If you’re getting quotes for a new roof or a repair, “valley” is going to come up. Contractors talk about them because valleys handle more water than any other part of your roof, and they’re one of the first places a roof starts leaking when something’s done wrong.
Here’s what a valley is, how it gets built, and what to watch for on a Chicagoland home.
The Basic Definition
A roof valley is the V-shaped channel formed where two roof planes meet at a downward angle. If you have a roof with any complexity — dormers, an L-shaped footprint, a garage that intersects the house, a gable over the front entry — you have valleys.
Water from both roof slopes drains into that channel and runs down it to the gutter. That means a single valley can carry runoff from hundreds of square feet of roof surface. Compare that to a random spot in the middle of a slope, where water only has to travel a few feet before it’s off the roof.
More water moving through a narrow channel, at higher speed, over a seam where two roof sections join. That’s why valleys leak.
Why Valleys Matter More in Chicago
Volume of water is only half of it. The other half is what our winters do.
Snow slides down both slopes and piles up in the valley. It sits there longer than snow on an open slope because it’s deeper and shaded by the intersecting planes. Then it goes through freeze-thaw cycles — melts during the day, refreezes at night — and you get ice buildup right in the channel.
Ice damming in a valley pushes water sideways and uphill, underneath the shingles. Standard shingle overlap only sheds water that’s moving downhill. Once water is being forced backward under the courses, the shingles do nothing. What stops it is the underlayment beneath them, and in a valley, that needs to be the right kind.
Debris makes it worse. Valleys collect leaves, pine needles, and shingle granules the same way they collect water. A packed valley dams up runoff and holds moisture against the roof deck.
The Three Ways Valleys Get Built
There’s more than one correct method. What matters is that whichever method your roofer uses, they do it completely.
Open Valley
The shingles stop short on both sides, leaving a strip of metal flashing exposed down the center. Water runs on the metal, not on the shingles.
Usually the metal is painted aluminum or steel in a color that matches the shingles, or copper on higher-end work. The metal is typically 24 inches wide, formed with a center rib or W-shape that keeps water from one slope from washing across into the shingles on the other slope.
Open valleys shed water and debris the best. They’re the most durable option and the easiest to inspect — you can see the metal from the ground. The downside is cost and the fact that some homeowners don’t like the look of a metal stripe.
Closed-Cut Valley
Shingles from one slope run all the way through the valley and up onto the adjoining slope. Shingles from the other slope are then run over the top and cut in a straight line a couple of inches off the valley center.
This is the most common method on asphalt shingle roofs in our area. Done right, there’s still valley flashing or a full-width ice and water membrane underneath, and the cut shingles are trimmed with a dubbed corner so water doesn’t wick sideways into the seam.
It looks cleaner than an open valley and costs less. It’s a legitimate method — the failures come from shortcuts, not from the design.
Woven Valley
Shingles from both slopes are alternated across the valley, weaving them together. No cut line, no exposed metal.
Weaving works with three-tab shingles. With architectural (dimensional) shingles, which is what most roofs get now, the shingles are too thick and stiff to lay flat through a weave. You end up with a bridged valley — the shingles don’t make full contact with the deck, they hold their shape, and traffic or snow load cracks them.
Most manufacturers no longer recommend woven valleys with architectural shingles. If someone proposes one on a modern shingle roof, ask why.
What Goes Underneath
The valley method you see is the visible part. What’s underneath is what actually keeps water out when things go wrong.
Ice and water shield. This is a self-adhering rubberized membrane that sticks directly to the roof deck and seals around nail penetrations. In every valley, on every roof, it should run the full length of the valley and extend at least 18 inches out on each side — 36 inches total width is standard, and wider is better on low-slope valleys or long runs.
Illinois code requires ice barrier along eaves in our climate zone. Valleys should get the same treatment regardless of what the minimum requires, because that’s where ice concentrates.
Metal valley flashing. On open valleys this is the visible surface. On closed-cut valleys it can be installed underneath as a second line of defense. Not every closed-cut valley includes metal — some rely on the ice and water membrane alone, which is acceptable if the membrane is properly installed. Ask your contractor which approach they’re using.
