A. A continuous underlayment is not normally used under shakes. Instead, 18-inch-wide strips of No. 15 or No. 30 felt are interwoven with the shakes. The bottom edge of the felt strips should start twice the exposure width above the shake butts (see illustration). For example, with 24-inch shakes having a 10-inch exposure, the " interlayment" should begin 20 inches above the butts. The felt interlayment should only be nailed to the sheathing along the top edge.
At the eaves, use a 36-inch-wide No. 30 felt eaves flashing or an ice membrane.
to read the full article.