Light-frame construction
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Light-frame construction is a building technique based around structural members, usually called studs, which provide a stable frame to which interior and exterior wall coverings are attached, and covered by a roof comprising horizontal joists or sloping rafters covered by various sheathing materials. Modern light-frame structures usually gain strength from rigid panels used to form all or part of wall sections, but until recently carpenters employed various forms of diagonal bracing to stabilize walls. Diagonal bracing remains a vital interior part of many roof systems.
Light frame construction has become the dominant construction method in North America and Australia because of its economy. Use of minimal structural materials allows builders to enclose a large area with minimal cost, while achieving a wide variety of architectural styles. The ubiquitous platform framing and the older balloon framing are the two different light frame construction systems used in North America.
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[edit] Materials
Light-frame materials are most often wood or rectangular steel tubes. Preferred woods for linear structural members are usually spruce, pine or fir woods. Light frame material dimensions range from 38 mm by 89 mm (1.5 by 3.5 inches — i.e. a two-by-four) to 5 cm by 30 cm (two-by-twelve inches) at the cross-section, and lengths ranging from 2.5 m (8 feet) for walls to 7 m (20 feet) or more for joists and rafters.
Wall panels built of studs are interrupted by sections that provide rough openings for doors and windows. Openings are typically spanned by a header or lintel that bears the weight of structure above the opening. Headers are usually built to rest on trimmers, also called jacks. Areas around windows are defined by a sill beneath the window, and cripples, which are shorter studs that span the area from the bottom plate to the sill and sometimes from the top of the window to a header, or from a header to a top plate.Diagonal bracings made of wood or steel provide shear (horizontal strength) as do panels of sheeting nailed to studs, sills and headers.
Wall sections usually include a bottom plate which is secured to the structure of a floor, and one, or more often two top plates that tie walls together and provide a bearing for structures above the wall. Wood or steel floor frames usually include a rim joist around the perimeter of a system of floor joists, and often include bridging material near the center of a span to prevent lateral buckling of the spanning members. In two-story construction, openings are left in the floor system for a stairwell, in which stair risers and treads are most often attached to squared faces cut into sloping stair stringers.
Interior wall coverings in light-frame construction typically include wallboard, lath and plaster or decorative wood paneling.
Exterior finishes for walls and ceilings often include plywood or composite sheathing, brick or stone veneers, and various stucco finishes. Cavities between studs, usually placed 40-60 cm (16-24 inches) apart, are usually filled with insulation materials, such as fiberglass batting, or cellulose filling sometimes made of recycled newsprint treated with boron additives for fire prevention and vector control.
[edit] Roofs
Roofs are usually built to provide a sloping surface intended to shed rain or snow, with slopes ranging from 1 cm of rise per 15 cm (less than an inch per linear foot) of rafter length, to steep slopes of more than 2 cm per cm (two feet per foot) of rafter length. A light-frame structure built mostly inside sloping walls comprising a roof is called an A-frame.
Roofs are most often covered with shingles made of asphalt, fiberglass and small gravel coating, but a wide range of materials are used. Molten tar is often used to waterproof flatter roofs, but newer materials include rubber or other synthetic materials. Steel panels are popular roof coverings in some areas, preferred for their durability. Slate or tile roofs offer more historic coverings for light-frame roofs.
Light-frame methods allow easy construction of unique roof designs. Hip roofs, which slope toward walls on all sides and are joined at hip rafters that span from corners to a ridge. Valleys are formed when two sloping roof sections drain toward each other. Dormers are small areas in which vertical walls interrupt a roof line, and which are topped off by slopes at usually right angles to a main roof section. Gables are formed when a length-wise section of sloping roof ends to form a triangular wall section. Clerestories are formed by an interruption along the slope of a roof where a short vertical wall connects it to another roof section. Flat roofs, which usually include at least a nominal slope to shed water, are often surrounded by parapet walls with openings to allow water to drain out. Sloping crickets are built into roofs to direct water away from areas of poor drainage, such as behind a chimney at the bottom of a sloping section.
[edit] Structure
Light-frame buildings are often erected on monolithic concrete slab foundations that serve both as a floor and as a support for the structure. Other light-frame buildings are built over a crawlspace or a basement, with wood or steel joists used to span between foundation walls, usually constructed of poured concrete or concrete blocks.
Engineered components are commonly used to form floor, ceiling and roof structures in place of solid wood. I-beam (closed web trussed) joists are often made from laminated woods, most often chipped poplar wood, in panels as thin as 1 cm (3/8ths of an inch), glued between horizontally laminated members of less than 5 cm by 5 cm (two-by-two inches), to span distances of as much as 9 m (30 feet). Open web trussed joists and rafters are often formed of 5 cm by 10 cm (two-by-four inch) wood members to provide support for floors, roofing systems and ceiling finishes.
[edit] See also
[edit] External links
- Canadian Wood Council - Wood building design tools, case studies and references.
