Flashing and gutter: two parts of the roof, and which one is leaking
A roof gutter carries water away. Flashing keeps water out. They are bent from the same sheet, sold on the same shelf and fitted by the same tinsmith, and that is the whole of what they have in common. When a wall is wet after rain, one of them has failed, and which one decides what gets fixed.
Both are roof work. We are plumbers, and the part of this that is ours starts at the bottom of the downspout. But we get sent for leaks that turn out to be flashing often enough that it is worth saying plainly what each part does, so you send for the right trade the first time.
What a roof gutter does
The gutter is a channel along the lower edge of the roof. Rain runs down the roofing sheets, drops off the edge into the channel, runs along it to an outlet and down the downspout to the ground. It is the alulod, and it comes in the shapes covered on our types of gutter page: box type, Spanish type, PVC half-round, and the valley, box and inside gutters that sit inside the roof's own shape.
A gutter fails by not carrying the water: it blocks and spills over its front edge, it sags and holds a pool, a joint opens and drips at one point. A gutter leak is water falling from the gutter where it should not, and you can usually see it from the ground.
What flashing does
Flashing is a strip of sheet, G.I. or sometimes aluminium, bent to cover a joint. A roof is only watertight where the sheets overlap each other, so every place the sheets stop is a joint that needs covering: where the roof meets a wall, where it runs over the ridge, where a pipe or a post comes through it, and at the eave where the sheets end. The flashing is lapped so that water running down meets metal and runs off it onto the next sheet or into the gutter.
In a Filipino hardware shop the pieces have plain names. End flashing goes at the eave and drops into the gutter. Wall flashing goes where a lower roof meets a wall. Ridge roll covers the ridge. Valley gutter, despite the name, does the flashing's job at an inside corner.
Flashing fails by letting water in: it was never fitted, it was cut too short, its top edge was sealed to the wall with a bead of sealant that has dried and opened, or it rusted through at a lap. A flashing leak shows up inside the house, as a stain, and you cannot see it from the ground.
Where the two meet: the end flashing
At the eave the roofing sheet ends over the gutter, and most of the rain drops straight off it into the channel. Some does not. Water also runs along the underside of the sheet and reaches its end, and in a wind it is blown back under the edge. The end flashing catches that water: a strip laid under the end of the sheet and bent down into the gutter, so what runs under the sheet drips into the channel and not behind it.
Drag the drawing sideways to see all of it.
A house with no end flashing, or one cut too short to reach the gutter, has water running down the fascia board behind the gutter in every heavy rain. The fascia rots or rusts, the paint on the wall under it peels, and the gutter gets blamed for leaking when it is doing nothing wrong. This is the single most common reason a new gutter does not fix a wet wall.
Which one is leaking
Drag the drawing sideways to see all of it.
Where the water shows up tells you which part failed. Look in the next heavy rain.
- Water spills over the front of the gutter along its length: the gutter is blocked or has no fall toward the outlet. Gutter.
- A steady drip from one point under the gutter: a joint or an end cap has opened. Gutter.
- Water runs down the fascia board or the wall behind the gutter: the end flashing is missing or short. Flashing, even though it looks like the gutter.
- A stain at the top of an inside wall where a lower roof meets it: the wall flashing has opened at the top edge. Flashing.
- A stain away from every wall, worse in long rain: a valley gutter blocked or too narrow, or a sheet lap. Roof.
- A drip around a pipe that comes through the roof: the flashing around that pipe. If the pipe is a vent or a water line, that is the one case where it is worth telling us.
- A drip that continues long after the rain has stopped: a pipe, not the roof. Our guide to water leaking from the ceiling sorts the three.
Who fixes which
Gutters, flashing, ridge rolls and valley gutters are a roofer or a tinsmith. Replacing a short end flashing means lifting the edge of the roofing sheets, and resealing a wall flashing means working at the top of a wall from a ladder or the roof. Neither is a plumbing job and we will say so if you send for us.
The one flashing a plumber does care about is the boot around a pipe that passes through the roof: the vent for the drains, which the plumbing code requires to end in the open air, or a water line to a tank on the roof. If that one leaks, it is sometimes the pipe and sometimes the flashing, and we can tell which.
From the bottom of the downspout onward it is drainage: the catch basin, the pipe to the canal, and the rule that none of it goes into the septic tank. That is our outdoor plumbing service, and our guide to the alulod and downspout covers it.
Questions about flashing and gutters
- What is a flashing gutter?
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Two different parts sold in one listing. The gutter is the channel along the roof edge that carries rain to the downspout. The flashing is the strip of sheet that covers a joint so water cannot get in. Shops bend both from the same G.I. sheet and list them together, which is where the phrase comes from.
- What is the purpose of flashing?
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To keep water out at the places a roof is not continuous: where it meets a wall, where a pipe or a post comes through it, along the ridge, and at the eave where the sheet ends. Each of those is a strip of sheet lapped so that water running down meets metal and runs off, never a gap.
- Do you put flashing behind the gutter?
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Yes. The end flashing, sometimes called a drip edge, lies under the end of the roofing sheet and turns down into the gutter. Without it, water running under the sheet drips behind the gutter onto the fascia and down the wall, and the gutter looks like the problem when it is doing nothing wrong.
- Is the gutter part of the roof?
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It is fitted to the roof edge and belongs to the roofer or tinsmith, along with the flashing. The pipe that takes its water down the wall is the downspout, and from the bottom of that pipe to the canal is drainage, which is plumbing.
- Who fixes a leaking flashing?
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A roofer or a tinsmith. The exception is the flashing around a vent pipe or a water line that passes through the roof, which a plumber may have put there and will look at. If you are not sure whether the leak is flashing, gutter or pipe, send us a photo of the stain and where it is, and we will say which it is likely to be.
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