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I need to make my own door jambs from 1x lumber because I have a salvaged antique transom (the whole door is actually restored antique) and couldn't find pre-made ones at Lowes/HD that are long enough. I did notice that the regular sized ones had a notch on each side jamb to set the header piece. I was wondering what the advantage of that is over simply laying the header piece on top of the side pieces. I can probably make notches in my homemade jamb but it would be easier to avoid the trouble and just set it on top of the side jambs.

Can anyone elaborate on the need to make the notches?

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Not sure what you are referring to by a "header piece" - can you post a pic? – Steven Jun 9 '13 at 0:48
The header jamb piece. The jamb has two sides and a header. – amphibient Jun 9 '13 at 0:57
as in the header piece of the jamb – amphibient Jun 9 '13 at 0:58
For clarification, it sounds like you're referring to the door casing, and not the rough framing (a header is typically a reference to the rough framing, along with the jack and king studs). – BMitch Jun 9 '13 at 1:21
casing is trim that is laid onto drywall around the door. the JAMB goes in between the door and the rough framing, it is what the stopper is nailed onto and holds a hole for the lock – amphibient Jun 9 '13 at 1:27
up vote 2 down vote accepted

The rabbet joint you refer to is done for strength, its a mini jack/king, where the side jambs support the head jamb.

Your plan for a butt joint will be sufficient.

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The rabbet joint on a door jamb has a couple of other important characteristics to take into account too. With regard to the strength issue the rabbet allows nailing of the corner from the top and side for strength and ensures that the corner stays closed up. The other purpose served by the rabbet is that if over time the door jamb shifts out of square some the corner will stay looking closed instead of opening to a gap that looks open. Both are important factors - I would never build a door jamb without the rabbet. Note that window frames are built with rabbets as well for the same reasons. – Michael Karas Jun 9 '13 at 13:01

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