle roofs and slate roofs when it comes to price. The main attraction is probably the efficiency of which they withstand the elements. Snow load areas benefit from a steel roof by that way they will shed the weight of snow 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 roofcover.
There are different types of integrations of solarsystems in roofs.
- integrated in the covering of pitch roofs b.e. solar shingle.
- mounted on an existing roof b.e. solar panel on a tile roof.
- integrated in a flat roof b.e. PVC.
- mounted on a flat roof with a construction and additional weight to prevent uplift from wind.
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.
A roof has different areas. For example, the eaves are the area where the rafter ends form the edges of the roof that run horizontally across the façade.
By extension one can speak of the roof of a tent, automobile, etc. 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.
- Galvanized roofs are normally used on sheds.
- "Pavillion" is a term used by architects to refer to any autonomous, self-contained structure, regardless of size or roof configuration.
Roof shapes
Roofs can be shaped to shed water easily. These include:
- lean-to
- single-sloped or shed roof
- ridged
- conical
- domical
- pyramidal
See also
Further reading
- Francis Ching; Building Construction Illustrated, Visual Dictionary of Architecture, Architecture: Form, Space, and Order."
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