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Key:1: ridge beam, 2: purlins, 3: common rafters. This is an example of a "double roof" with principal rafters and common rafters. A timber roof truss is a structural framework of timbers designed to bridge the space above a room and to provide support for a roof.
Invented in 1969, the I-joist is an engineered wood product that has great strength in relation to its size and weight. The biggest notable difference from dimensional lumber is that the I-joist carries heavy loads with less lumber than a dimensional solid wood joist. [1] As of 2005, approximately 50% of all wood light framed floors used I-joists.
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.
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]
By supporting the rafters they allow longer spans than the rafters alone could span, thus allowing a wider building. Purlin plates are very commonly found in large old barns in North America. A crown plate has similarities to a purlin plate but supports collar beams in the middle of a timber-framed building.
The top ends of the rafters often meet at a ridge beam, but may butt directly to another rafter to form a pair of rafters called a couple. Depending on the roof covering material, either horizontal laths , battens , or purlins are fixed to the rafters; or boards, plywood , or oriented strand board form the roof deck (also called the sheeting or ...
A hammer-beam is a form of timber roof truss, allowing a hammerbeam roof to span greater than the length of any individual piece of timber.In place of a normal tie beam spanning the entire width of the roof, short beams – the hammer beams – are supported by curved braces from the wall, and hammer posts or arch-braces are built on top to support the rafters and typically a collar beam.
[5] [6] The term truss has often been used to describe any assembly of members such as a cruck frame [7] [8] or a couple of rafters. [ 9 ] [ 10 ] One engineering definition is: "A truss is a single plane framework of individual structural member [sic] connected at their ends of forms a series of triangle [sic] to span a large distance".