We have a winner! Yes, ladies and gentlemen, Jen, Mary, Liz and Andi (except for the gum) are right. Sue Ellen, I apologize for not stating what to me is so familiar: these tanks are wood. (And I thank you for your kind words about the blog! JL's sister, huh? Cool!)

A tank is built thusly: the bottom is affixed to the steel or masonry. (Steel that holds structures on roofs, by the way, is "dunnage.") The bottom has a track to hold the staves. Two people, one outside and one inside, go around the tank installing the pre-cut tongue-in-groove staves. When they get to the last one the inside guy comes out (lucky for him) and they fit that one in, which can be kind of a pain in the neck to do. Then they put on the roof. No sealant and very little in the way of nails, because they rust. When it's built they fill it, and when the staves get soaked they expand and that's what keeps the water from leaking out.

Of course, nothing whatsoever keeps the water from leaking out until then. If you watch the filling process, it involves turning on the tap and getting this fabulous 360 degree waterfall gushing from every joint. Eventually the swelling stops the leaks and you're left with puddles on your roof, which dry.

Unless you have, as we had once in my architect days -- well, more than once, but I'm only telling this one story right now -- a contractor who's both corrupt and incompetent. Many contractors are neither, and some are one or the other, but both is very undesirable. These guys were eager for a payment they weren't about to get until the plumbing system passed various inspections. So after Rosenwach installed the tank -- this was a fire plus domestic water tank -- they filled it immediately. It must have irritated them to have to do this at all, seeing as how the plumbing inspector was one of the few inspectors they hadn't been able to buy off. They were so badly irritated they weren't thinking straight. The flashing hadn't been installed on the roof yet. Can you see where this is going? Gallons and gallons and gallons, hundreds of gallons, of water leaking through every joint where a horizontal roof surface met a vertical one, pouring into the top floor, soaking the sheetrock, the floorboards, the electrical wiring; and contractors racing through the building turning on every tap in every bathroom and kitchen, in a desperate attempt to drain the tank before this one dumb move ruined six months of work. And me, arriving in the middle of it all, making a desperate attempt not to laugh myself silly.