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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.
Common purlins in wood construction, also called a "major-rafter minor-purlin system". [3] Common purlins are typically "trenched through" the top sides (backs) of principal rafters and carry vertical roof sheathing (the key to identifying this type of roof system).
This is accomplished by using metal ties fastened to each rafter or truss. This is not normally a problem in areas not prone to high wind or extreme weather conditions. 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.
Compass rafter: A rafter curved or bowed on the top (the top surface of a rafter is called its "back") or both the top and bottom surfaces. Curb rafter: The upper rafters in a curb (kerb, gambrel, Mansard roof) roof. Hip rafter (angle rafter): The rafter in the corners of a hip roof. The foot of a hip rafter lands on a dragon beam.
Planar roof trusses The roof trusses of the Basilica di Santa Croce in Florence The simplest form of a truss is one single triangle. This type of truss is seen in a framed roof consisting of rafters and a ceiling joist , [ 13 ] and in other mechanical structures such as bicycles and aircraft.
The first rafter is fitted between the wall and the temporary central support and then further rafters are added, each resting on the last. The final rafter fits on top of the previous rafter and under the very first one. The rafters are then tied before the temporary support is removed.
The gable roof [2] is so common because of the simple design of the roof timbers and the rectangular shape of the roof sections. This avoids details which require a great deal of work or cost and which are prone to damage. If the pitch or the rafter lengths of the two roof sections are different, it is described as an 'asymmetrical gable roof'.
The simplest form of roof framing is a common rafter roof. This roof framing has nothing but rafters and a tie beam at the bottoms of the rafters. The next step in the development of roof framing was to add a collar, called a collar beam roof. Collar beam roofs are suitable for spans up to around (4.5 meters). [5]