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A roof valley is formed where two roof slopes meet. Water collects in a valley to flow off the roof. During winter, snow will accumulate and rest on a roofing valley until it melts. Therefore it is vulnerable to leakage.
Therefore, the proper installation of roofing valleys is important. For a valley system to work at its best there must be underlayment installed: Ice/water shield alongside metal flashing. Then shingles are installed for full water protection.
To seal any valley, a layer of underlayment and an ice/water shield is installed over it. Afterward, the shingles are installed, starting with the lower slope and then on the steeper slope. This creates an overlap effect which will resist water. The excess part of the shingles on the steeper slope are cut off so that the overlap appears seamless.
In an open valley system, the ice/water shield and the underlayment are installed under the shingles. However, instead of overlapping shingles like the closed valley method, a pre-bent metal flashing is installed over the valley area. This effectively seals off the vulnerable middle area of the valley and allows you to install the shingles 3-4 inches out from the middle.
Using this system has several benefits, this is thanks to the metal flashing installed.
First, since valleys have a high traffic of water flow from the roof they are inherently vulnerable spots for water leakage. Over time the exposed metal can handle the corrosive effects of the water much better than shingles. We often find that valleys are the first places to deteriorate in the event that a closed valley system has been installed.
Secondly, shingles are installed using nail fasteners. This method keeps fasteners from being installed too close to the valley. Using metal flashing also creates a nice water basin to keep the water flowing on and off the roof. It is highly recommended for this method to be installed on any home to make certain the shingles will last their lifespan.