In our opinion, a metal valley system is best. That’s because water, snow and ice shed from opposing slopes collects in the "valley" and, as a result, the valley is prone to more wear.

If you have a "closed" valley system, the heavy volumes of water collected causes premature wear of the shingles in those areas. Twenty years ago it was common to see mineral surfaced valley materials (which had ceramic granual covering, like shingles). We would always observe that the valleys would wear out before the shingles in the field of the roof.

Having said this, alternative valley methods are acceptable to shingle manufacturers. The two other systems are a closed valley system as you described, where shingles are extended from the lesser slope up and under the shingles from the opposing steeper slope. A "western" or "weaved" valley consists of every six shingle courses being run under the opposing slope in opposite directions.

As some customers prefer the appearance of closed valleys, Avenue Road Roofing still insists on installing metal valleys and appropriate underlayment materials underneath such assemblies. Some roofing contractors install closed valleys without underlays. You can’t see the difference when the roof is installed, but there is a big difference in workmanship.

When a very steep slope adjoins a much lesser pitched slope, it is recommended to manufacture the metal valley with a "centre stop" detail. This involves forming the metal with a standing seam in the centre of the valley. This is designed to divide the flow of water and prevent water from being forced under the lesser slope.