Why does a leak show up far from where the water gets in?
Water follows the slope of the boards and the lines of the rafters, so the stain can appear several feet from the entry point. The diagnosis follows the wet wood upward until it finds the joint where the water first passed the roof covering.
A drop of water that enters at the top of a valley may run along a board for several feet before it reaches the ceiling. The stain you see is at the lowest point of that run, which is why the stain is a poor guide to the source on its own.
The attic is the place to follow the path. Dark, damp wood shows where the water has been, and the highest wet point usually marks the joint where it came in. That is where the flashing check starts.
What does a flashing failure look like from the ground?
From the ground, a failing flashing can show as rust streaks, a lifted edge, a gap at the wall or a dark line where the sealant has pulled away. Many failures are hidden under the shingles, so a ground view is only the first look.
Look at the roof where it meets the walls and the chimney. Metal that has bent out of line, gaps between the flashing and the masonry, and shingles cut or lifted at a flashing edge are all worth a call.
Most flashing failures are not visible from the ground. The top edge of a flashing piece is hidden under the shingles above it, and that is where water usually gets behind the metal first. The technician lifts the shingles to check it.
Can sealant alone fix a flashing leak?
Sealant can close a small crack, but it is not a long-term fix when the flashing itself is lifted or worn. Caulk laid over a wet joint can trap the water behind it, which makes the rot worse.
Sealant is useful as part of a repair, not as the repair. It stays flexible for a few seasons, and then it shrinks and cracks under the freeze and thaw cycle. A flashing that has been caulked several times is usually a sign the metal needs replacing.
Never caulk a joint while it is wet. The sealant bonds to the water and not to the metal, and the first thaw breaks the bond. A technician dries and tests the joint first, and then decides whether sealant is part of the fix.
How do I tell a flashing leak from a gutter leak?
A flashing leak usually shows up in wet weather and in the attic at a roof-to-wall line, while a gutter leak runs down the wall outside. The two can look alike, so the diagnosis checks the gutter and the flashing separately before deciding.
Watch the water during the next rain. If it runs down the outside wall from a gutter seam or a downspout joint, the gutter is the cause. If it appears on the ceiling near the wall line, the flashing is the more likely source.
Ask the technician to check both. A flashing repair that leaves a gutter overflowing will not stop the water reaching the wall, and a gutter repair that ignores the flashing will not stop a leak at the roof.
What should be done before the next thaw?
Before the next thaw, clear the gutters, keep the attic dry and place a bucket under any active drip. Avoid climbing onto the roof, and let a technician close the joint once the surface is safe to work on.
Snow that sits on a roof melts into the flashing joints at the first warm spell. Clearing the gutters and the valleys from the ground, where it is safe, reduces the water that is pushed toward the weakest joint.
Protect the rooms below the drip with a towel, a tarp or a plastic sheet, and move valuables out of the way. A temporary cover is not the repair, but it keeps the damage from growing while the diagnosis is arranged.
