What is the best way to build a roof valley?
It depends on the geometry, and there are three constructions in use. An open metal valley leaves a formed channel of metal exposed down the centre with the shingles cut back either side, and it is the most durable choice on unequal planes, low pitches, long runs and anywhere under heavy tree cover, because water and debris move through a smooth metal channel instead of over shingle edges. A closed-cut valley carries one plane through and trims the other to a line just off centre, and it is a perfectly sound choice on equal, moderate pitches with a short run. A woven valley interlaces both planes course by course, and while it survives on old three-tab roofs it is impractical and often not permitted with modern laminated shingles, which are too thick to lie flat in a weave. Under all three, the same two things are non-negotiable: full-length self-adhering membrane on the deck, and no fasteners anywhere near the water channel.
Why a valley is not just another part of the roof
Stand in a valley during a heavy rain — from a window, not on a ladder — and the difference is audible. The field of the roof makes a hiss. The valley makes a sound like a stream, because it is one.
The arithmetic is unforgiving. A valley collects the entire catchment of both planes above it and funnels it into a channel a few inches wide. Where the two planes are unequal — a big main roof meeting a small dormer, or a steep plane meeting a shallow one — the water does not merely combine, it arrives at different speeds and gets thrown sideways across the seam. Anything a valley does poorly, it does poorly at several times the volume of anywhere else on the roof.
Every valley on your roof is a small river with a shingle bed.
Then add what the water is carrying. Valleys are where leaves, pine needles, seed pods and shingle grit collect, because that is what a channel does. Debris in a valley builds a dam, the dam holds water, and standing water on a shed surface is the same failure mode as an ice dam at the eave — just in September. In winter the same channel is where ice forms first and melts last.
And a valley is the one place on a roof where the covering has to be cut. Every shingle crossing or terminating at a valley has a cut edge, and a cut edge has no factory sealant, no granule coverage on the exposed cut, and a capillary path along it. How those cuts are made and where they land is most of what separates a good valley from one that will be back.
What goes underneath, whatever the construction
Before any decision about metal or shingles, the valley gets lined. This part is not a style choice and it does not vary between the three methods.
Self-adhering membrane, full length
A bonded ASTM D1970 sheet centred in the valley, running from the eave to the top of the valley in one continuous run, wide enough that it extends well past where any cut will land. It is the layer that makes every fastener and every cut edge in the vicinity survivable.
Fails when: Membrane in the lower half only, or ordinary underlayment folded into the valley instead. Also, a valley lined in short overlapping pieces rather than a continuous run.The membrane goes in before the field underlayment
Valley membrane first, then the field synthetic laps over it from each plane. That way water on top of the field layer runs onto the valley lining, not under it.
Fails when: Field underlayment run through the valley first and membrane stuck on top of it, which puts a lap facing uphill in the highest-flow part of the roof.Valley metal, where the construction calls for it
For an open valley, a formed W or V channel over the membrane. The W has a raised rib down the centre, which stops water from one plane shooting across and up under the shingles on the other side — the failure that happens when the two planes are unequal.
Fails when: Flat sheet used instead of a formed profile on unequal planes, or metal too narrow so the shingle cuts land inside the water path.Nothing in the channel
The clear rule everything else follows from: no fastener within about six inches of the valley centreline, in any construction. In an open valley the metal is held by clips or nailed only at its far outer edge.
Fails when: Nails driven through valley metal in the middle of the flow, sometimes with a dab of sealant over each. Every one of those is a hole in the fastest-moving water on the roof.The old roof tells you what the new one needs. Rust holes in the existing valley metal, granule loss concentrated in a stripe down the valley, cupped shingle edges along the cut line, or a repair history at one valley and nowhere else — all of that is evidence about flow volume and debris load at that specific valley, and it should be what drives the choice of construction rather than crew habit.
