Shop made door jamb

[Pappy]

One of the things I have found wrong with my house is that the rough-in for the front door is too big. That, coupled with not wanting to pay the outragous price for a commercial jamb, has me thinking of making my own from 2x6.

My problem with the store bought jambs, other than the price, is 2-fold.

First, as I said, I need something of thicker stock to fill the opening more. Even with the extra thickness of the 2x6, I may add a 1/4" ply filler strip on each side of the rough opening.

Second is the only ones I have found are made from finger jointed boards. This allows cheap manufacturing from what would othrewise be scrap. Even this wouldn't bother me if the joints were glued with a water proof glue.

The face cuts/profile is simple enough to cut. Essentially a 2" wide x 1/2" deep rabbet on the door side with a groove at the inside corner to hold weather stripping in place. I plan to use a magnetic weather strip to seal against the door face and may add the folded metal strips to seal against the edges.

I do have one question. Ther are grooves cut in the back of the jambs to help prevent cupping. Looking at the in grain of the stock, which side would I want the grooves on to be most effective?

[mpc]

Are those grooves there to reduce cupping? Or are they more like the shallow depression in base board molding that makes it easier to get the molding edges against not-quite-perfect walls? If the molding backsides were dead flat, any bump in the walls would mean the edges of the moldings would sit proud of the wall. It's similar to making a slight back-cut in miters or other woodworking pieces to help guarantee a tight fit at visible joints.

I've seen quite a few door jambs mounted "off" the support structure; the attachment screws/nails just pass through shims to adjust the spacing. The add-on decorative trim then covers the gaps. My house has many interior doors this way and I'd bet the front door is that way as well... I just haven't pulled it apart (yet) like I did on some of the interior doors to change the trim profiles. For an external door, filling this gap with plywood or spray-in expanding foam would improve the insulation qualities. Just like mounting windows... shims and fill in the gaps with insulation are the typical technique.

As for your question on what side of a board is best to make grooves to minimize cupping... no idea. I'd think though keeping paint on both sides is more important - minimizing the difference in moisture penetration rates just as you'd do on any woodworking project.

mpc

[Pappy]

The rough-in for a door or window is supposed to be bigger than the jamb to allow for shimming and squaring. This one is just too wide. With a standard jamb installed centered in the opening there is about 3/4" on each side. Since the jamb is not normally going to be against the structural members the grooves wouldn't serve the same purpose as the depression in the back of trim boards.

[dbhost]

You know, I never gave that much thought. That is a VERY good question! I am gonna watch this thread now because I wanna know...

[cabinetman]

Since the jamb is not normally going to be against the structural members the grooves wouldn't serve the same purpose as the depression in the back of trim boards.

That's pretty much the way it is. You do need some shim space. My front door has a jamb that's 2x's. There is concrete block behind. To install`a new 3'0" door, there's no room for a prehung.

The 2x's have no relief behind. No need for that there. Once shimmed, and fastened, there shouldn't be any movement, other than across the grain if at all. If you are painting, then the fastener holes can be filled and sanded. If you are doing a wood finish, You could fasten behind the door stop that you add.

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[Pappy]

CMAn, a door stop isn't needed on an exterior jamb. The shoulder of the rabbet serves as the stop. I could add the weather strip aftr the jamb is installed and use it to hide the fasteners.

The finish will be paint showing on the inside and stained on the outside. The stained part will be topped with spar varnish. To prevent any wicking effect I plan to seal the bottom end grain and up about 6" with fiberglass resin.

[cabinetman]

CMAn, a door stop isn't needed on an exterior jamb. The shoulder of the rabbet serves as the stop. I could add the weather strip aftr the jamb is installed and use it to hide the fasteners.

If you're making your own, you can make it any way you want. The door can sit against a rabbet that you mill. Or, you can add a stop as in this detail. If you hang the door to a machined rabbet, any deviation in the plane of the door will be apparent in the spacing to the rabbet (which is in actuality a stop). A well fitted stop (with weatherstrip in groove)added after the door is hung produces a good seal. This is how my exterior doors were originally done.

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[LarryG]

I do have one question. Ther are grooves cut in the back of the jambs to help prevent cupping. Looking at the in grain of the stock, which side would I want the grooves on to be most effective?

Generally speaking (because there are exceptions to everything), a board will cup toward the bark side of the tree. If you look at the end grain of a flat-sawn board, cupping will tend to flatten out the grain rings. IME the grooves are usually on the face toward which the board will cup:



The wood moves most on the face where the rings are most widely spaced, so I presume cutting the stress-relieving grooves on that face is more effective at reducing cupping. Also, the heartwood side of the grain usually has a more pleasing appearance than the bark side, but that factor is probably given more consideration for something like expensive oak flooring than for an inexpensive pine door frame.

[Pappy]

After too much time looking for info/pics on the web, I couldn't find a difinitive answer. Most jambs I found good pics shoing end grain were either finger jointed or glued up from vertical strips. Stopped by JeldWen doors yesterday and they didn't know. The one cabinet shop I deliver to that has done a lot of custom doors uses all kiln dried hardwoods and doesn't make the relief cuts.

Bottom line, still not sure which side is best to control cupping in a solid jamb, but I am going to go with Larry's suggestion.