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Switched from a 3/8 inch mortar joint to a 1/2 inch on a big chimney job

I always thought a tighter joint looked cleaner and was just as strong. On this big chimney rebuild in Springfield, the foreman insisted we use a half inch joint for the firebrick liner. After the first day, I saw why. The wider joint gave us way more room to work the mortar in fully behind the brick, leaving no voids. It also handled the heat expansion better without cracking. We finished the whole stack in three days with zero callbacks. Anyone else find a wider joint works better for high heat areas?

3 comments

3 Comments

miles_sanchez1mo agoMost Upvoted

Guess I'm just not seeing the big deal here. A quarter inch difference in a joint feels like splitting hairs. If the brick is laid right and the mortar is mixed right, it should hold up fine either way. Heat expansion is real, but most chimneys from the last fifty years have thin joints and they're still standing. Sounds like maybe the foreman just liked the way it looked or had an old habit.

richard_dixon1mo ago

You think a quarter inch doesn't matter? It's the difference between a repair and a rebuild.

quinn_wood1mo ago

Honestly, a fifty year old chimney with thin joints is a ticking time bomb. I've seen three of them fail just last year in my area, all from cracked mortar that let heat into the outer wall. That quarter inch is the difference between a joint that can flex and one that just splits. Your foreman wasn't being picky, he was saving you from a rebuild in five years.