1956 home with original aluminum casement windows. In the master bedroom, two windows (each 36”H x 48”W) meet at a corner in what appears to be a custom join. Cool window, but broken in multiple areas, prone to mold and not energy efficient. Stucco exterior.

I cut the stucco to expose the mail fin, and bought two standard PlyGem vinyl windows. My question is, how do I join the windows at the corner?

Other houses in my neighborhood have the same build, but with upgraded windows (retrofit and new construction). For the new construction replacements, there appears to be a plastic/other corner cap (the windows are not meeting at a stud support).

The video below shows my scenario where they use a corner mullion/cap that inserts into the nail fin slots o of a certain kind of window. I can’t find that type of mullion anywhere.


Anyone know of a solution/product here? I will be calling PlyGem but suspect I won’t get much help there.

On another note, since the custom join is so small, and neighboring houses with new construction windows don’t have corner stud supports, i presume the existing join is not load bearing. May have to open up the walls to see.

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  • All outside walls are load bearing. The window corner does not seem to be able to hold a load. Even with steel or cement headers I would expect a decent load bearing structure in that corner and I do not see one. See how your neighbours did it.
    – crip659
    Jul 24, 2023 at 20:21
  • 1
    Corner windows exist with no corner post. But you can't assume your house was built that way. The big steel plate at the foot of the steel corner post in the first picture makes me wonder if it is supporting the roof. You need to figure this out before you select windows. If in fact there is a cantilevered beam above the window so there is no weight on the corner post, you just need to buy a corner window system from the maker, rather than try to retrofit it. BTW this forum doesn't permit specific product recommendations.
    – jay613
    Aug 24, 2023 at 13:11
  • florianglass.com/union-city-sunrooms-glass-service - no affiliation, other than at some point our solarium was on their webpage, as they weren't even done printing instructions for their parts kits yet 20y ago, let alone had any pictures to post. Remember what a Wendy's used to look like; yeah, it's that stuff. It has DiY rubber glazing strips you smash in with the back of a wooden hammer, otherwise it wouldn't be. And... it leaks anyway.
    – Mazura
    Aug 24, 2023 at 20:36
  • @crip659 Looks like there is a small steel post in the corner (pic #1), probably 2" box tube and looks like it has a plate welded on the bottom. Still looks somewhat undersized though.
    – matt.
    Aug 24, 2023 at 23:41

1 Answer 1


Once you've figured out the support issue, which might be achieved by running steel up the centre of your corner post [not my forte, ask a local specialist], if you replaced those with double-glazed units, you could set them slightly further out, keeping the indoor appearance identical.

You've got a couple of inches of room to play with on the outside. Any glazing company will have set pieces to turn corners.
You can also get modern 'fake' leaded glass to match the existing panes exactly. See How thick should a stained glass window in a door be?

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