Recently I endeavored to renovate a bathroom. There was some apparent leakage in tub that manifested it’s self as a stain in the ceiling below. We pulled the ceiling down to access the pipes, and in the process discovered quite a mess. It’s no secret that plumbers are often notorious for hacking through floor joist to lay pipe, but the mess that this plumber made was pretty unbelievable. it turns out he hacked thought all 4 of the floor joist that were supporting the tub for the toilet pipe to fit through. Rather than cut a hole, he notched one joist so far that only an inch of material was left. The result was the remaining joists provided little to no support for the bathtub and bathroom floor. Since we will be putting a layer of plywood, a layer of concrete board, and tile atop the existing floor, we figured it would be a good idea to over engineer and sister all four joists. The existing joists were 2 x 8 x 16, actual dimensions (lumber used to be actual dimensions). The new 2 x 8 x 16 are actually 1.5 x 7.5 x 16 so it makes it easy to fit them up next to the existing joists. We pre- drilled 10 holes, then used construction cement and 3″ lag bolts to “glue and screw” each joist. Where the pipes were involved, we made a shallow notch and capped the top with a specially designed “mending plate” that sits over the top of the pipe. The resulting floor is strong enough to hold 4 bathtubs. If your interested in loads and spans, you can check out this pdf from the american wood councel or this online calculator
