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Construction of a ridged roof
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. Heavier under purlin are used to support longer rafter spans. Tie beams or ceiling joists, are connected between the lower ends of opposite rafters to prevent them from spreading and forcing the walls apart. Collar beams or collar ties may be fixed higher up between opposite rafters for extra strength.[1]
The rafters, tie beams and joists serve to transmit the weight of the roof to the walls of the building. There are a number of structural systems employed to facilitate this, including the use of wall-plates set at the top of the wall, hammer-beams, which spread the weight down the wall and create an equilibrium between outward and upward thrust, king posts which transfer the weight of the roof ridge, and various types of trusses.
In cyclone and hurricane prone areas the main engineering consideration is to hold the roof down during severe storms. Every component of the roof (as of course the rest of the structure) has to withstand the uplift forces of high wind speeds. This is not normally a problem in areas not prone to high wind.
Modern roofing technologies, apparent in the accompanying photo of a house under construction in a cyclone-prone region of Northern Australia, include the purpose-made steel hook bracket which is bolted to the truss with M16 bolt. The bracket is bolted to an M16 bolt cast in situ, embedded 300 mm into the reinforced concrete block wall. This system is typically in place every 900 mm around perimeter.
Commercially available 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:
- Thatch is roofing made of plant stalks in overlapping layers.[2]
- Shingles, called shakes in North America. Shingles is the generic term for a roofing material that is in many overlapping sections, regardless of the nature of the material. The word is also used specifically to denote shingles made of wood.
- Redcedar. Life expectancy, up to 30 years.[4] However, young growth redcedar has a short life expectancy. High cost. Should be allowed to breathe. [5]
- Hardwood. Very durable roofing found in Colonial Australian architecture, its use now limited to restorations.
- Slate. High cost with a life expectancy of up to 200 years.[6] Slate cleaves into thin sheets, making it much lighter than concrete tiles, though heavier than sheet steel and other light roofcoverings.
- Stone slab. Heavy stone slabs (not to be confused with slate) 1"-2" thick were formerly used as roofing tiles in some regions in England. Stone slabs require a very heavyweight roof structure, but their weight makes them stormproof. An obsolete roofing material.
- Ceramic tile. High cost, life of up to 100 years.[7]
- Imbrex and tegula, style dating back to ancient Greece and Rome.
- Metal shakes or shingles. Long life. High cost, suitable for roofs of 3/12 pitch or greater. Because of the flexibility of metal, they can be manufactured to lock together, giving durability and reducing assembly time.[8]
- Mechanically seamed metal. Long life. High cost, suitable for roofs of low pitch such as 0.5/12 to 3/12 pitch.
- Concrete, usually reinforced with fibres of some sort. Concrete tiles require a stronger roof structure than slate, as some owners have found to their cost.
- Asphalt shingle, made of bitumen embedded in an organic or fiberglass mat, usually covered with colored, man-made ceramic grit. Cheaper than slate or tiles. Various life span expectancies.
- Asbestos shingles. Very long lifespan, fireproof and low cost but now rarely used because of health concerns.[9][10]
- Membrane. membrane roofing is in large sheets, generally fused in some way at the joints to form a continuous surface.
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- 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.
- Thermoplastic (e.g. PVC, TPO, CSPE). Plastic sheets welded together with hot air creating one continuous sheet membrane. 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.
- 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.
- 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.
- Fabric
- polyester.
- PTFE, (synthetic fluoropolymer) embedded in fibreglass.
- Metal roofing. Generally a relatively inexpensive building material, unless copper is used.
- Galvanised steel frequently manufactured with wavy corrugations to resist lateral flexing and fitted with exposed fasteners. Widely used for low cost and durability. Sheds are normally roofed with this material. Known as Gal iron or Corro, it was the most extensively used roofing material of 20th century Australia, now replaced in popularity by steel roofing coated with an alloy of zinc and aluminium, claimed to have up to four times the life of galvanized steel. [11]
- 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.
See also
- Roof garden
- Tented roof
- Cool Roof
- Building construction
- Building envelope
- Green roof
- Metal roof
- Metal Roofing Alliance
- Roofer
- Roofing felt
- Tensile architecture
- Thin-shell structure
- Tile
References
- ^ Illustrated Architecture Dictionary on Buffalo Architecture and History
- ^ Thatching Information.[1]
- ^ as claimed by McGhee & Co. Roof Thatchers.[2]
- ^ Wood shakes and shingles.[3]
- ^ Hometips - Wooden shingle roofing, with good diagrams. [4]
- ^ Slate Roof Central.[5]
- ^ Renaissance Roofing.[6]
- ^ HomeTips: Metal shingle roofing.[7]
- ^ Asbestos and Your Health, Victorian Government.[8]
- ^ Asbestos Diseases Advisory Service.[9]
- ^ Bluescope.[10]
Further reading
- Francis Ching; Building Construction Illustrated, Visual Dictionary of Architecture, Architecture: Form, Space, and Order."
External links
