A winter stain, snow on the shingles, and no way to see the roof

It feels as though a leak has to wait for spring. In fact a cold morning shows things that a summer afternoon hides. Frost on the roof boards, a damp hatch cover and a stain that grew overnight all appear most plainly in the cold, which is exactly what a diagnosis needs.

The Department of Energy guide says a vented attic with insulation at the attic floor makes roof leaks easier to detect and solve. Your job in winter is to gather clues, not to climb on the roof. The roofer will do the climbing when the visit comes.

The first clue is the weather. Did the stain grow after snow or a thaw, or after a clear night with nothing falling? A stain that grows after a clear night with nothing falling probably did not come from the sky.

Write the date and the weather beside each change. After a few weeks the pattern will tell you which way the roofer should start, and you will have it ready to send.

Frost on the underside of the roof boards is water that froze out of the air. Condensation forms whenever a surface is cooler than the dew point of the air around it, and in cold weather it can first show as frost, then turn to water when it warms. The roof deck is the main surface where this happens.

If you see frost, take a photo before it melts, and look again in the afternoon. A frosty morning followed by a dripping afternoon is a strong sign that the attic, not the shingles, is making the water. Tell us when you book.

A thaw is a different test. When snow melts, water runs down the roof all at once, and it shows you where the water path is weak. A stain that appears during a thaw may come from water that has nowhere to go at the roof edge, or from heat escaping into the attic and melting snow from below.

At the eaves, insulation can be thin or compressed, and in winter that can create cold spots along the ceilings. If your stain runs along an outside wall, say so. It points the roofer to the edge of the attic first.

If you share the house or rent a flat, ask your neighbor or landlord to check too. A stain in one unit can come from a bath fan or a kitchen in the other, and the attic above both is shared. Compare notes on the dates and the weather before anyone books a visit.

• Open the attic hatch on a cold morning and shine a light across the roof boards.
• Look for frost, white crust, dark wood and matted or damp insulation.
• Feel the hatch cover; a cold, damp cover means warm air is leaking past it.
• Check where each bath fan vents, and whether the duct is attached.
• Read the humidity in the room under the stain; most sources recommend 30 to 50 percent in winter.

Do not walk on the insulation or the ceiling below it. A light and a camera from the hatch are enough, and a dated photo is worth more than a guess.

You can see a lot from the ground without touching the roof. Look at the gutters and downspouts after a thaw. Is water leaving them, or sitting in them? A gutter that holds water, or a downspout that ends against the wall, can add to a problem that starts higher up.

The guide asks for a drop of 1/16 inch or more per foot of gutter run. You cannot measure that from the ground, but you can see whether water runs toward the spout or pools along the run. Send a photo of anything that looks wrong.

When the roofer comes, the attic is the first stop, not the roof. They look at the boards, the insulation, the ceiling gaps and the bath fan ducts. If the stain follows rain, they move out to the roof, the joints and any flat sections. In snow, the roofer may leave the roof for a dry day and still give an answer from the attic.

That is why your log and photos matter. They tell the roofer which part to start with, and they stop the visit from going to the wrong place first. The written proposal then names the cause and the repair, so you can decide at your own pace.

A phone camera often fogs or misses frost in a dim attic. Use the flashlight on the phone from the side, so the frost throws a shadow, and take one wide shot and one close shot. Put the date in the name of the file, and write down the outdoor temperature if you know it.

Leave the stain unpainted until the cause is known. Put a bucket or a towel under a drip, and keep clear of light fittings and switches near the wet area. If the ceiling bulges or sags, keep people out of the room and call the roofer, or an electrician for a wet fixture. Keep the log going. If the stain grows fast, say so when you book, and we will make sure the roofer hears it. Avoid chipping snow or ice off the roof yourself, which can damage the covering and is risky on a ladder.

If a stain is stable, there is time to gather the facts. Send the log, the photos and the answers to the five questions beside the form, and the visit can begin with the right question.

A good entry takes a few seconds and has four parts: the date, the stain (bigger, smaller or the same), the weather the night before, and anything unusual indoors, such as a long shower or a laundry day. Short lines are fine.

• Jan 12: stain bigger. Clear and very cold overnight. Two long showers.
• Jan 15: same size. Light snow all day. No showers.
• Jan 18: stain bigger. Thaw and rain. Dripping near the chimney wall.

Even three entries like these point the roofer in a direction. Add a photo whenever the stain changes, and send the lot in with your request.

• Does the stain follow rain, a cold night or a thaw?
• Is there frost on the roof boards on cold mornings?
• Does a porch or flat roof sit above the stain?
• Where does each bath fan vent?
• Where do the downspouts end?

Those five answers cover most of what a first visit would have to find out. The roof leak diagnosis page shows the steps, and the attic ventilation guide covers what happens if the attic turns out to be the cause.