When professional home inspectors examine a roof, they start not in the attic with a flashlight but on the ground outside the house, with a pair of binoculars. This is not because they are lazy or afraid of heights (if the latter is true, they've made a very bad career choice); they're looking for obvious signs of trouble, like missing shingles or holes made by animals, as well as standard red-flag areas, like skylights and plumbing vents. Later, in the attic, they check for any signs of water, double-checking all of the trouble spots noted in their ground inspection.
If you've noticed some water damage on your ceiling, don't assume it's coming from a hole in the roof directly above. Thanks to gravity and liquid adhesion (yes, there's a name for it), the water could be getting in just about anywhere and wending its way down the underside of the roof deck before it drops onto the back of the ceiling. To sleuth your own leaks, do what the inspectors do: start outside, looking for any of the obvious trouble areas listed below, then head to the attic with a flashlight.
1. Plumbing Penetrations: Builders know that running a pipe through a roof is like drilling a hole in a boat and patching it with a rubber plug, but it has to be done. Common penetrations include plumbing vent pipes, exhaust flues for appliances, attic vents and ventilation fan ducts, among other necessities. Worn or cracked rubber boots and metal flashing around pipes and flues are among the most common leak sources.
2. Flashing: Flashing is any thin metal sheet, strip, angle or fitting that covers a seam or transition point in a roof. Transitions are hell on roofing materials, and flashing is the best band-aid we've come up with so far. Leaks tend to start where flashing is either damaged or corroded, or where it's missing altogether (how much did you pay for your last roof?). In addition to most penetrations, flashing should be present and intact at all dormers, valleys (where two different roof planes meet) and where a roof meets a wall.
3. Skylight: Skylights and roof windows are wonderful for bringing light into a home. If you also like a little rain now and then, they'll eventually provide you with that, too. Evidence of water anywhere near the downhill side of a skylight is a sure sign that the unit (or more likely, its installation) is performing its less-advertised function: leaking.
4. Chimney: If you think it might be hard to seal every millimeter of airspace between a brick wall and a plywood floor, you're pretty smart, or at least you're getting a sense of what roofers have to deal with when it comes to chimneys. A chimney stack not only creates a huge hole in a roof, it also collects water on its surface, which can trickle down and get into the smallest breaches. Failed flashing is the leading cause of chimney-related leaks. Also, if your chimney is anywhere but at the peak of the roof, it should have a cricket-a triangular metal "tent"-on the high side where the stack meets the roof; without this, water can pool at the base of the chimney, and pooling water is never a good thing on a roof.
5. Shingle Damage: Unless you've been walking around on your roof in your golf spikes, the shingles themselves might be the last things to cause a leak, but they don't live forever. While really old shingles can simply wear enough to fail, more common problems are cracked, cupped, damaged or missing shingles. Cap shingles, along the peak and ridges, are the most prone to wind damage and often blow off in a storm. Nails driven through shingles and left unsealed (with roofing cement) also tend to leak. Because nails make holes. If your roof is metal, watch for corrosion, especially where dissimilar metals are in contact, such as with an aluminum vent cap on a steel roof. With tile and slate roofs, broken and missing shingles and failed repairs are cause for concern-regarding your roof and your bank account.
6. Ice Dams: People in cold climates can't wait to see local TV news coverage of ice dams each year...just like they can't wait for the next flu virus. For those of you who missed the most recent newsflash, ice dams are caused by a warm attic melting snow on the roof, and the water runs down toward the cold eave and freezes. Eventually, the ice backs up under the shingles, where it melts and drips into your house. A vicious cycle, indeed. And the solution? Better insulation and air-sealing above the home's living space, and better ventilation in the attic, to keep it cold (while lowering your heating bill-a happy cycle, indeed).
7. Meteorites: This reason for a roof leak should be considered common only if you are Radivoke Lajic, whose house in northern Bosnia has been struck 6 times by meteorites since 2007. If you are not Mr. Lajic, the chances of a meteorite causing a roof leak in your home are close to 1 in 2 trillion, which means even lottery players can stop worrying about it.
How to Trace a Leak
In the attic, leaks typically show up as dark streaks on the underside of the roof decking, the structural plywood or board layer under the shingles. Follow these streaks up the roof to where they appear to start. Unfortunately, you might not be there yet; the water can travel along the top of the roofing felt (tar paper) before it penetrates to the sheathing, and even then it has to find a crack or seam somewhere before it travels to the underside...In any case, following the water markings will get you closer to the leak source, and, with any luck, will keep you from using an entire case of roofing cement (now available in convenient tubes!) trying to stop the leak.