Monday, 27 February 2017

Dovetailed Collars

Using a raised collar to tie the rafters together in an oak truss gives a greater sense of space and headroom than a lower tie beam, but across wider spans the loading on the oak pegs can get too much for the structural engineer's liking. We often use dovetailed joints in heavy oak framing, and this is a good example of clever yet simple, well tested timber design; used where many might reach for a steel strap and metal fastenings instead.

The wedged dovetail joint shown here works well in green oak, and in conjunction with 2 large oak pegs allows us to work over larger spans.
The dry oak wedge will need tapping in more as the green oak timbers dry and shrink over the years, but its a massively strong joint from day one.


We have also built a couple of wide span trusses in slightly smaller section kiln dried oak, which allow us to use this classic dovetailed collar joint. Here the 3" deep collar is housed into the 4" rafter, and the 2 are offset slightly to allow the nice stepped reveal. The engineer here insisted on using stainless bolts to hold the joint together, but the overall aesthetic is pleasing and gave a really secure brace to the rafters. It's a very different material to work with compared to green, unseasoned oak, but it's rather nice to know there's now no shrinkage to consider, and the joint will stay as tight as the day it was cut.

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