I have a bunch of doorways to make jambs and casing for. These are not going to have actual doors hung, just trimmed out. These are standard 2x4 framed walls with finished gypsum board surfaces.

I'm sure there will be some irregularity in the wall thickness at these doorway openings and surface variations. What's a good guideline for what measurement to use for the jambs?

(Assume that the walls are as sanded flat as they're going to get.)


  • Large: Use the maximum thickness from all the way around the opening. May have to caulk/fill between casing & walls.

  • Medium: Subtract the relief at the back of the casing from the max measurement

  • Small: Use a measurement of minimal wall thickness, and caulk between the casing & jambs

These are just preliminary thoughts... other options welcome!

I suspect that caulk would be less noticeable in option 1 than 3. Option 2 probably requires caulking both sides of the casing. (Maybe that is inevitable).

This will be painted so the caulk or filler itself is not a problem in that sense.

2 Answers 2


First, cut a measuring jig from half-inch plywood. Make it U-shaped, 12 by 12 inches with a large notch to match the wall thickness. Use the jig to quickly scan each door opening. If the notch is cut medium, then you can gauge thin wall (jig fits loosely) and medium wall (jig just slides over.) A thicker wall will prevent the jig from sliding over. Then decide if you want every jamb the same exact width, or if you might fine-tune the widths for each opening. I would err on slightly larger than smaller widths. Then the casings would always be parallel to the wall surface. The slightly larger gap is easily caulked.

It is best to caulk both sides of each casing strip. Even when tiny, a black line aways shows, and paint is inconsistent for covering the gap.


I have installed frames in walls that even vary in width over the length of the jambs. The trick I have used is to make the jambs too wide, dry assemble the jambs with a single nail to loosely hold the head and jambs together, and set them in place. I then score a line on the jambs to match the contour of each wall edge over the perimeter of the opening. I can then trim or plane the jambs to the exact width necessary. The only problem I have found is if I trim the jambs with hardwood that is to thick to gently deflect to the contour.

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