Although the dampness is most likely from a roof leak, it may also be from a ventilation problem, particularly during winter. With inadequate attic ventilation, perhaps from clogged or too few vents, moisture from indoor air can condense on the rafters. It can drip down on the insulation and appear to be a leaky roof.
If you determine the moisture is coming from a roof leak, you will have to play detective. Even if you see the water dripping from a rafter or truss during a rainstorm, you can’t be sure where it’s penetrating the roof. The water can run quite a long distance along a rafter until it hits a bump or edge and drips from there.
During the next heavy rain, go up into your attic armed with a flashlight, paper towels, and a tape measure. Wear a breathing mask and a long-sleeved shirt because you will be disturbing the insulation and years of dust. It’s wise to protect yourself from inhaling the airborne particles.
Wear a tool belt or baggy pants with big pockets. You should hook the tools to the belt or put them in your pockets so that your hands are free. It’s difficult to keep your balance when you are walking on the attic floor joists, and you will need your hands for balance. If you lose your balance and step off the joists, your foot will probably go right through the drywall to the room below.
When you find a damp area in the insulation or actually see some water dripping from a roof rafter, start your search there. The water is most likely coming through the roof to a spot on the rafter further up. Starting at the dripping or damp spot, run your hand up the rafter to a point where it no longer feels wet.
Since this can get a little tricky to find, lay a paper towel against the rafter. You will be able to see if there is any dampness on the paper towel. Once you find the water entry point, measure from there to the peak of the roof and to a side wall. These measurements will be used up on the roof to locate the same spot.
Wear appropriate safety gear if you go up on the roof. Better yet, let a professional fix the spot. Some typical leaky spots to look for are slots between shingles, valleys (where two roof angles come together), any metal flashing, plumbing vent flashing (usually a rubber material), and chimneys. If you see any nearby spots where the sand granules are missing, give them a closer look, too.