A Complex Asphalt Shingle Roof With Two Roof Valleys

Roof Valleys: What They Are (+ Do You Need One)

Roofs have a tough job. They keep out the rain, handle the summer heat, and (hopefully) stand up to whatever the weather throws their way.

But some parts of a roof work harder than others.

And one of the hardest-working parts whose role tends to get underappreciated is exactly the roof valley.

This guide is there to tell you how roof valleys and their flashing work, how much they tend to cost, and why you should care.

What’s a Roof Valley?

Roof Valley In Between Two Slopes

In simplest terms, a roof valley is the channel at the intersection of two slopes. At first glance, it looks like something that doesn’t really do much apart from just being there.

When it rains, all the water from those slopes gets funneled down the valley and into your gutters. Without a properly functioning valley, rainwater would just flow wherever it pleased—down walls, under shingles, and into places it really shouldn’t be.

Not every roof has or even needs a valley, though.

Simpler designs, like a straightforward gable roof with just two slopes, may not need one. But for more complex roofs with multiple slopes, valleys are a must, and not just because you can’t physically avoid having them.

What’s Roof Valley Flashing?

Because roof valleys handle a lot of water, they need both extra protection and a way to siphon all this water away as fast as possible.

This extra protection—we’re saying extra because gutters also do their part here—is what the roof valley flashing is there for.

It’s a thin, waterproof layer of metal that lines the valley, allowing the water to smoothly slide off the roof.

In other words, flashing is what protects the valley (if it’s open—more on that below), and lets it do its job more efficiently.

Materials Used for Roof Valley Flashing

No plastic or PVC in the valley flashing department! The most common materials used in Australia as well as the rest of the world are always metal:

  • Aluminium – Lightweight, relatively affordable, rust-resistant, and easy to work with. A solid all-rounder for so many things around your home, it’s the option #1 for flashing too.
  • Copper – Much more expensive than aluminium, but incredibly durable. It also develops a nice greenish patina over time, what with all the moisture.
  • Galvanised steel – Tough and probably the most cost-effective of all materials, but can eventually rust if you give it no attention at all. (Admittedly, it will take many years to rust.) It’s cheaper but heavier than aluminium.
  • Lead – We’re only mentioning it here because it’s traditional, lasts forever and is absurdly cheap. But you should keep in mind that lead has fallen out of favour within the past couple of decades, and for good reasons. It can seriously affect our brains, kidneys or even cause cancer. No wonder it earned the label of a construction hazardous substance!

Most Australian homes use either aluminium or galvanised steel flashing, as they balance durability with affordability.

Open vs. Closed Valleys: What’s the Difference?

Some typologies go into far more detail, but there’s no need for that here. The differences boil down to open and closed.

The choice between the two is entirely up to your personal preference. Both types can go well with any roof design, so the choice mostly ties to your budget and how much effort you’re willing to put into maintenance.

Open Valley

Open means exposed. The metal channel that directs water off the roof is visible and can even be relatively wide.

While some homeowners won’t like the look of a metal channel cutting through their roof, this type is more efficient at keeping water moving and less likely to trap debris.

Closed Valley

A closed valley is covered with shingles or tiles, blending seamlessly into the roof. It looks cleaner but has a downside—debris can build up in the overlap, which means it might need more frequent maintenance.

So, which one’s better?

That depends. Open valleys tend to last longer because water runs off more easily, while closed valleys are more budget-friendly upfront and offer a more streamlined look.

Naturally, a high-rainfall area will call for an open valley that can deal with all the downpour.

But if you can’t afford spending extra on flashing, a closed valley should work just fine as long as you’re willing to give it a quick once-over from time to time.

How Wide Should a Roof Valley Be?

A valley’s width depends on your roof pitch, the expected rainfall in your area, and the type of materials being used. As a general rule:

  • 100mm–150mm (10–15cm) is standard for most Australian homes. Note that we’re only talking about the visible part of the valley, not the underlying valley boards!
  • Wider valleys (up to 300mm) are used in areas with heavy rainfall or on roofs with a lower pitch, where water drainage needs extra help.

If a valley is too narrow, water may overflow during heavy rain. Too wide, and it may not blend well with the roof design.

Why Do Roof Valleys Leak?

A leaking roof valley is bad news, but it’s a common problem. Here’s why they fail:

  • Debris buildup – It sounds next to impossible, but leaves, twigs, and dirt can pile up even down this channel. In turn, this causes water to back up and seep under shingles or tiles. Even though the valley doesn’t require nearly enough maintenance as gutters do, it’s still a nice-to-do once a year or so.
  • Worn-out flashing – Over time, flashing can crack, rust or dislodge. Such damage is easy to detect, though.
  • Improper installation – If the flashing isn’t laid correctly, or if the valley is too shallow, water won’t flow properly.
  • Sealing failure – Some valleys (especially closed ones) rely on sealants that degrade over time. Once they crack, leaks start.

Regular inspections and gutter cleaning go a long way in preventing valley leaks before they become a bigger issue.

Roof Valley Maintenance: Keep It Flowing

Roof Valleys On A Historic Stone Roof Home In Halifax, Uk
A set of valleys on a historic stone roof in Halifax, UK. Crownbuild, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons

Want to avoid costly repairs? Stay on top of maintenance. A little attention now can save you a roof replacement later.

  • Clear debris regularly – Especially if you have a closed valley. Leaves and twigs are the enemy.
  • Check for rust or cracks – If your flashing is metal, keep an eye on corrosion.
  • Watch for water stains – If you notice water spots on ceilings or walls inside, your valley might be leaking.
  • After storms, give it a once-over – Heavy rain and wind can loosen materials or dislodge flashing.

How Much Does a Roof Valley Cost in Australia?

The exact price will depend on things such as materials, the length of the valley itself, and whether you’re looking at a repair, replacement or a brand-new installation.

Plus, since the upper part of the valley begins at the ridge, some of the ridge capping will need to go. That means you’ll need rebedding of the ridge too!

Still, here’s a very rough guide to what the valley itself is likely to set you back:

  • New valley installation: $500–$2,000 per valley, depending on material and complexity.
  • Valley flashing replacement: $300–$1,000 per valley.
  • Small repairs and resealing: $150–$800, depending on the extent of the damage.

If your roof valley needs replacing, it usually makes most sense to do it alongside other roofing work to save on labour costs.

Final Word: Do You Even Need a Roof Valley?

Not every roof has or needs a valley. If your roof has a simple design, like a single ridge with two slopes, a valley isn’t just unnecessary but also impossible.

But if you’ve got a more complex roof with intersecting sections, valleys are essential to control water flow and prevent pooling.

Some homeowners ask if they can remove roof valleys during renovations. In most cases, the answer is no—at least not without redesigning the entire roof. If you’re building a new home or planning major roof work, talk to a professional about whether valleys are required for proper drainage.