The three constructions, compared honestly
| Open metal valley | Closed-cut valley | Woven valley | |
|---|---|---|---|
| How it is built | Formed metal channel over the membrane; shingles from both planes cut back a few inches either side of the centre, leaving the metal exposed. | One plane runs its shingles through the valley and past the centreline; the other plane is trimmed to a line about two inches off centre, on top. | Courses from both planes are interlaced through the valley, alternating over and under, with no cut down the middle. |
| Water path | Down smooth metal. Nothing to catch on, nothing to dam against. | Down over the shingles of the through plane. Rougher, slower, but continuous. | Down over interlaced shingles. Roughest of the three. |
| Debris behaviour | Best. Leaves and needles flush through. | Moderate. Debris catches on the cut line. | Worst. The weave holds material. |
| Best suited to | Unequal planes, low or mixed pitch, long valley runs, heavy tree cover, and any transition from shingles to metal roofing. | Equal or similar pitches, moderate valley length, clean exposure. The workhorse valley on a straightforward roof. | Old three-tab roofs, and repairs matching existing woven work. |
| Cosmetics | A deliberate architectural line. Some people love it, some do not want to see metal. | Nearly invisible from the ground. The default aesthetic expectation on most DMV houses. | Invisible, with a slight hump along the seam. |
| The honest weakness | It is metal, so it can be dented by a falling branch, and a poor termination at the eave or a fastener in the channel ruins the whole benefit. | The through plane is asked to carry the flow of both, and the cut edge on the trimmed side is a capillary path if it is not dubbed and set correctly. | Laminated shingles are too thick to weave flat, so the seam humps and does not seal. Many shingle manufacturers do not permit it on their laminated products at all. |
The details that make a closed-cut valley good rather than adequate
Closed-cut is the most common valley in this region and there is nothing wrong with that, but it is the construction most sensitive to workmanship, because all its protection is in how the cuts are made.
- The through plane goes first, and goes all the way. Shingles from the lower-sloped or lesser plane run through the valley and at least a foot up onto the adjoining plane. Running them just to the centreline leaves the seam unprotected.
- The cut sits off centre. The trimmed plane is cut to a line roughly two inches back from the valley centre, so the cut edge is never in the middle of the flow.
- Every corner is dubbed. The upslope corner of each shingle crossing the valley gets clipped at an angle. That tiny cut breaks the path water would otherwise wick along the top edge of the shingle and under the course above it. It is one snip per shingle and it is the difference between a valley that stays dry and one that seeps in wind-driven rain.
- The cut edge is set, not just laid. A bead of the correct sealant under the cut edge, kept out of the water channel, holds it down against wind and wicking.
- No fastener within six inches of the centre. Same rule as everywhere else. On a closed-cut valley this is where it is most often broken, because the nail line of the course is inconvenient there.
The dead valley, and why it is a different problem
Not every place two planes meet is a valley in the useful sense. A dead valley is a low spot with nowhere to drain — a roof plane running into a wall, a valley that terminates against a dormer face, a flat pocket behind a chimney chase, or the notch where an addition roof dies into the main house. Water arrives and has nothing to do.
A dead valley is not a shingle problem and it cannot be solved with shingles, because shingles need slope and there is none. What it needs is a genuinely waterproof pan: a soldered or welded metal pan, or a fully adhered low-slope membrane system, turned up the adjoining walls behind the wall covering and drained deliberately to somewhere it can leave. Frequently it also needs a cricket built to create the slope that was never there.
This is the single most common source of a chronic, unfixable leak. If your house has one spot that has been sealed four times by three companies and still leaks, walk outside and look for a place where the roof has no way to shed. Dead valleys get shingled over because shingling is what the crew was there to do, and then they get caulked annually for a decade. The fix is to stop treating it as roofing and build it as flat-roof waterproofing.
What we specify, and why
Every valley is counted, measured, and named by construction
Valley work is priced by the linear foot and the count, and both numbers come off the same measurement as the rest of the roof — so there is no reason for either to be absent from a quote.
- Number of valleys stated, and the linear feet of each
- Construction named per valley — open metal or closed-cut — with the reason
- Full-length ASTM D1970 membrane in every valley, continuous, under the field layer
- Valley metal profile named where open, W or V, with its width
- No fasteners within six inches of the centreline, clips or outer-edge nailing on metal
- Upslope corners dubbed on every shingle crossing a closed-cut valley
- Dead valleys identified and quoted as waterproofing, never shingled over
- Photographs of each valley lined before the shingles cover it
How to tell whether a contractor is cutting this corner
Valleys are countable from an aerial measurement before anyone leaves the office. An open-ended phrase for a countable quantity is a change order waiting to be written.
