- Q. What underlayment should I use under cedar shakes?
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.
When installing wood shakes, interweave 18-inch-wide strips of No. 15 roofing felt between each course. Nail the felt strips to the sheathing along the top edge, and keep each strip two exposure widths above the butt edge of the course below. This way the felt isn’t visible in the 1/2" gaps between the shakes.