That’s a photograph taken in an attic of a mid-1800s house. The good news is that the wood is dry as a bone, so it’s not rotting. The maybe bad news is that brick chimney…isn’t quite straight.
The reason for the chimney running diagonally is, simply, fakery. The fireplace below didn’t, for whatever interior layout reasons, line up with where the builder though the visible exterior chimney should be, so he moved the chimney over a bit by corbelling the brick. In the language of the École des Beaux Arts, the plan doesn’t match the elevation. It doesn’t match for a good reason, which is that people care about the appearance of both the interior and exterior of their houses, and if they have to fudge a chimney location to get both to work, they will.
If you see a brick wall that far out of plumb, you assume it’s going to collapse, so why am I talking like this chimney is maybe okay? In short the chimney is not stable on its own, so it has to be supported by the wood structure of the roof. If we assume that the roof is providing resistance against the chimney moving to the left (by rotating, most likely) then the chimney works, structurally. But that assumption runs up against another given of new-building design: we’re not supposed to support large masses of masonry on wood.
There’s no physical reason why we can’t support masonry on wood but it seems wrong. First, one of the advantages of masonry is that it’s non-flammable, but that advantage disappears if the masonry is supported on flammable structure. Second, masonry seems much, much stronger than wood. This appearance is based on the fact that masonry is much stronger in compression than wood and that we tend to use masonry in much larger cross-sections. Comparing apples to apples – comparing similar sized sections – leads to the conclusions that (a) unreinforced brick masonry is significantly weaker in tension, and therefore weaker in bending, and (b) it’s not much stronger in compression, if it is at all. In other words, it’s not necessarily surprising that a wood-framed roof can support the lateral kick of a tilted brick chimney.
Ultimately, all of this discussion is after-the-fact rationalization. The chimney and the roof are both performing well, with no signs of movement. I trust that observation more than I trust a numerical analysis of this structure.
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