Re: 42" build in Virginia
I would try to use full bricks on either end, no notch cuts - and use two bricks tying together either end of your vent opening. you want a good bond between what is essentially two arches standing side by side. If the bond between them breaks your front arch becomes freestanding. you can also tie together across the top with your rectangle-->flue transition

oh, and terracotta works fine. Many builders use that. Do a search and you will fine examples. 9 x 12 will work, it just may be a little large for the vent in terms of looks. I have seen builders cut them down by cutting down the middle and removing a few inches of vent, then mortaring back together in order to have a more reasonably sized flue
I would try to use full bricks on either end, no notch cuts - and use two bricks tying together either end of your vent opening. you want a good bond between what is essentially two arches standing side by side. If the bond between them breaks your front arch becomes freestanding. you can also tie together across the top with your rectangle-->flue transition
oh, and terracotta works fine. Many builders use that. Do a search and you will fine examples. 9 x 12 will work, it just may be a little large for the vent in terms of looks. I have seen builders cut them down by cutting down the middle and removing a few inches of vent, then mortaring back together in order to have a more reasonably sized flue
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