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All  |   Discussion (0)Roof (General)

Ridge beam vs. ridge board: These are very different treatments to the apex of the A-frame, and should not be confused.
- A ridge beam is a hefty structural member, like 300 x 100mm, that supports the weight of the rafters, sitting at the peak of the A-frame underneath birds-mouthed rafters that rest on top of the beam. It should be one whole length, but if joined the joints of a ridge beam must be made over a support.   kellyjones00 (593)
 
 


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- A ridge board is a smaller timber like 190 x 35mm, which does not carry the weight of the rafters but provides a connection for them. The weight of the roof pushes out at the bottom of the rafters, so they must be coupled to ceiling joists and be tied with collar ties in the top third. It may be joined in the very centre, between rafter pairs, without requiring a support.   kellyjones00 (593)
 
 


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Rafters: Getting the angles right is important for structural strength and lack of movement. Create a template rafter with the angles of the plumb cut against the ridge board, of the birdsmouth against the wall top plate, and of the end where the fascia board will be affixed. Using dressed timber makes this process much easier.   kellyjones00 (593)

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Erecting the ridge beam on gable roof: Temporarily fix two vertical timber studs, longer than the roof height, at either end of the ridge beam position, with a short stud placed placed nearer the top for the ridge board to rest on. Screw the ridge board to these two studs. Then place the rafters.   kellyjones00 (593)

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Coupled vs. uncoupled rafters: Coupled rafters are securely fastened to ceiling joists and the wall plates, meaning the weight of the roof does not push the wall plates outwards. Uncoupled rafters, as in a cathedral ceiling, do not have this connection to the ceiling joists, and may only be used with a ridge beam that carries the weight of the roof.   kellyjones00 (593)

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A solution for a ridge beam treated as a ridge board: If a ridge member called a ridge beam, with some coupled and some uncoupled rafters, has been given smaller dimensions by a structural engineer that resemble a ridge board and is consequently joined halfway without a support underneath the join, a solution is to create a king post that sits under the join, or a nearby king post extending a strut up to the join.   kellyjones00 (593)

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