Q. I have been asked to determine the cause of black shadow marks on a cathedral ceiling in a new home. The black marks correspond to the bottom sections of the rafters, near where the rafters meet the wall. The black shadows do not resemble stains from water leaks. There is a gas fireplace on one of the outside walls; the fireplace has been tested for leakage, and it checked out fine. The owner burns a large candle, which sits on the fireplace mantle. Could the ceiling shadows be caused by soot from the candle?

A.Frank Vigil, senior building science specialist at Advanced Energy in Raleigh, N.C., responds: Reports are increasing of problems with black stain deposition on interior surfaces. Since particulate is attracted to cooler surfaces, it is not uncommon to see black staining following the framing members behind sheetrock. The source of the particulate varies. We have documented sources ranging from furnaces and water heaters to fireplaces, candles, outdoor cooking grills, and even automobile tires.

Although you say the fireplace has been tested for leakage, not all technicians test for all possible problems. Was the fireplace tested under pressure or simply examined? Is the fireplace a "sealed" unit? Some units appear to be sealed, but when tested under negative pressure can leak significantly. Is there a standing pilot light? If so, how large is it? Does it impinge on any of the logs? Has the gas pressure been tested? Was the orifice examined to make sure it is the correct one for the fuel being burned? These items should all be checked, since we have found that any one of them might be the source of particulate matter.

We also know that some candles — as well as certain candle-burning practices — can be the source of black stain deposition. Unfortunately, it can be difficult to predict beforehand which candles will lead to problems. Many types of jar candles, imported candles, and highly fragranced candles can burn dirtier than other types. Candles that are poorly designed or have improper wicks can also be problematic. Improper burning practices include failing to keep the wick trimmed, burning a candle anywhere near a draft (for example, near a heating register or ceiling fan), and burning a candle for too many hours. Any of these practices can lead to staining.

If you suspect that a candle is the cause of the staining, tell the homeowner to stop burning the candle for a few months. If the stains return to the ceiling after repainting, you know the source is from something else. Covering the stains requires a good quality sealer, such as Kilz, prior to painting. Otherwise, the stain will bleed back through the paint.

Because particulate is attracted to cooler surfaces, there is a strong likelihood that the ceiling insulation is poorly installed — perhaps because the insulation is compressed at the edges or because there are voids or gaps along the edges of the insulation. The owner of the house may wish to improve the ceiling insulation.

For more information, see my article "Black Stains in Houses: Dirt, Dust or Ghosts?" in the Jan./Feb. 1998 issue of Home Energy magazine (510/524-5405).