Why the stain lies to you
Water follows the path of least resistance, and inside a roof assembly that path is horizontal for a while. It enters at a penetration, runs down the underside of the decking until it meets a rafter, tracks along that rafter, soaks into insulation, then finally drops through the ceiling at whatever point the drywall gives first.
Five or six feet of lateral travel is completely ordinary, and on a low-slope roof it can be much more. So the first move is not to go and look above the stain. It is to work out which way is up-slope of it, and start there.
The four usual suspects, in order
Pipe boots. The rubber collar around a plumbing vent is the single most common roof leak there is, and it has a shorter life than the shingles around it. The rubber cracks at the base from UV and thermal cycling and lets water straight down the pipe. It is cheap to put right and it is the first thing worth checking.
Flashing at a wall or chimney. Where a roof meets something vertical, the seal depends on formed metal laced into the shingle courses. Where sealant is doing that job instead, it works for a couple of seasons. This is the second most common cause and the one most often misdiagnosed as a masonry problem.
Valleys. A valley carries more water than any other part of the roof. Debris packs in, water backs up sideways under the shingles, and the underlayment beneath is what is holding the line.
A nail that missed. A fastener driven outside the nailing zone, or one that backed out over time, is a hole in the deck under a shingle. Individually tiny, collectively the reason old roofs leak in several places at once.
How it is actually located
From inside first, if there is attic access. Trace the wet path back to its highest wet point with a light, and mark it. Dry insulation below a wet rafter tells you the water came from further up.
Then from the roof, with the interior mark measured off a fixed reference — a gable end, a chimney, a vent. Guessing at the position from outside is what produces a repair in the wrong place.
A water test where the cause is not obvious: run a hose on the roof in sections, starting low and working up, with somebody inside watching. Ten minutes per section. Crude and completely reliable.
Moisture meters and infrared have their place on flat roofs and on large wet areas, but on a shingle roof with one stain, the hose finds it faster.
What we do, and the honest part
We inspect free across Maryland and Northern Virginia, usually within twenty four hours, and you keep a photo report of the whole roof either way — not only the leak.
The honest part: United Developers does full roof replacements. If the leak is a boot or a flashing detail on a roof with years left, we will tell you that and tell you what it needs, because a page that diagnoses everything as a new roof is not worth reading. What we will not do is sell you a patch on a roof that is finished, and the photo report is what lets you see which one you have.
Get it looked at, free
We inspect across Maryland — Montgomery, Prince George's, Howard, Anne Arundel, Frederick, Carroll and Baltimore counties. Free inspection, photo report within 24 hours, and the report is yours to keep whether or not you hire us.