Roof leak leads to strange shadow on ceiling below
A: It’s possible that your ceiling simply stained in a way that’s more noticeable when the light hits in a certain way. Test with a moisture meter. When the ceiling looks wet, compare the reading you get there with one from a ceiling you know to be dry. Or press a sheet of paper to the troubled area and check whether the paper darkens, a sign of moisture.
If there is moisture, ask the roofer to come back and check the flashing on all exhaust pipes, skylights, chimneys or other things that extend through the roof. If the roofer refuses, call a home inspector. Be aware that water leaking at a flashing doesn’t always drip directly down. It can travel along a rafter or the underside of the roof for some distance first.
The attic inspection should also include checking the insulation directly over the stained area of the ceiling. If the insulation is waterlogged, it obviously needs to be replaced. If it’s black, whether dry or moist at the time, that’s a sign that an air gap in the ceiling, perhaps around a light fixture or over cabinets where the space above wasn’t boxed in, is letting warm indoor air leak into the cold attic. Or there could be a vapor barrier improperly installed. Warm air holds more moisture than cold air, so as this air cools in the attic, moisture condenses and allows mildew to grow, making the insulation black. Enough moisture could even be condensing to cause the ceiling to become damp, especially when the indoor air is especially humid. The moisture could also be activating stains that are water-soluble.
If there are air gaps, plug them with caulk or other materials, depending on how big they are, then see if the dampness returns.
Once you are sure you have solved any moisture issues, press on the drywall or plaster to make sure it still feels solid and tightly fastened to the ceiling joists. If it’s spongy, you will need to cut out the damaged area and patch. (Cut along the middle of the ceiling joists so you can fasten the patch to solid wood above.)
If the ceiling is still OK, paint it with a stain-blocking primer. Oil-based primers used to be the only reliable way of blocking water-soluble stains, but today there are waterbased primers that do the job, such as Kilz Max, $34 a gallon at Home Depot. (Regular primer won’t do, because water-soluble stains can seep through.) When the primer dries, repaint the ceiling.