Ask instead: how many valleys there are and how many linear feet in total.Good, and incomplete. The question is whether it runs the full length in one continuous piece, and whether the field underlayment laps over it or under it.
Ask instead: full length, continuous, and laid before the field layer.On a modern laminated shingle a weave will not lie flat, and many manufacturers do not permit it on their laminated products. It is a three-tab technique being applied to a product it was not designed for.
Ask instead: closed-cut or open metal, and which one suits this geometry.A fastener in the water channel is a hole in the fastest-moving water on the roof, and the sealant over it has a shorter life than the roof does. Valley metal is held with clips or nailed only at the outer edges.
Ask instead: how the valley metal is fastened, and whether anything penetrates the channel.A pocket with no slope is not a roofing detail and sealant will not make it one. This is the classic chronic leak: sealed repeatedly, never fixed.
Ask instead: for a pan or a cricket that gives that area somewhere to drain.A preference presented as a fact. On unequal planes, long runs or heavy tree cover, open metal is the more durable construction, and whether you want to see it is a decision you should get to make knowingly.
Ask instead: which construction is technically better on this roof, and then decide the aesthetics separately.Valley questions DMV homeowners ask
Which is better, an open metal valley or a closed-cut valley?
Neither, in the abstract. Open metal is the more durable construction where flow or debris is high: unequal roof planes, low or mixed pitch, long valley runs, heavy tree cover, and any transition between shingles and a metal roof. Water and leaves move through a smooth channel instead of over cut shingle edges. Closed-cut is a sound and very common choice on similar pitches with a moderate run and clean exposure, and it is nearly invisible from the ground, which is why most houses in this region have it. The right question is which suits your geometry, decided before the aesthetics.
Is a woven valley bad?
It is not bad on the roofs it was designed for, which were three-tab shingles. It is impractical on modern laminated architectural shingles because they are too thick to interlace and lie flat, so the seam humps and does not seat properly, and a number of shingle manufacturers do not permit weaving on their laminated products at all. A woven valley on a laminated roof is usually a sign the crew is applying a habit rather than reading the product instructions, and it is worth asking about before work starts rather than after.
Why does my valley keep leaking after it has been repaired?
Three causes account for most of it. Fasteners in the water channel, which is a hole that sealant covers temporarily and water finds repeatedly. Missing or short valley lining, so there is nothing bonded to the deck under the cuts. Or it is not really a valley at all but a dead valley, a low pocket with nowhere to drain, which cannot be fixed with shingles or sealant and needs a soldered pan, a fully adhered membrane, or a cricket to create the slope that was never there. A repair that has failed more than once is usually pointing at the third case.
How much does a valley add to a roof replacement?
It is priced by the linear foot plus the labour of the cuts, and both come off the same aerial measurement the rest of the roof is quoted from, so it should always appear as a stated quantity rather than a contingency. What that means in practice is that a complicated roof with six valleys is genuinely more expensive to build correctly than a simple gable with none, and a quote that treats them as an afterthought is either absorbing that cost somewhere invisible or intending to raise it later. Ask for the count and the linear feet, and compare that number between quotes.
Can a valley be repaired without replacing the whole roof?
Often yes, and it is one of the more worthwhile targeted repairs on a roof that otherwise has life left. A valley can be opened up, relined with membrane, refitted with new metal or recut properly, and closed back in, provided the surrounding shingles are still pliable enough to lift without breaking. On a roof near the end of its life the shingles will crack rather than bend and the repair becomes a patch on a failing surface, which is the point at which replacement is the honest recommendation rather than the profitable one.
Where to go next
Valleys, walls, chimneys and pipes are the same category of problem: water being turned rather than shed. The flashing guide covers the rest of them.
Flashing and penetrations
Step, kick-out, apron, counterflashing, crickets and boots. Sealant is a finishing detail and never was a water barrier.
Read the standard →The thesisWhy a roof fails at its details
Ninety-something percent of your roof is the easy part. Take the inventory of the hard part and you can read any quote.
Read the standard →Ice barrierIce-and-water shield: where it must go
The membrane that lines every valley, and the 24-inch rule at the eave that most quotes get wrong.
Read the standard →ServiceRoof leaks and storm damage
A valley that has been repaired twice is telling you something. Documented, photographed, report yours to keep.
See the service →