thanks for the replies--Jim--yes standard layout rule 2" off the layout side-- right--but when you look at the rafter laying beside the one in question notice that the cog seems to be centered,
Timbeal--cog--I suppose does have the same function as a tenon but in a different way, for instance it is not pinned, but it does fit into a mortise cut into the plate, other than that I really don't know what to say.
Cogs do come in different forms, with smaller rafters the cogs will extend right across the whole base of the rafter, in the case of these fairly large rafters the cogs are situated as you see. One thing that struck me though was that for the job that they were doing these cogs seem fairly small, and being shaped on the face would have been harder to cut without an adze.
The thrust fase of the cog no matter where they are used are always are at a right angle to the foot of the base of the rafter, and are a little tricky to fashion. I used an adze as mentioned above predominantly to shape the cogs on the feet of the reconstructed buildings that i was involved with at UCV.
Whenever you are examining existing buildings for reconstruction it is almost impossible to know for sure just what the cog or tenon on a hidden joint truly looks like sometimes you have to make decision based on your experience or if you are lucky you may be able to come across it in research document or historic book written by an author dating to the period that you are working in.
