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For longer span rafters, building materials manufacturers have created laminated veneer lumber (LVL) rafters that can be 2–5 times longer than typical wood rafter. In the US, most wood rafters have maximum length of 20 feet (6.1 m). [5] If a longer rafter is needed, LVL is an alternative.
They function as beams to provide support over large spans, such as removed support walls and garage door openings, places where dimensional lumber is insufficient, and also in areas where a heavy load is bearing from a floor, wall or roof above on a somewhat short span where dimensional lumber is impractical.
Light frame material dimensions range from 38 by 89 mm (1.5 by 3.5 in); i.e., a Dimensional number two-by-four to 5 cm by 30 cm (two-by-twelve inches) at the cross-section, and lengths ranging from 2.5 metres (8.2 ft) for walls to 7 metres (23 ft) or more for joists and rafters.
Rafter ties are designed to tie together the bottoms of opposing rafters on a roof, to resist the outward thrust where the roof meets the house ceiling and walls. This helps keep walls from spreading due to the weight of the roof and anything on it, notably wet snow. In many or most homes, the ceiling joists also serve as the rafter ties. When ...
First developed in the 1960s, they have become the most common form of enclosure for spans of 20 to 60 meters. [2] Because of these very strong and rigid joints, some of the bending moment in the rafters is transferred to the columns. This means that the size of the rafters can be reduced or the span can be increased for the same size rafters.
The Tomlinson Lumber Co sold pre-cut materials for a 34 by 50 feet (10 m × 15 m) dairy barn with a Gothic-arched roof supported by three-ply rafters in 1958 throughout Minnesota. [10] The first published plans by an architect for a Gothic-arch barn appeared in 1916. [5]
Greek: the ridge height is 1 ⁄ 9 to 1 ⁄ 7 the span (an angle of 12.5° to 16°); Roman: the ridge height is 2 ⁄ 9 to 1 ⁄ 3 the span (an angle of 24° to 34°); Common: the rafter length is 3 ⁄ 4 the span (about 48°); Gothic: the rafters equal the span (60°); and; Elizabethan: the rafters are longer than the span (more than 60°). [7]
A knee wall is a short wall, typically under three feet (one metre) in height, used to support the rafters in timber roof construction. In his book A Visual Dictionary of Architecture, Francis D. K. Ching defines a knee wall as "a short wall supporting rafters at some intermediate position along their length."