The drip that only comes in rain, and the six places to look

The open face of a shingled roof sheds water as long as the slope is right. The trouble starts where the roof is interrupted: a chimney pushes through it, a wall rises out of it, a pipe passes through it. Each interruption needs a separate piece of flashing, and each one is a place where a drip can begin.

The code describes how those pieces are built. It does not promise how long any of them lasts, and nobody can read a life span off the age of a house. The useful question at every joint is whether it is built the way the code describes. Take that question to the roofer, one joint at a time.

The code requires flashing wherever a roof meets a chimney. A chimney more than 30 inches wide, measured across the slope, also needs a cricket on its uphill side, a small peak that splits the water and sends it around the masonry.

Without a cricket, water piles up behind a wide chimney and works at the flashing. Your next step is to ask the roofer to measure the chimney across the slope, and to say whether yours has one.

From the ground you can often see the chimney against the sky. Look for gaps along the mortar line where the flashing meets the brick, and for a bead of sealant that looks recent and thick. A lot of sealant at a joint is a sign that someone has been here before, and the roofer should hear that history.

Where a lower roof runs into a taller wall, the base flashing should be a single piece or a series of steps. A kick-out at the bottom turns the water into the gutter so that it does not run down behind the siding.

Siding can hide the failure. Newer vinyl, fiber cement or brick veneer does not peel the way old paint does, so a wall joint can leak behind it for years while the outside looks clean. If your drip sits under a wall line, ask about the kick-out by name.

Look at the wall joint from the yard and from inside. A damp patch on the ceiling at the point where the wall meets the roof line, or a stain on an upstairs wall near the roof edge, are both worth a dated photo. They show where the water travels once it is behind the siding.

A vent pipe leaves the roof through a boot or collar. The code sets no size for it. It says the pipe flashing must follow the shingle maker's printed instructions, which is why this joint is judged by how it was installed, not by how old it is.

Ask the roofer which instructions were followed for your pipe. If the answer is vague, ask to see the flashing and how it overlaps the shingles around it.

You can find the vent pipes from inside. From the hatch opening, look for pipes that rise to the roof, and match them to the plumbing below. Each one is a hole in the roof, and a stain that sits right under a pipe is a good clue. Photograph the spot where the pipe meets the roof boards only from the hatch opening, and keep off the insulation.

A bath fan hood on the roof is another hole, and it needs flashing all the way around, like any opening. There is a second point here. A bath fan should vent outdoors, and one that ends in the attic can cause a stain that looks like a leaking hood.

If a fan sits close to your stain, tell the roofer where its duct goes, then ask them to look at both the hood and the duct.

A skylight is a large opening, and the code requires flashing around the whole of it. The uphill side is the one that matters most, because water running down the roof meets it head on.

If your stain is near a skylight, say which side shows it first. Ask how the uphill flashing is built and whether it ties into the roofing above the glass.

Skylights add a second cause to check: condensation on the glass itself. A wet frame after a cold, clear night may be room air meeting a cold pane, not a joint. Note the weather when you see it, and tell the roofer.

A valley is where two slopes meet and send their water down one channel. Valley lining goes on before the shingles, and an open valley lined with metal must be at least 24 inches wide.

A valley carries more water than the open roof beside it, so a small flaw shows quickly. Ask whether your valley is open or closed and what lines it.

Valleys are hard to judge from the yard. If your stain sits under the line where two slopes meet, ask the roofer to show you the valley and explain what lines it.

Water does not drop straight down. It follows the roof boards, the rafters or the top of the ceiling for some distance before it finds a way through. The stain can therefore sit several feet away from the joint where the water entered, and usually lower down the slope.

That is one reason a quick patch at the nearest fixture so often misses. When you describe your stain, give the roofer the room, the wall it is nearest, and any fixture on the roof above. Let them trace the route instead of guessing at it, and ask them to tell you what they found along the way.

Stand in the room under the stain and look up. Then picture the roof above that spot. Is there a chimney, a wall, a vent, a fan hood, a skylight or a valley near it? Water often travels a short way along the roof boards before it drops, so the joint can sit a little uphill of the stain.

Take a dated photo of the stain and note which joint is closest. Send both with your request, and the roofer arrives knowing where to start. The flashing page sets out the visit in four steps.

• Which pieces of flashing are in place, and which are missing?
• How do they overlap the roofing and the wall around them?
• Is there anything behind this joint that needs to be checked too, such as a cricket or a kick-out?
• What will the repair change, and will it be written into the proposal?

If a roofer proposes to seal over the top of a joint without naming the pieces underneath, ask what the sealant is covering. The answer tells you whether the joint is being rebuilt or only closed for now.

Not every stain beside a fixture is the fixture's fault. A stain that grows after a cold, clear night near a bath fan hood may be steam. A stain that returns beneath a flat porch roof may be a slope problem. Those cases are covered in the leak or attic guide and the porch roof guide.

The weather you noted is your best way to tell them apart. A drip that follows rain and sits beside an opening is a joint until the roofer shows you otherwise.