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Cossack 02-28-2010 04:48 PM

Door jamb grooves necessary?
Manufactured pine door jambs I removed have three grooves running the length of the rear of the board. Do these grooves actually serve a necessary purpose? I made the opening height higher, and I will have to make my own door jambs out of clear pine. Do I have to route three grooves on the back of my boards?

jaros bros. 02-28-2010 05:19 PM

Sounds like the grooves for the weatherstripping. You don't have to use the insert type of weatherstripping but it would be easiest to do so. If you have a tablesaw that should be hard to do with a pass or two. Try a piece of the weatherstripping out to make sure it fits nicely. It should be snug but not too tight so that you can replace it if you want.

oh'mike 02-28-2010 06:10 PM

Those grooves are machined on the back of most larger trim(base-casing and jambs.)they are there to make the trims stay flat and add a little flexibility.

You do not need to add them to your custom jambs. You will be using nice flat stock ,I'm sure.


Shamus 02-28-2010 06:10 PM

I haven't installed a prehung door. We make our own casing/framing. They may be there to allow for easyer planing/fitting of the door into the opening or as mentioned above for weather stripping. I do not know.

In my experience we do this on wood that might be exposed to dampness, we backcut to relieve grain stress and prevent warping.

Maybe someone else will chime in with some experience and a better answer.

tpolk 02-28-2010 06:16 PM

keeps them from cupping. did on all our flat jambs at a custom millwork shop

Cossack 03-01-2010 04:31 PM

Thanks to all who answered. This is an interior door, so I do not think it has to do with weather stripping. They are the same grooves you see on the back of pre-hung door jamb.

One person said they are to stop cupping. Is it necessary for me to route these in or can I just install the jamb. Yes, the boards I selected are straight and flat.

Thanks again.

jlhaslip 03-01-2010 09:33 PM

Dry would be the critical aspect of keeping them flat.
If in doubt as to the dryness, I would run them across a table saw and make saw kerfs down the length of the back.
Approximately 1/3rd of the material thickness. (minimum)

stevesandsons 03-04-2010 09:39 AM

Several posters are correct; these are kerfs that keep the jambs straight. Remember all wood come from round trees and want to return that way. My suggestion is to kerf it, if not over time they will have the tendency to "try" to return to their natural state.

mr makita 03-04-2010 09:56 AM

kerfs are the right answer
Saw kerfs are the right answer. and the reasons given are correct as well. round trees dont want to be flat boards and the trees we are getting lumber from is smaller and smaller. The old norwegian I apprenticed with an eon ago would have told you to prime the backside of your jambs for the same reason, to help prevent cupping.

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