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
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...
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