Ask the Carpenter: Mold in the attic and trouble in the garage

Ask the Expert
A section of this reader's garage is showing wear. Handout

Q. In the attic, condensation builds up on the copper tubing to our air-conditioning system and drips onto the insulation. There are vents on both ends of the attic. I believe the condensation has caused mold. Is there a way to prevent the dripping and to remove the mold?


A. Don’t sweat it! The condensation you are seeing is caused by outside humidity getting into the attic through the vents and condensing on your cold AC lines. The most basic corrective measure is to insulate the lines to prevent the dripping.

The more proactive approach would be to insulate the attic space with closed-cell spray foam. This will keep the humidity from getting into the attic and will significantly reduce the amount of run time needed to keep the house cool or warm by taking the load off the home.

Many homeowners who insulate the roof deck with closed-cell spray foam typically see utility savings of at least 40 percent. It also helps prevent ice dams, among other problems.

If you have less than 10 square feet of mold, you can remediate the area yourself, following Environmental Protection Agency guidelines ( If you have more than that, the EPA recommends its guide titled “Mold Remediation in Schools and Commercial Buildings’’(


Q. My 50-year-old garage has a section of its foundation, about 10 feet long, where the concrete is deteriorating. That batch of cement must have been bad. What do you recommend as a treatment that can arrest that deterioration? The garage floor is in pretty good shape overall.


A. A masonry contractor can tear it out and pour a new floor or just pour a new floor over it. Otherwise, the walls and vertical surfaces would benefit from parging work (a skimcoat).

Dear Rob

From Mike DeBlasio: I read your column about the chimney gap (“Is this chimney falling down?’’ Aug. 18). I am a mason contractor, and the movement in the chimney away from the sidewall is caused by “oxide jacking.’’ When the mason built the chimney, he or she installed a steel lintel above the finish fireplace opening. Typically this lintel is set 8 to 12 inches higher than the finished fireplace opening. The placement of this “roughing in’’ lintel allows the mason to leave out the firebox, which would be completed as a separate operation. This lintel supports the masonry against the side wall and forms the base of the smoke chamber.

In many cases the ends of this steel lintel extend to the extreme ends of the chimney. Over time, moisture in the masonry will cause the ends of the steel lintel to rust. As steel rusts, there is a volumetric gain in dimension. A square inch of iron oxide will generate a force in excess of 500 pounds per square inch.

Once this oxide jacking has started, there is no way to stop it without removing the lintel. Removal of this lintel is not easy, and in most cases the chimney has moved enough that it has to be taken down.

About 10 years ago I was an expert witness for an insurance company regarding a large chimney claim. I see at least 15 claims per year were the insured thinks high winds moved the chimney away from the house. Ninety percent of the time it was oxide jacking. I have gone out to several structural fires were the source was the gap within the smoke chamber caused by the rusted roughing in lintel.

From Rob: Thank you, Mike. I’ve read about some of your work in trade magazines, and I’ve forwarded your advice to the reader to ensure she sees it.

Rob Robillard is a general contractor, carpenter, editor of, and principal of a carpentry and renovation business. Send your questions to or tweet them to @robertrobillard. Subscribe to our free real estate newsletter at