Finding The Leak
The stain on the ceiling is where the water came out. The entry point is almost always somewhere else, and frequently several feet up the slope.
Short answer
Water enters at the roof, runs along the underside of the sheathing and down the side of a rafter, and drops where something interrupts it. Trace it from the attic upward from the stain, in or just after rain. Flashings, penetrations and valleys account for most of them; the field of the shingle rarely does.
Why it is not above the stain
Water follows the underside of the sheathing and the framing, so it travels laterally before it falls.
It drops where there is an obstruction — a nail, a joint, a piece of strapping, a wire.
Insulation absorbs and holds it, which both delays the appearance and moves it.
In a Toronto house with a finished attic or a cathedral ceiling there is no access at all, which is where thermal imaging and a hose test earn their keep.
And with ice damming the entry is at the eave while the stain is at the interior wall line, which is a particular Toronto pattern.
Go up during rain
The single most useful thing is to be in the attic with a light while it is actually raining, or within an hour of it stopping. Look up-slope from the stain for rust on nails, dark streaking down a rafter, wet insulation and daylight. A dry attic in August tells you much less.
The usual entry points
Chimney flashing, which is the most common single source on a Toronto house.
Step flashing where a roof meets a wall, particularly at a dormer.
Valleys, which carry the most water.
Plumbing stack boots, where the rubber has perished and cracked.
Skylight perimeters and the flashing kit around them.
And ice damming at the eave, which is seasonal and is the reason a leak that appears only in February is not a shingle problem.
What not to do
Do not have somebody caulk a general area in the hope of catching it. Roofing cement smeared over a flashing is the signature of a guess, and it makes the real repair harder.
Do not paint over the ceiling stain until the leak is confirmed fixed, because the stain is the only indicator you have.
Do not assume the shingles have failed, because they usually have not.
Do not ignore a stain that has stopped growing — a seasonal leak comes back with the season.
Do not let anybody replace the roof to fix a flashing leak.
And do not skip the attic, because the attic is where the evidence is.
Common questions
Is the leak above the stain?
Usually not. Water runs along the sheathing and the framing before it drops.
What is the most common source?
Chimney flashing, then step flashing at a wall, then valleys and stack boots.
Why does it only leak in February?
That is ice damming at the eave rather than a shingle problem.