Did that stain come through the roof, or off the attic boards?

Think back to the last time the stain appeared or grew. Was it raining, or had it just rained? Or was the night clear and cold with nothing falling at all? A roof cannot let in water that never landed on it, so a dry night points away from the covering and toward the attic.

That second case is common in winter: at Syracuse Hancock Airport an average January has about twelve days with no precipitation at all. Write down the date and the weather each time your stain changes, and you will see your own pattern within a few weeks.

Water collects on any surface cooler than the dew point of the air around it. In cold weather that water can show first as frost and turn to drips when it warms. The Department of Energy guide calls the roof deck, the boards under the shingles, the main condensing surface in a roof.

So the attic is not outside. It is connected to your rooms by every gap in the ceiling, and warm air from your kitchen and bath rises into it. You can act on that today: tell the roofer which rooms sit under the stain, and say whether anyone showers or cooks heavily near it.

Condensation in an attic needs four conditions at once. There has to be a surface below the dew point, indoor air pushed through a gap in the ceiling, humid indoor air, and too little ventilation or drying to clear it.

That list is useful because you can break the chain at more than one place. Seal the gaps, keep humidity in a sensible range, and move bath steam outdoors, and the surface has less to collect. Most sources recommend indoor humidity between 30 and 50 percent in the heating season, so a humidity gauge in the room under the stain is a fair first step.

A fresh stain is dark and damp to the touch, and it may drip. An old one is dry, brown at the edge and paler in the middle. A ring that gets a little bigger each time tells you water has arrived more than once. That is why a pencil line and a date are worth more than a photo taken once.

If the spot is damp on a day with no rain, the water probably did not come from the weather. It may have come from the attic, or from a pipe or fixture above the ceiling. Keep clear of any light fixture under it, note the date, and tell the roofer which day you saw it.

A stain under a bathroom, kitchen or laundry can also be a plumbing leak. If the stain is under one of those rooms and the roof was dry, call a plumber as well. The roofer can rule out the roof or refer you on.

If you have an attic hatch, open it on a cold morning and shine a light along the roof boards. Do not climb in or step on the insulation. You are looking for white crust or frost on the underside of the boards, dark patches in the wood, and insulation that looks damp or matted.

Photograph whatever you see, with the date. Then look at the bath fan duct, if you can see it. A duct that ends in the attic instead of going outdoors is a strong clue, and the guide is clear that a fan should vent outside.

When the attic is the answer, the work runs in a fixed order. First the gaps in the ceiling get sealed, because that is where the warm air gets up. Then the bath fan duct is sent outdoors, and the soffit path is opened so air can enter at the low edge. Vents come last, and only as many as the sum for your attic calls for.

That order is our advice. The Department of Energy guide backs the first step, sealing at the ceiling plane. If a proposal starts with new vents and says nothing about the ceiling, ask why, and ask what it does about the air that is already getting up there.

Houses are not tidy. A roof can have a weak joint and a leaky ceiling at the same time, and the two stains can land in one room. The weather clue then gets muddy, because the stain grows after rain and after cold nights alike.

If that sounds like your house, say so when you book. The roofer can sort the two out by looking at the attic and the joint on the same visit, and the proposal can list them as separate lines so you can decide which to do first.

Painting over the stain hides the one clue you have, so leave it for now. Adding vents before the ceiling is sealed asks the vents to cope with a steady supply of warm air. Pulling out insulation to let the attic breathe makes the ceiling colder and the cold spots worse.

Waiting is fine if the stain is not growing, and it costs nothing to ask while you wait.

A stain that shows only in or just after rain does point at the roof. The likely places are the spots where the roof is cut through: a chimney, a wall line, a vent, a skylight. The code requires flashing at every one of them, so check which fixture sits nearest the stain.

The other rain-only suspect is a porch or addition roof that is too flat. Shingles are allowed only at 2:12 or steeper, and a flatter roof can leak in rain no matter how often it is patched. If your stain sits under such a roof, the low-slope roofing page explains what to ask, and the flashing page covers the joints.

If you are not sure which cause you have, keep a simple log for two weeks. It takes a minute a day and turns a feeling into a pattern a roofer can read.

• Each morning, note whether the stain looks bigger, smaller or the same.
• Note the outdoor weather the night before: rain, snow, or clear and cold.
• Note whether the bath fan or a humidifier ran, and for how long.
• Once a week, take one dated photo from the same spot.

After two weeks, look for the link. A stain that grows after cold, clear nights and sits still after rain is an attic stain. One that grows after rain and sits still after clear nights is a roof stain. Send the log with your request.

In a two-unit house, one roof and one attic cover both homes, and the moisture from both kitchens and baths rises into the same space. A stain in the upper unit can come from steam made in the lower one. If you manage such a house, tell us which unit sits under the stain, and which unit has the bath fan.

That detail changes what the roofer looks at first, and it saves a second visit when the answer turns out to be shared.

• The date and the weather each time the stain appeared or grew.
• The room, and what sits above it: a porch, an addition, an attic or a flat roof.
• Whether it has grown, with a pencil line round the edge and a date.
• Where each bath fan vents to.
• What was done the last time, and by whom.

Send that list with the form, or call and read it out, and the visit starts with the right question.

Ask where the water gets in, and how they know. A roofer who answers before naming a covering is working on your stain, not selling a product. Ask whether the cause could be the attic, and what they saw there. Then ask for the answer in the written proposal, with the cause named next to the repair.

The roof leak diagnosis page shows how the visit runs, and the attic ventilation page covers what happens if the answer is the attic.