Corbeled Controversy +1

[Kyle Kubs]

It's funny how a recent thread can make you look twice at something or even change your perspective. After John's recent post I took a harder look at this fireplace and noticed the corbeled brick was a little stretched. By stretched I mean the edge of one brick looks like its hardly sitting on the lower brick at all. It was a 1977 house and there was no sign it's had any issues from this but isn't there a limit to the overlay, or lack there of?

In regards to John's post, you'll notice quite a bit of creosote on the corbeled area, the flu & rest of the smoke chamber was practically spotless.

Click to Enlarge

78.41 KB

[Jeremy]

It's funny how a recent thread can make you look twice at something or even change your perspective. After John's recent post I took a harder look at this fireplace and noticed the corbeled brick was a little stretched. By stretched I mean the edge of one brick looks like its hardly sitting on the lower brick at all. It was a 1977 house and there was no sign it's had any issues from this but isn't there a limit to the overlay, or lack there of?

In regards to John's post, you'll notice quite a bit of creosote on the corbeled area, the flu & rest of the smoke chamber was practically spotless.

Click to Enlarge

78.41 KB

I believe as a rule of thumb, the projection of the brick should be no more than half the height of the course.

[AHI in AR]

Technically, what you have there is not a corbel. A corbel is where the lower courses of brick support an area of masonry (or other part of the structure) that gets wider as it goes up. You have a reverse corbel. I'm sure it has a proper name, but I'm too lazy at the moment to look it up.

Structurally, (and others may disagree) I think what's happening is that the outer ends of the "corbelled" brick are being restrained (clamped, in effect) by the front and rear walls of the chimney. While the exposure of the brick exceeds a properly done corbel, keep in mind that we're not seeing a corbel here. I believe that the interior edges aren't likely to fail due to what minimal load they carry. Admittedly, the adhesive strength of mortar is minimal, but the brick load is cantilevered to such a degree that I don't foresee any real structural problems -- assuming that all other factors remain the same. That said, if you introduce any water intrusion into the chimney and you have to deal with freeze damage, all bets are off.

Lastly, the brick should be parged....

[Chad Fabry]

The stepped courses are supported at their ends by the walls of the chimney; they bear vertically there and the centers can't fall because mortar isn't compressible.

They don't need the support of the stepped course below and each of the stepped courses would be fine on its own.

[Darren]

Lastly, the brick should be parged....

Actually, besides being parged, the bricks should be solid or grouted solid.