Making my own external door frame

[disco_monkey79]

I've got a replacement door for my garage, but the hole is bigger than normal, so I need to make my own frame, rather than buying a ready-made one from B&Q (or similar).

As it's going on a pre-cast garage, there won't be any shoulders (if that's the correct term - where the header extends further than the jambs).

Because of this, should the mortices in the header and threshold be open on one side, i.e. should I essentially be cutting "u-shapes" in there? Or should the tenons on be surrounded on all sides by the wood of the mortice in the header/threshold?

I hope that makes some sense, and apologies if the terminology is wrong. Logic suggests it doesn't matter if the mortice is "open", as the weight of the door will pull the jamb tighter against the header/threshold. However, the screws that fix the jambs will be pulling the other way...

Your advice (and patience!) is much appreciated.

EDIT: FYI, this is a 30" door at the back, not the big one at the front

 

[Bradshaw Joinery]

Usually just mortice full jamb width or with setbacks if the mortice is in the rebate.

If your worried about the screws pulling it away pack behind the frame or use window screws with a 6.5mm hole through the wood and building. The screw then holds the position of the frane as it screws through both surfaces.