A while back I started ran a series of blog posts1 on inherent incompatibilities in building design that we all politely pretend don’t exist. Today’s is almost but not quite on that list: installing modern plumbing in a wood-joist building. Obviously, that is something that is done all the time, so it’s possible…on the other hand, it is quite easy to get designers and carpenters to tell stories about the havoc that plumbers have wreaked using saws. Here’s an illustration2:
The plumbing risers are at the top, just to the left of the light-gage steel track: a big black drain pipe and two small copper supply pipes. All the stuff in the floor is drain branches to take water from fixtures to the riser.
The specific reason I was there was that a previous plumber – not the one working on the construction project that was taking place in 2021 – had cut the joists, and there was a question as to whether that was a structural problem. On a semi-related note, I’m impressed by the way that the plumber has squeezed a bunch of large (for a residence) diameter pipes into the narrow inter-joist space.
Why do plumbers cut joists? Because there is typically little or no excess space in a joist floor (including the subfloor and ceiling) to run pipe, because the inter-joist spaces are somewhere between 10 and 14 inches wide and so require convoluted layouts like the one in the photo, and because modern bath and kitchen layouts do not always have fixtures clustered near the risers. Meanwhile the joist geometry is determined by purely non-plumbing concerns like the maximum possible span of subfloor, and keeping the joists spaced closely enough that their required size does not get too big.
We have been wood-joist floors for much longer than we have been installing indoor plumbing. The two systems have been developed with little regard for one another, and it shows.
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