On Mon, 17 Dec 2007 17:48:49 -0000, "ALex" wrote:

what make of garage door are you fitting,if a torsion spring do not use a
half lap joint as they are too weak.a through and through mortice and tenon
is normal.

Thank you everyone for your help so far.

I'm a bit confused because I've got one vote for a half lap joint and
another against it. Can anyone break the deadlock?

I'm not sure that the existing frame has a mortice. I think it's just
two bits of wood screwed together; I think that's known as a full lap?
My worry was that this might twist

You can buy ready made frames in timber

I have seen these for sale but they cost £80, whereas the wood to make
them probably costs a quarter of this.

Or if a new door pay a little extra and have a metal frame door comes
prefitted and it makes installation much easier.

That was my first choice but they are not available until February.