Roof flashing around chimneys, vents, and valleys

If you have read that most roof leaks start at the flashing, this is the article that shows you where and why. Flashing seals the joints where a roof meets something else, and three spots in particular do the hardest work: the chimney, the vents and penetrations, and the valleys where two roof planes meet. Each is sealed differently, each fails in its own way, and understanding them explains most of the leaks a roof will ever have.

Chimney flashing

The chimney is the biggest interruption on most roofs, and its flashing is a layered system. Base flashing tucks under the roofing on the low side, step flashing weaves up the sides, and counter-flashing sets into the masonry to cap it all. That is a lot of pieces, and any one of them lifting, rusting, or being replaced with caulk lets water into the gap. Chimney flashing is the single most common leak point, covered on our page on chimney leaks.

Vent and penetration flashing

Every pipe, vent, and duct that pokes through the roof needs its own seal. Plumbing vents typically use a boot, a collar that fits around the pipe, and those boots crack and dry out over the years, especially under the LA sun. Other penetrations are flashed individually. Because there are often several of these on a roof, they add up as leak points, and a cracked vent boot is a classic small leak, covered on our page on common leak sources.

The three trickiest joints

Joint	How it seals	Common failure
Chimney	Layered base, step, counter-flashing	Lifted or caulked-over metal
Vents / penetrations	Boots and individual flashing	Cracked, dried boots
Valleys	Valley flashing channels water	Clogs, corrosion, debris

Valley flashing

A valley is where two roof planes meet and a huge volume of water funnels down. Valley flashing is the channel that carries that water safely off the roof. Because so much water runs through, valleys are punishing: they corrode, and they clog with debris that dams the water and forces it under the roofing. A leak that shows in heavy rain often traces to a valley, covered on our page on leaks only in heavy rain.

Why they fail

Across all three, the failures rhyme: metal that flexes with heat and lifts, corrosion over the years, debris that dams water, and the recurring sin of caulk used in place of proper flashing. Caulk dries and cracks in a season or two, so a joint "sealed" that way leaks again soon, which is why the lasting fix is always proper metal, covered on our page on flashing failures.

Getting it fixed right

The lasting repair for any of these is to reflash the joint correctly, layered and tucked, not to smear on more sealant. On a reroof, new flashing at every one of these spots is worth insisting on, which is why it is a line to check on any estimate, covered on our page on reading a roofing estimate. If you have a stain near a chimney, a vent, or a valley, our flashing repair and leak detection pin and close it, and a roof inspection checks all three while there.