Q. Our house was flooded and we’re now adding a second floor. The plumbing inspector just passed our second-floor boiler installation, with one exception. He wants proof that our hot water tank is OK where it is. He’s concerned about the weight. I weigh over 200 pounds and the floors seem fine to me. The water tank is 100 gallons, so we’ll have enough heated water for three bathrooms, including a whirlpool master bathtub. Since water only weighs a few pounds per gallon, I really don’t see what the problem is. But the plumbing inspector needs a letter from an engineer or architect, and won’t let the rest of the heating system be inspected without it. What do you think?
A. I think you grossly underestimated the weight of your hot water tank, and your plumbing inspector has good reason to insist on a letter and special attention to the problem. Water weighs 8.35 pounds per gallon, so, conservatively, your tank weighs 835 pounds plus the tank weight, which could be another 150 to 200 pounds. The average second-floor load requirement is 40 pounds per square foot. Even if your tank covers an area of 2 feet by 2 feet (4 square feet), with a floor capable of 160 pounds of dead load, the tank exceeds the capacity by approximately 835 pounds. Although the floor would flex (sag) for a long time, until the moisture in the fibers began to reduce, the floor joists could eventually give way.
Many people unknowingly assume that all the extra ballast weight makes the floor feel stiffer. This is the same logic used by tile installers who put a really thick mortar (“mud”) base down on a bathroom floor. I often see the old system of concrete installed, called dunnage, set over 2x4 lumber laid flat between floor joists. It holds beautifully until the lumber ages and becomes brittle. Then the wood doesn’t just sag, it cracks. In some cases called to my attention, the floor has given way.