A valley in the roof looks, functions and is essentially just like a valley between two mountains in a landscape. In nature, rain hits the mountains, is pulled downwards by gravity to the lowest point and forms a river that flows away. On a roof, water hits the shingles, is pulled by gravity until it hits a valley then flows downwards into the eaves trough.
By its very nature, a valley receives higher concentrations of flowing water which makes them particularly important on a roof system. This is why the Alberta Building Code and most shingle manufacturers require roofers to install leak barriers along valley centers from top to bottom.
In addition to installing leak barriers, there is a particular way to install shingles within a valley center to help limit and prevent the likelihood of moisture entry related issues. There are two general ways to install shingles within the valley area.
The first and most common way, traditionally, is what is referred to as a closed valley. A closed valley consists of weaving shingles together along the center of a valley to provide coverage on both sides as water splashes around. This is an easy way to install shingles in a valley and keep moisture out, and is generally accepted as an installation style by most shingle manufacturers and still commonly used by many contractors today. The problem with this installation style is that, by default of valleys having higher concentrations of water flow, there is a more rapid rate of deterioration to the shingles. What this means is that shingles inside a valley center will typically deteriorate quicker then shingles anywhere else. And this is where the second type of shingle installation procedure for valleys comes into play.
The second type of shingle installation procedure in the valley is referred to as ‘open valleys’. An open valley has a piece of sheet metal extending from top to bottom, which carries moisture into the eaves trough while keeping shingles free from the deteriorating effects of flowing water. The Alberta Roofing Contractors Association particularly recommends fthis type of installation procedure as a good roofing practice, and most professional roofers now use this installation style as regular practice.
There are different types of valley metal in the market used at different times for different reasons. A basic and entry-level type of valley metal is about 30 gauge in thickness, galvanized, and comes in a roll for easy carrying in storage. This metal can also come in a variety of colors to match your shingles and provided a nice finish.
An upgrade from this would be a thicker gauge metal such as 26 gauge. The thicker the metal, the less likely there will be issues such as tracking, splitting, punctures and listing of valley metal.
The final, and best type of valley metal is referred to as “W” valley. A “W” valley is basically a valley with a jut out bent into the middle of it. This jut out serves to prevent water from running down one slope and up under shingles on the adjacent slope. “W” valleys are typically always fabricated in the 26 gage thickness and colored to match or accent shingle colors. Due to its nature it cannot be stored or transported in a roll which makes it more difficult to transport, store and get up onto a roof system. From every angle, installing a “W” valley takes more time to install which translates into more money in an overall roof price. On average it is common to see about $3 – $6 per foot extra for a “W” type valley metal installation, and in our professional opinion; it’s worth it for every roof system with a pitch greater than 8/12 and for every roof slope that meets with another roof slope equal to or greater than 3/12 in difference ie: a 4/12 slope meeting with a 7/12 slope.
Note that installing “W” Valley is a requirement by Alberta Building Code for any area where two roof slopes meet that are equal to, or greater than 3/12 and pitch.