Why does nobody talk about the difference between a 3/8 and a 1/2 inch mortar joint on a chimney?

I built two identical chimneys for a client in Boise, one with each size, and the 3/8 joint cracked within a year while the 1/2 inch is still solid. What's your go-to joint size for freeze-thaw areas?

2 comments

2 Comments

evanh274d ago

Your point about the 3/8 joint cracking is really telling. I always used the smaller joint thinking it was stronger, but your real world test changes my mind. For freeze thaw cycles, I'm going with the half inch from now on.

lisa6714d ago

Yeah, the "real world test" part is key. I read a whole thing from a mason in Minnesota, @evanh27, where he tracked repairs over ten winters. Every 3/8 joint on a north-facing wall failed. The half-inch joints held, even with cheaper bricks. It wasn't about strength, but letting the material move. That small extra space makes all the difference when the water inside expands.