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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.
Two king post trusses linked to support a roof. 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.
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.
Historically, roof pitch was designated in two other ways: A ratio of the ridge height to the width of the building (span) [6] and as a ratio of the rafter length to the width of the building. [7] Commonly used roof pitches were given names such as: Greek: the ridge height is 1 ⁄ 9 to 1 ⁄ 7 the span (an angle of 12.5° to 16°);
A view of a roof using common purlin framing. The purlins are marked in red. This view is from the inside of the building, below the roof. The rafters are the beams of wood angled upward from the ground. They meet at the top of the gable at a ridge beam, which has extra bracing to attach it to the rafters.
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.
In the UK, a concrete tiled roof would normally have rafters at 600 mm (24 in) centers, roof battens at 300 mm (12 in) centers and ceiling joists at 400 mm (16 in) centers. The United States still uses imperial units of measurement and framing members are typically spaced sixteen or twenty-four inches apart.
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.