It’s a long-standing bit of wisdom among designers that plumbers will cut anything that is in their way when installing pipe. Give a plumber a saw, and structural engineers expect him to cut through the nearest piece of structure. I need to correct one mis-perception: it’s not just plumbers. (And, of course, it’s not all plumbers.)
The picture above shows some very nice wood joists in a 1920s rowhouse, as exposed at the beginning of a project after plaster demolition. I don’t know what was installed there – maybe a light fixture, maybe a small air-handler, maybe something else – but whoever installed it carefully cut a joist in two and left it like that. They also drilled a hole at the bottom of the next joist over, well below the recommended portion of the joist for drilling holes. They also cut one of the two joists that serve as a trimmer.
Notice how straight everything is? There was no sag, no indication that there was a problem here. One joist effectively removed, a trimmer’s capacity cut in half, and another joist weakened, and nothing bad happened because (a) the subfloor served to distribute load from those joists to their neighbors, (b) the actual average live load in a private house is usually on the order of one-quarter of the code load, and (c) the old joists may have been a bit overdesigned to begin with. Of course, this line of thinking inevitably leads to someone saying “See? It wasn’t so bad”, to which my response is that Russian roulette is safe until it’s not.
My experience is that interior demo, exposing the structure, inevitably shows some problems like this. In other words, every building has some minor structural flaws that are unsafe in the sense that there is an overloaded member or a bad load path. (Another common problem in rowhouses is circular framing, where a stair header carrying a bunch of floor joists is carried by a trimmer that is carried at the end opposite the stair by a chimney header, which is carried by a trimmer that is carried by the stair header. Oops.) Which is why when we uncover a problem like this, it’s not a cause for panic: it’s simply another thing that has to be fixed.
