Our bathroom was partially demo’d for renovation. We hired a plumber, who came highly recommended, to add the plumbing for a salvaged clawfoot tub. While the plumber was out, his assistant decided to cut through floor joists directly under the tub to accommodate the new drainpipe! Sadly, we don’t use that plumber anymore. —John and Erna McCants
Floor joists help carry the weight of the house and its contents by distributing loads to structural beams, girders, headers, and sills. A 300-pound tub (never mind the weight of the water, when it’s filled) will place extra strain on aged joists, even those not compromised by holes, notches, or insect damage. If anything, the existing joists should be reinforced or replaced before a new tub goes in. Cutting through these essential supports is a recipe for disaster.
Now that damage is done, resist the temptation to under-engineer the repair. First, double up (or “sister”) the existing joists to the full extent possible. (Since they’ve been cut through, you can “scab” between the cuts by fastening strips of ”-thick plywood over or under the new piping as a stopgap measure.)
Use metal joist hangers and make sure each of the sistered joists is supported by at least one sound, loadbearing header.
Then add solid blocking or doubled-up cross braces between (that is, perpendicular to) the joists on either side of the cut area, again using metal hangers. In effect, you are creating a braced box around the damage; a good plumber would have created such a structure before cutting or notching any joists to add plumbing.
Bridging between the compromised joists transfers the load away from the damaged area to sound timbers. When the blocking fits tightly, neither joist can flex or bow. Because the area must support such a heavy load, it’s a good idea to add cross bracing on the next joist over, for additional reinforcement.