Seek clues in shuddering house mystery

Q.My house shudders at night, or at least it seems like it happens at night because all is quiet and I am trying to sleep. It is a intermittent vibration of sorts, in my half of a two-family house with a puddingstone foundation. The house was completely rebuilt 20 years ago from the foundation up after a devastating fire. The subway is two blocks away. I am having a little trouble with my heating system, and I plan to replace it.

TOM, Jamaica Plain

A. Ah, the house that shudders at night. 'Tis a riddle, wrapped in an enigma, inside a mystery. Or something like that. It could be vibration from the heating system, or something outside, telegraphing underground until it hits that foundation. For starters, I don't think the house is falling down. Check to see if the vibrations start and end when the heat comes on and off. It could be something as simple as a tree branch scraping against wires coming into the house. So, check the outside of the house to see if anything is touching house or wires. A fan in a hot air system may be just a bit off balance.

There are other possibilities, but if you don't find anything with a casual check, find an architect, engineer, or home inspector to take a hands-on look.

Q. I have an in-law apartment, partly underground, and I am getting tiny little ants, on the counters, floor, and everywhere, it seems. Where are they coming from? How can I get rid of them and how can I keep them away? How can they come in when the apartment is very tight? Also, my townhouse has a loft with no source of heat or air-conditioning. How can I get it warm and cool when necessary?

CONFUSED

A. Ants are so small they can come in the tiniest openings, or even make them, particularly in basements. Here is what you can do: Sweep them up and throw them outdoors. Put ant cups around the kitchen, but not if there are pets around. Do not spray indoors. Do not let anyone spray indoors. Here are a couple of natural techniques: Spread the plant tansy around, or bay leaves. Or, sprinkle boric acid around the baseboards, but do it very, very lightly. Do not use these deterrents if there are pets around.

As for the cold/hot loft, install a ceiling fan in the loft. Adjust it so it blows air up, bringing warm air from the first floor in winter, and cool air in summer.

Q. I bought a new house in St. Paul that has a fresh-air intake designed to provide the heating system with enough air to burn properly. It went from the wood sill on the foundation to the heater. Lo and behold, when I turned on the heat last fall it did not work. You can only imagine what the temp was even in early fall. There was no hole cut in the sill. Could I cut a hole in the sill to provide that necessary air?

ST. PAULY GUY

A. Without expressing shock that the builders forgot an important thing that you described, you can cut a hole in the sill to accommodate the pipe or cut a hole in a window, or in the outer joist. Any way will do. To cut a hole in the window, I suggest you take out a pane of glass and replace it with a piece of 1/8-inch hardboard, with the proper hole cut in it. Put a screen on the opening to keep out pests.

Q. I bought a house in Gadsden, Ala., and discovered moss on the roof. It's dark green, and looks like a little grove of evergreen trees. How can I take off the moss, and how can I keep it away? I have heard there is a zinc strip that can be put on the roof to deter the growth of moss. Where can I find it?

ALABAMA

A. Scrape off the moss with a wide putty knife or wood spatula. You can keep away the moss with those zinc strips. Install them under the second-from-the-top row of shingles the full length of the roof, with up to 3 inches exposed. Rain will run over the zinc strip and some of it will wash down the roof, deterring the moss growth. It will also prevent black mold (sometimes white), green algae, and light gray-green lichen from growing. Get it at Z-stop.com.

Q. A couple of burners on my old Thermador cooktop are heavily caked with cooked-on food. How can I clean those parts?

NATICK

A. Soak removable parts overnight in a strong solution of baking soda and water. Or, if it is allowed, spray the parts with foaming Easy-Off.

Q. How can I get rid of the nonslip bottom of a cast iron bathtub?

SLIPPERY

A. I don't think you can because it is built into the cast iron and porcelain enamel. If it seems difficult to keep clean, try this: Make a strong solution of Spic and Span and water, and put in the tub for half an hour or so. This soaking will give the detergent a chance to dissolve the dirt. Then scrub and rinse.

Q.You ought to know how contractors are doing a job on homeowners in New Orleans. The ceramic tile on my kitchen floor is in very good shape after a year in place. But the sandy grout is not. It keeps falling out, and installers have come back twice to fill it. It still comes up, all sandy, making a mess. What can I do? The contractor seems to be hiring incompetent installers.

HESTER YOUNG, New Orleans

A. That is the story of fixing up. Contractors and sellers of products are pretty good, but they seem too often to hire incompetent installers. Since the tile is in good shape, you have two choices: Hire an able installer who knows what the heck he is doing, to do the job one more time. Or do it yourself.

The fault with the grout is that it is simply not compacted enough. To do it right, chip out all traces of old grout and install new floor grout, either sanded or unsanded. And put it in very, very compactly. It is simple, really, but takes just enough know-how to do it right.

Q. I spilled hot wax on a Corian sink top. I tried scraping it without success. How can I remove it safely?

JOAN BANGS, Winthrop

A.Corian can be sanded, so try sanding with very fine sandpaper or emery cloth. Don't get carried away so that you sand a depression in the Corian. Then wet it with paint thinner to dissolve anything left. Pouring boiling water on the stain, then wiping it off, may do it, too. I don't think even boiling water will hurt the Corian. But if in doubt, don't.

Q. When I put up a plasterboard ceiling, I butted some of the sheets end to end, which did not have the depressed ends. I then applied paper tape and joint compound, but the tape and the some of the compound is coming down. What did I do wrong? Also, I put the board up with blue dry wall nails, but some of them are rusting through the joint compound. How can I fix those rusty spots?

TOM HINES, Malden

A. I don't think you did anything wrong, except not put three coats of compound on the tape. But whatever you did, it should come down and be redone. When you do that, make sure there is at least a 1/4-inch gap between plasterboard sheets. Then you can fill that joint with joint compound, smoothing it out very thinly. The gap will allow the joint compound to key itself in a little. Then apply tape and another thin coat of compound. Repeat this twice, sand smooth, and hope for the best. You can use nylon mesh tape if you like, but it might be a little too thick for this purpose.

As for the rusty spots, try this before you do anything drastic: Paint the spots with clear shellac, then repaint the ceiling.

Globe Handyman on Call Peter Hotton is also in the Styles Section on Thursdays. He is available 1-6 p.m. Tuesdays to answer questions on house repair. Call 617-929-2930. Hotton also chats online about house matters 2-3 p.m. Thursdays. To participate, go to Boston.com. Hotton's e-mail is photton@globe.com.