Imagine the most simple roof you can. It has two sides that meet at the top, creating a ridge line. You’ve probably seen many sheds, garages, or barns with roofs like this.
Now start complicating the roof. Let’s say you want to add a porch area. You’ve probably seen a house like this many times. Their roofs are like a very large shed, but since they also have a porch, a small “additional” roof joins the main big one.
Now the roof has more than just two sides that meet at the top ridge. It will probably have two ridges, the second being where the two sides of the porch meet at the top.
Because this porch roof is perpendicular to the rest of the house where it meets the main house roof is a new type of roof component: the valley. In simple terms, a valley is where two non-parallel roof decks meet and the meeting is concave, like a valley is concave into the earth (hence the name).
Why does this matter?
Because valleys get a ton of rain.
In the most simple type of roof without valleys, rain water falls nearly uniformly down the slope, from the ridge to the gutter. But when valleys are present, rainwater from two non-parallel roof decks flow from ridge to valley. Then it travels down the valley. Water accumulation at roof valleys is substantially larger than any other parts of the roof on most homes.
This means that when your roof gets a leak, it’s often at the valley because of its increased weathering.
How can you prevent valley leaks?
Some roofers won’t do anything special to try to bolster waterproofing performance at valleys. They’ll just put felt paper down like they do on the rest of the roof, then weave the shingles across the valley.
This can work okay for a time period, but if you want your roof to last a long time, it won’t.
The first step is to get a heavy duty leak barrier in the valley instead of standard underlayment. GAF WeatherWatch ice and water shield is the best. It’s almost like having an additional layer of shingles beneath your shingles because it’s thick, sturdy, and seamless.
A good roofing contractor will apply one of these leak barriers along the entire valley.
The second step is to install 26 gauge steel to the valley.
The type of metal your roofing contractor should use is often called “W-valley” because it’s preformed like a “W”. While the appearance of where the roof decks meet to create the valley is a “V”, you want a “W” flashing instead because the middle hump stops water flowing down each side from shooting up under the shingles on the opposite side.
The metal can withstand about as much weathering as you can throw at it. A properly installed system with metal valley flashing can be the most robust part of a new roof.
Since one of the first points of failure in any roof is the valley, when you want a roof that will last for decades to come, you’ll want to make sure your roofing contractor will install steel flashing along your valleys.