Nailing placement. Nails should stay out of the valley center. A general rule is no fasteners within 6 inches of the valley centerline. Every nail in that channel is a hole in the water path.
Signs Your Valley Is the Problem
Ceiling stains that line up with a valley above. Go outside and look at where the valley runs. If the interior stain is roughly beneath it, that’s your first suspect.
Missing granules or worn shingles along the valley. All the water moving through the channel wears shingles faster there than anywhere else. If the valley shingles look noticeably more weathered than the surrounding field, water has been concentrating and the roof surface is losing its protection.
Rust or holes in exposed valley metal. Older galvanized valley metal eventually rusts through, especially where debris holds moisture against it. Small pinholes leak.
Lifted or curling shingles along the cut line. On a closed-cut valley, shingle edges that have lifted let water get under the cut. This often shows up after a windstorm.
Debris packed in the channel. If you can see leaves and needles sitting in the valley, water isn’t draining through it. That’s a maintenance issue that becomes a leak issue.
Nail heads visible in the valley. Shouldn’t be there. Every one is a potential entry point.
Repair or Replace?
If the rest of your roof has years left and the problem is isolated to one valley, a valley rebuild is a reasonable repair. That means tearing off the shingles a few feet back on each side, removing the old flashing and underlayment, installing new ice and water shield and new metal, and reshingling the area. It’s not a patch — you can’t fix a valley by dabbing sealant in it and expect it to hold.
If the shingles across the whole roof are at the end of their service life, or if you have leaking at multiple valleys plus other issues, rebuilding one valley is money spent on a roof you’ll be replacing soon anyway.
Something to understand about sealant: caulk and roof cement in a valley is a temporary measure at best. It doesn’t move with the roof, it fails in a couple of winters, and it makes a future proper repair messier. If someone quotes you a valley “repair” that consists of sealant, that’s a stopgap, not a fix.
Questions to Ask on a Roofing Quote
When you’re comparing bids, valleys are one place where estimates differ in ways that aren’t obvious on paper:
- What valley method are you using, and why?
- How wide is the ice and water shield in the valleys?
- Is there metal valley flashing, and what gauge and material?
- How far back from the valley center do you keep nails?
- Is valley work itemized, or bundled into the square footage?
A contractor who does this work will answer those directly. Vague answers about valleys usually mean the crew handles it however they handle it that day.
After a Storm
Chicagoland gets hail and straight-line wind, and valleys take damage differently than the open field of a roof.
Hail hits valley metal and leaves visible dents that are easy to document. It also strips granules from shingles that are already thin from water flow, which accelerates failure even if the shingle isn’t punctured.
Wind lifts the cut edge on closed-cut valleys and gets underneath. This doesn’t always show from the ground.
If you’re filing an insurance claim, valleys should be part of the inspection. An adjuster looking at a roof from a ladder at the eave isn’t seeing what’s happening in the valleys.
Maintenance
Valleys aren’t a fix-it-and-forget-it component. A few things help:
Keep them clear. Especially if you have trees near the house. Leaves and needles in the valley in October become an ice dam in January.
Look at them in spring and fall. From the ground with binoculars is fine for most homeowners. You’re looking for debris, obvious wear, lifted shingles, and rust on exposed metal.
Trim back overhanging branches. Less debris going into the valley, and less shade holding snow and ice in place.
Deal with attic ventilation and insulation. Ice dams in valleys are a symptom of heat escaping into the attic, melting snow from below, and letting it refreeze at colder points on the roof. Fixing the valley doesn’t fix the reason ice forms there.
The Short Version
A valley is where two roof slopes meet and all the water from both goes. It carries the highest water volume of anywhere on your roof, it collects snow and ice, and it’s built with more layers and more steps than a plain field of shingles.
That makes it the part of a roof most sensitive to how carefully it was installed. If a valley on your house is leaking, staining ceilings, or looks noticeably worn compared to the rest of the roof, get eyes on it before the next freeze-thaw cycle.
If you’re in the Chicagoland area and want someone to look at yours, AST General Contracting handles roof replacement, repair, and storm damage inspection. We’ll tell you whether you’re looking at a valley rebuild or a full roof, and why.