How to fill this gap between window and brick wall? (too deep for backer rod?)

brickcaulkgapwindows

I'm renovating the windows of my 1936 house. Doing the first window and I notice there's a big gap between the window frame and the wall and I was wondering the best way to fill it in so I can caulk it.

I bought a 1/2 inch backer rod, and that seems the right size, HOWEVER, the hole is really deep and gets a lot wider about 1/2 inch in. This means the backer rod keeps falling through the gap when I try to install it.

Is there something else I can use to fill the gap or should I put something behind the backer rod to stop it falling through?

Any advice here is very much appreciated!

Here's a picture of the gap:
gap next to window

And here's a picture of the gap with a backer rod 'falling through' the gap where it moves around freely:

falling through gap

Best Answer

If I were you I'd try this

  1. Fill the gap with expanding foam. Give it a full day to cure (it expands so doing it too soon will potentially leave it uneven) and cut any bubbles off so it's flush
  2. Buy some really good outdoor (oil based) caulk. The local big box sells something called Quad but if that's not available you can probably find something similar in grade. Caulk over the joint (1/2 inch is about the limit so be liberal). That will seal the window and protect the foam.
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