Oxfordshire Woodland Group

There is a misconception amongst some timber-framers that a mortice and tenon joint should form a perfect fit and historically that is far from the truth. What really matters is that shoulders should mate perfectly and then the peg be able to draw the joint together. To acheive this the standard practice when working with green timber is to leave a deliberate gap of about 0.25" between the end of the tenon and the bottom of the mortice to ensure that the tenon can pull up fully and so during the drying process that cross grain shrinkage does not result in the tenon bottoming out in the mortice thereby beginning to load the peg in reverse which will reduce shoulder bearing load and consequent friction generated by same.

When cutting a mortice in green wood a perfect fit tenon will in due course cause a check to develop in the mating beam especially if this is a boxed heart timber. The standard traditional practice is to introduce "slop" in the joint by making the mortice cheeks bell mouthed for example a 1.25" hardwood tenon might have a 1.375" wide mortice opening with the "slop" about the width of a thick coin.

The bottom of the mortice could be up to 2" wide to allow for shrinkage, ease of assembly and alignment during assembly and certainly should not be parallel.

Chilterns