How a Roof Valley Works, and Why It Leaks

A valley is where two roof planes meet at an inside angle, and it is the busiest few feet on any roof. Everything that lands on both slopes funnels through it. That concentration is why valleys leak more than their share, and why the detail is worth understanding before you approve a quote.

Open versus closed

An open valley runs a formed metal channel down the intersection with the roofing stopped short on either side, so the metal stays exposed. Water runs on metal the whole way down. It sheds debris well, and it is what we use on metal roofs almost without exception.

A closed valley carries the roofing material across the intersection, so the water runs on shingles or panels. It is common on asphalt roofs and cheaper to do. It also holds debris and gives water more chances to find a seam.

On a metal roof the open valley is not really a choice. Panels end at the valley in a hemmed edge that hooks onto the valley metal, so the valley is a piece of the system rather than a place where two pieces of the roof overlap. How the seam is formed matters here as much as anywhere.

Why valleys fail

• Debris damming. Leaves and needles collect, water backs up sideways past the edge of the channel, and gets under the roofing. This is the most common valley failure, and it is a maintenance problem more than a construction one.
• The channel is too narrow. A valley on a large roof, or one where a big slope drains into a small one, needs more width than a standard detail. Undersizing works fine until the first hard rain.
• Nothing underneath. A valley should have self-adhered membrane run full width under the metal. When the metal alone is doing the work, one fastener through the channel is a leak.
• Fasteners in the flow path. Nothing should penetrate where water runs. Valley metal is held at its edges, outside the channel.
• Ice. In Indiana, Ohio, and Michigan, valleys freeze and back up the same way eaves do, and a valley carrying two slopes' worth of meltwater freezes harder than a straight run.

What a quote should say

It should name the valley metal, say how wide the channel is, and say that self-adhered membrane runs the full width underneath. If a quote for a complicated roof does not mention valleys at all, it has not been priced properly. Valleys are where the labor is. Our guide to reading a quote goes through the rest of the line items.

Where this matters most

Simple gable roofs have no valleys and this article barely applies to them. Story-and-a-half houses, homes with dormers, additions tied in at different times, and anything with an L or T footprint live and die by their valleys. The more of them a roof has, the more the installer's flashing work decides the outcome, and the less the brand of panel does.

A valley that is leaking right now

Keep it clear. Most valley leaks I hear about are debris dams on a roof that was built correctly, and clearing it once or twice a year is the whole fix.

If it is leaking with a clear valley, the problem is underneath and the valley needs opening up. Patching the surface of a valley rarely holds. On a roof with life left in it, that is a repair contractor's job, one detail on a sound roof, and I would say so. If the valley is one of several places an aging roof is giving up, that is a different conversation, and the repair-or-replace guide walks through how to tell which you have.

Now you know what that channel is doing and what to ask about it.