How to Caulk Large Gaps


Have you ever been in a position where you had an extraordinarily large gap to fill and you were really not sure how to do it?

This may have been the case with wood, concrete, on the interior, or on the exterior, but the challenge was still the same!


A gap large enough to seemingly be able to fit three tubes of caulk, but incorporating that particular solution just did not seem correct.

I used to wonder about the exact same thing while in this position.


So what is one to do when confronted with this daunting quandary?

Enter backer rod.

Backer rod is a foam-type, cylindrical “string” that comes in various diameters.

It could be ½”, ¾”, 1”, etc.

How backer rod is used, is by taking the most appropriate thickness of backer rod and with the help of a putty knife or a 5-in-1, stuffing it into the crack that you are attempting to fill, leaving a very small portion of the gap unfilled.

Once the backer rod is set in place, you can then caulk the gap as you normally would.

Once the caulk is dry, you can paint or finish over it as you may have already planned to do.

Keep in mind this is meant in situations where caulk would normally be used, it is not meant to be a solution for gaps that were ultra-wide and would require finishing with a material other than some type of caulk.

I have seen backer rod used for a number of different scenarios ranging from gaps in trim, to cement, to window and door applications and on and on.

If for some reason, backer rod is not an option, you may indeed have to go the multiple layer of caulk route.

Though certainly not what I would prefer, one could also fill the gap with layers of caulk, letting the caulk dry or set each time before covering it again with another layer of caulk.

This would be a painstakingly slow approach but, in theory, could get you to where you need to be.


When using backer rod, it is important to be sure to use a backer rod whose width is a tad wider than the crack you are using it to fill.

Backer rod can easily be cut with a utility knife or a razor to trim it to the length that is needed.


It is important to not fall into a trap of thinking that backer rod is ok to solely stuff and leave in a gap without filling around it.

By using the caulk, you will not only be able to form a proper seal around the backer rod, but you will also set yourself up for being able to leave a smooth finish over the gap itself.

Seeing what at first appears as a monstrous gap can be a little bit of a head scratcher as to how to correctly fill it, having backer rod at your disposal, however, will make this initial discouraging nature of the task quickly go away as fast as the gap that you are filling in the process.




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