Q. The hardwood flooring closest to an outside wall in a bedroom is cupping downward. It does this mostly in the winter: Center bowing up, edges down. The floor is always dry but cold in winter. PETER, in Hotton’s chat room
A. Cupping usually occurs when the wood takes on moisture and expands, which is unlikely to happen when the wood is dry. But you say the wood is cold in winter, which leads me to believe that water vapor under the boards condenses on the cool boards and enters the unfinished part (bottom) of the board, causing the cupping. Cupping also may occur when the boards get moist and expand, but have no place to expand to except up. I think the cupping will go down in summer, when the water vapor under the boards is less likely to condense.
Another phenomenon caused by moisture is buckling, when the bottom of the board expands, but has no space to expand into except by buckling. This buckling also will go down when the board dries out.
There is one possible cure, which can affect cupping and buckling. It is to build an expansion joint at the edge of the floor next to the wall. Cut half an inch off the board nearest to the wall or baseboard. This will allow a board to expand into the space and stay flat.
This goes against all logic, but I think that is what happens and, as I described, how to cure it. If it doesn’t work, it will cause no harm.
Q. My hardwood floors have three coats of polyurethane, and are holding up very well. They’ve only been vacuumed; how can I clean them when necessary? STEVE CURTIN, Lakeville
A. I have found that a wet Swiffer (pad on a long handle) does well, although I have heard that it sometimes dulls the finish, but not mine.
Q. I had leakage from my Christmas tree stand and now the joints in the hardwood floor are stained black. How can I clean those black spots? JOHN, in Hotton’s chat room
A. The water leaked into the cracks and penetrated the unfinished hardwood, turning it black. Those stains are indelible. But try this: Treat the stains with straight household bleach. If it helps a little, do it again. Then rinse and dry.
Q. I read your answer on installing a gas fireplace which said not to use propane. Why is that? I have two fireplaces that I would like to put gas inserts in. Your answer concerned me. I don’t have natural gas in my community, so I’d like to use propane. LEO APPEL, Rye, N.H.
A. I’m sorry I did not give a reason. It is this: Propane is simply very expensive as a fuel. But if you use the fireplaces as auxiliary heat, romance, or to warm up one room, go ahead. It won’t break your budget and it will be satisfying.
Q. The metal joist hangers that are holding together my deck are rusting a lot. Should I be worried? ARTHUR, in Hotton’s chat room
A. No. Joist hangers are made of galvanized steel, which is rust resistant. That rust may be just a patina that may keep the metal from rusting further. If you discover deeper rust, it will still take years for the hangers to weaken. If the deck shows signs of shaking or sagging or moving in any way, replace the hangers.
Globe Handyman on Call Peter Hotton also appears in the Sunday Real Estate section. He is available 1-6 p.m. Tuesdays to answer questions. Call 617-929-2930. Hotton (photton @globe.com) also chats online 2-3 p.m. Thursdays. Go to www.boston.com