Filling large gaps between window/door frame and molding?

[Bert McMahan]

I have some drywall up on some walls that used to be thin panels, and the gaps between the walls and the window frames are pretty large- 1/4"-1/2" in areas depth-wise (aka, when looking at the window head on, the drywall is closer to you than the window frame by 1/4"). It seems to be variable in a few areas as well, so ripping a straight piece would work for some but not all.

I can think of a few ways to fill the area, but what's the easiest way to get a good fill? I'll be filling in the tiny gaps with caulk, so it doesn't have to be perfect, but the closer the better.

I've thought about holding up a thin piece, marking the line with a pencil, then following it on the bandsaw, or putting up a reasonably thick piece and using a power hand planer to get it down to the right thickness. I suppose there are lots of ways to skin a cat, but does anyone have any thoughts? Any pitfalls I should watch out for regarding wood movement or similar?

[Jim Becker]

Backer rod (a flexible foam product) is typically used to fill larger gaps like this prior to caulk. You could also use non-expanding foam sealant from a spray can. Both of these methods assume the area will likely be covered with trim, etc., although compound or plaster could cover it, albeit with risk of cracking.

[Bert McMahan]

That may work, but I think I need an actual structural piece to nail into for some of the areas. Since the original wall was thin panels, the new drywall is thicker, so I'm trying to take up the gap on the "window" side of the joint. I may be able to caulk it but I don't think I have quite enough to nail into, so I need something rigid to take up the space.

[Charlie Velasquez]

That may work, but I think I need an actual structural piece to nail into for some of the areas. Since the original wall was thin panels, the new drywall is thicker, so I'm trying to take up the gap on the "window" side of the joint. I may be able to caulk it but I don't think I have quite enough to nail into, so I need something rigid to take up the space.

If I understand, you are trying to bring the edge of the window frame even with the sheetrock so the window trim sits flush with no gaps.
If so...what I did when working on my first house, built about 1890, when they used real 2x4’s and nothing was square or flush.

First step is ensure the Sheetrock around the window is co-planar, a long straight edge or level will work.
If not co-planar, find the “high spot”, then carefully pull the sheetrock with a putty knife till co-planar, shim under it (playing cards work well) where needed, drive a screw to stabilize it.

Next, add wooden lath to the edge of the window frame. I first ripped a bit to make sure I had a square edge, then eased the facing edge with some sandpaper.
I glued and pinned the lath, then drove the pins down after the glue set.

Then plane the lathe flush.

[Tom Bender]

Be very careful about flexing your window frames if you want them to open.