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Gap to wide for caulking

I am trying to do some caulking between 2 surfaces where the gap is too wide to caulk. I read somewhere where, when this occurs, you need to use some backing of some kind to fill in the gap and then caulk. Can someone tell me what to use and how to do this (aside from using spray foam)

Re: Gap to wide for caulking

One good product is the backer rod (made out of foam). You push into gaps wider than 1/4" with a putty knife, then caulk on top of it.

Re: Gap to wide for caulking

How wide is too wide? Their are still limitations for width depending on the type of caulk.
The one good thing about backer rod placed at the correct depth is that it will maximize the attachment of the caulk at the edges and minimize the depth in the middle. This allows the caulk the stretch and still stay attached at the edges. This is more important in exterior locations where temperature extremes will cause more movement in the materials being caulked.

Re: Gap to wide for caulking

I've used 1 1/2" backer rod for large building expansion joints before but that was many years ago and I was working for someone else so I don't know where they got it. It works as described above so there is no real limit to width, only a point where practicality dictates lessening the gap some other way before caulking.


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