By Norman Becker, Popular Mechanics

Roof Truss Uplift

There is a problem with cracks forming in my house. In cold weather, cracks appear at the corners where walls and ceilings meet. I had molding installed at the top of the walls two years ago, but this didn't help. What causes this, and how can it be repaired?

SHIRLEY ZIMMERMAN
ST. MARYS, OH

The seasonal opening and closing of the joint between the ceiling and the wall is caused by roof truss movement. This is commonly known as truss uplift. During the winter, the part of the truss immediately above the ceiling is exposed to higher temperatures and lower humidity than the parts of the truss immediately below the roof deck. These parts see lower temperatures and higher humidity. They pick up condensation that forms on the underside of the roof deck. As a result, the lower part of the truss tends to shrink slightly during the winter, while the parts immediately below the roof deck tend to swell. In turn this causes the upper parts of the truss to bend the bottom of the truss upward. This lifts the ceiling below, and cracks develop where the wall and ceiling meet. This condition can be prevented with construction techniques, but the best way to deal with the problem now is with a cosmetic repair. You were correct to have moldings installed, but the moldings should have been nailed to the ceiling, not the wall (as is typically done). The moldings should be free to move up and down with the ceiling. As long as the molding is wide enough, it will cover the crack at the ceiling.

Erratic Oilburner

We had a new nozzle and filter installed in our old oil furnace. After being shut off overnight, it fires for 20 seconds and then stops for 12 seconds. Then it fires like it should until the thermostat causes it to shut off. If the burner is shut down for 5 or 6 hours, you have to press the reset button to get it going again. The technician who installed it is stumped. What is the remedy for this?

JOE MCKEIRNAN
POMEROY, WA

The problem may be caused by an air leak in the oil supply line (the suction line). This can cause the formation of an air pocket by the burner nozzle, which could account for a 5- or 6-second delay. After running for a while, another air pocket could cause the safety control to shut down the burner. And when the burner sits idle, the air leak could cause the fuel pump to lose its prime. In this case, when the burner is turned on but doesn't fire within the lockout time on the primary control, it will be shut down by the safety control.

I checked with burner manufacturer R.W. Beckett Corp. The company says this is often caused by an air leak around the filter or a valve stem, or in a compression fitting, a flare fitting or the nozzle adapter. Another cause is a loose nozzle. The company points out that compression fittings on an oil supply line are liable to create an air leak. They should be replaced with flare fittings. To find the leak, an oil heat service technician will have to pressurize the oil supply line.

Cleaning Concrete

My house's concrete driveway and walkways have turned black, and besides the black areas, rust spots have formed. I have tried without success to remove the stains with trisodium phosphate and bleach. What cleaner will work?

HARRY FRANKEL
BOYNTON BEACH, FL

The spotted black areas are caused by a fungus, probably mildew, and the rust color stains are iron oxide, which may come from a lawn sprinkler, lawn fertilizer or iron particles in the concrete.

The cleaners that you used are correct for removing mildew, but you may not have used the right proportions. According to the Portland Cement Association (PCA), a solution for removing mildew from concrete is: 1 ounce of laundry detergent, 3 ounces of trisodium phosphate, 1 quart of laundry bleach and 3 quarts of water. Apply this to the area with a soft brush, and let it stand. Then hose the area. The rust stains can be removed with a solution of 1 pound of oxalic acid per gallon of water. Rinse the area off after 3 hours using clean water and by scrubbing with a stiff bristle broom. Oxalic acid, otherwise known as wood bleach, is sold at hardware stores, paint stores and in some woodworking supply catalogs. The PCA booklet, "Removing Stains and Cleaning Concrete Surfaces" (IS 214), costs $14.50 postpaid from the PCA, P.O. Box 726, Skokie, IL 60076; 800-868-6733. You can order it from the PCA Web site: www.portcement.org.

Failed Bulbs

For many years, I have noticed that the light bulbs in our home burn out every year when cool weather arises in early fall. What causes this, and what can be done to solve the problem?

CHARLES ROMER
LORAIN, OH

It's possible that there is an increase in the voltage supplied to your house when and just before you notice the bulbs are failing. A bulb that operates at just a few volts higher than its rated voltage will have a drastically reduced service life. According to General Electric, you can determine the projected life of a bulb by using the following formula: Actual life = rated life (rated voltage/actual voltage) 13.1. If, for example, you're using a 120-volt bulb, rated to last 750 hours, and the voltage it's running on is 125 volts, then you can expect the bulb to last about 440 hours. You should call your power utility company and have it check the house's electrical supply voltage over a one-year period. Also, ask your neighbors if they've experienced this. If so, an overvoltage is likely.

Service Tips: Maintenance

Sump Pump Check
A sump pump may go for long periods without being activated. The sump pit will dry out, and the pump will sit there high and dry. It couldn't hurt to check the pump to be sure that when it's needed, it will operate as it should. Otherwise, you're liable to have a wet basement after a heavy rain. To check the pump, either fill the sump pit using a garden hose or gently tip in a couple of large buckets of water to check the motor and pump operation. If you notice that the ground is wicking the water up before the pump has a chance to activate, wrap a sleeve of plastic sheet around the wall of the pit. You may have to line the pit bottom with plastic, as well. Don't just check the pump by lifting the float. All that will tell you is whether the float switch and motor are working. You need to know whether the pump works, too.

Parts Source: Window Repair

Boy, have we got a parts source for that old casement window opener that doesn't work: Blaine Window Hardware. For the last 45 years, the company has specialized in repairing doors and windows (including storm doors and windows), and it sells repair parts. The company stocks 20,000 window and door parts and can custom make parts because it has a foundry, plastic molding machinery and a machine shop. If you'd like, you can send in a part, and the company will try to match it. Send parts to Blaine Window Hardware, 17319 Blaine Dr., Hagerstown, MD 21740, Attn: Research Department. Or write Attn: Customer Service Dept. You can also call 800-678-1919.

-- Roy Berendsohn

Review: Multiuse Reference

We like reference books. They're fun to browse through, and they have what we need in a pinch, especially stuff that we just can't remember. This little book fills the bill nicely. It measures 3 X 5 in., and it's 5/8 in. thick, but it has 540 pages with all manner of helpful stuff -- span tables for framing lumber, weights and measures of materials, conversion factors, mechanical, engineering and scientific data, area and volume formulas. We can't imagine how anyone had the patience to gather it all, but we're glad someone did. It costs $13 postpaid from Dan Holohan Association, 63 N. Oakdale Ave., Bethpage, NY 11714; 800-853-8882. You can also order the book from the company's Web site: www.heatinghelp.com.

-- Roy Berendsohn

Do You Have A Home-Maintenance Or Repair Problem?

Just ask Norman about it. Send your questions to

Homeowners Clinic, POPULAR MECHANICS
810 Seventh Ave.
New York, NY 10019

While letters cannot be answered individually, problems of general interest will be discussed in the column.