Week 8 - Day 39
At end of the week we are preparing drawings to record the roof structure.
There is a lot of interest, but an unusual detail is the arrangement of the Purlins to the Principal Rafters, the basic skeleton of the roof structure that hold the roof together and support the slates.
The Purlins are the horizontal timbers that link between the Principal Rafters, there are many different ways that fasten the elements together, in the case of George Street Chapel a tusked tenon was used.
The diagam shows how the pattern of Purlins locks together the Principal Rafters.
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| The trusses run across the chapel supported on the outside walls. Each truss comprises a Tie beam, Principal Rafter, Collar and King Post. The Purlins run the length of the chapel between the Trusses. A tenon at the end of each Purlin passes through the Principal Rafter and is locked in place by a wedge. |
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| Underside of the Principal Rafter with a Purlin connected to it from each side. A tenon projects through a mortice hole and is locked in place with a wedge - commonly called a tusked tenon. |
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| The top face of the Principal Rafter and two Purlins. |
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| The side view from slightly below showing a tusked tenon with locking wedge - this is the protruding tenon of the Purling from the other side of the Principal Rafter. that joins the Principal Rafters. |
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