Doors and Windows - casings - tight 90 degree bends

Forum has been a huge help. I now have 14 rough openings re-fitted with new vinyl / double / argon. Now doing jamb extensions + trim.

Using a compound miter and finished 2 windows so far. The casing corner angels are tight but not air tight - the best corners have a visible 1/32 seem along them. Will paint fix this (I can check this myself)?

Question: is there any trick to make these 45 degree cuts air tight? Like undercutting by a degree so that only the surface touches?

Yes, if you do the top piece first and install it, cut the angle on your stiles and fit. If the angle needs adjusting, do it now by under or over cutting the angle. You may be left with a 1/16" gap where the bottom meets the stool, but it can be caulked. But, in reality, if your saw is properly set, making 45 degree angles around a window built in a jig at 45 degrees, should work without adjustment. Now, you talked about air tight, and I hope you were referring to the gap, and not the actual air coming in. You should insulate between the window and framing with non expanding foam or fiberglas (not packed too tightly), then trim installed.

I start off doing the head casing first. I don't back cut the 45* on the miter saw but I use a small hand plane to dress the back of the cut.

Once the head casing is in place I install the side casing (always cut to 45*) aligning and nailing at the miter joint first. That way I get it perfect tight every time.


If the jambs are off square a bit you can usually tweak the side casing as you nail it off. The reveal may vary a small amount but it will look better than a gap at the miter joint.

If the window or door is really out of level or square you may have to cut the miter at an odd angle

A good adjusted miter saw will make the job easier. If the wall is not exactly flush I use shims behind the trim or when nailed the miter will open up. If the miter is a little open caulk is the best bet and I always run caulk around the outside of the trim.

Chalk? You mean "chaulk" chaulk? The stuff you write on black boards with? That's an interesting idea.

No...no...no... "caulk"! Don't "chalk" it. "Caulk" seals cracks...fills gaps...

Done.

Did stay with uniform 45* cuts throughout but did not dress the backs of cuts with a hand plane. I just could not do this properly - I had not seen how it is done and the couple times I tried it just left too much of a gap at the outer or inner angle of the cut! So I gave up.

Biggest news to me was tweaking the casings - this worked a treat. Match it tight on the top and then just bend the casing to your reveal as I tacked the board down.

Thanks again! I'm ready to do more windows but I am finit!