The most common type of valley construction used with a slate roof system is an open valley lined with sheet metal (see figure). Open valleys permit clear, unobstructed drainage and are advantageous in locations where debris from surrounding foliage settles on a roof's surface and accumulates in valleys and around penetrations.
NRCA suggests the metal used in an open-valley slate roof system have a gauge or weight that reflects the anticipated service life of the slate roof system. The valley should be fabricated from one of the metal types listed in The NRCA Roofing and Waterproofing Manual, Fifth Edition.
In climates prone to accumulations of snow and ice or with regular freeze-thaw cycling, open-valley construction should be enhanced. The valley area can be lined with a self-adhering polymer-modified bitumen underlayment material before installation of the valley metal. In addition, the flanges on either side of the valley metal can be stripped in with 9-inch- to 12-inch- (229-mm- to 305-mm-) wide strips of self-adhering polymer-modified bitumen underlayment material. The self-adhering strips are bonded onto the valley metal flange and self-adhering underlayment material.