The source of most roof leaks is hard to find because it originates away from where the leak shows up. In order to find the source of a leak, follow a roofer’s advice and “think like water.”

Water typically comes in through worn, broken, or missing shingles; where nails have worked loose; or through corroded or poorly sealed flashings around vents, skylights, or chimneys or along the intersections of roof planes. Once water passes the roofing, it flows along the sheathing, roof rafters, or topside of ceilings until it finds a place to drip down—inevitably onto your favorite piece of furniture.

Look for a roof leak during the day. Go into the attic with a bright flashlight; step only on secure framing members and never on the insulation or topside of the ceiling below, neither one of which will support you. Start above the place where the drip has occurred and work your way uproof.

Look for wetness along the framing members. Or, if the weather has been dry for a while, look for water marks, stains, or discolorations on the wood made by moisture. Then switch off the light and try to find a hole where daylight shows through the roof. (With a wood-shingle roof, you’ll see many such places, but while the overlapped shingles let light show through they shed water.) If it’s still raining, put a bucket under the leak in an area with proper support.

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