Caulk Caulking crack around fireplace

Published on June 8th, 2018 | by Henri


Caulking around fireplace

Q.  The cold weather over the past winter caused a great draft from the space between the brick fireplace and the abutting wood. The wood is part of the shelving. What would you recommend as a caulk or filler to stop this draft?

A.  I assume that the crack is rather narrow and not too deep – only as deep as the thickness of the siding.

For a successful and lasting caulked joint, first make sure that both of the opposite surfaces are clean and free of loose material. You may need to use an old toothbrush to do this.

Then, very carefully, insert a backer rod, making sure that you do not break its skin. Select one of the right size to fit the crack, no bigger or smaller. Push the backer rod gently into the crack with a wooden stick, but only as deep as the width of the crack. The purpose of the backer rod is to ensure that the caulking bead will adhere only to the opposite surfaces of the crack. It should not adhere to any material in the bottom of the crack, as the caulking compound should be allowed to to stretch and compress without interference.

Use a polyurethane caulking compound and apply it to the crack, but don’t overfill the crack. When done, tool the bead of caulking with a gloved finger dampened with your saliva. The end result should be a bead that is slightly concave.

You should be able to buy a backer rod and polyurethane caulking in specialty masonry stores, and in large box stores in their masonry department.
I have used Sikaflex-1a for decades and consider it from experience to be the best polyurethane caulking/sealant compound. As far as I remember, it was the only product of this type available in the 1950s. Since then, other brands have been on the market, and I have tried a few, but I have always returned to Sikaflex-1a.

Some stores may carry essentially the same product with a slightly different name, such as Sikaflex Compound. You can also order on-line.  A.H. Harris, (, which has stores in several eastern states, will ship anywhere.

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