Friday, August 27, 2010

Scarf

The roof project requires a few timbers longer than I have on hand, so I have to make four scarf joints like this one.
These are about the most complex joints I get into. The end on the left shows how a little nose of timber grips into a recess in the corresponding part. The key to the joint is the diagonal edge to the two flats of the scarf surfaces. The joint starts off slack, but as you drive the two sides together the diagonal center forces the two noses into the mating part and makes for a very tight bond. Set squares in japan are commonly 15mm wide, so the width of the square can be used to draw the nose and recess as well as the two mating flats of the scarf.
The lower picture shows the joint when driven home as well as a full scale drawing I did to plan it out for this size of timber. These pieces will hold up the rafters and I will aim to get the joints between two rafters. The joint can be oriented either way I think, but if you want to assemble it on site orient it like this so that it can be driven in on top of any tenons the timber mates with. This is also a bit of a show off if the roof timbers are visible from below. In the upper picture the piece on the right would have to be laid first, then the left part driven home on top of it. I have three mortises to cut in this piece now, then I will cut off the excess timber and use that for struts in the roof.