I am planning on re-rroofing a tool shed with cedar shingles. I always thought it would be a simple job of laying the #15 underlayment (tar paper) over the sheathing (or 1 x 3s) in overlapping layers and then simply nailing the cedar shingles in courses over the previously laid underlayment. After watching several youtube videos, it appears that a 3ft wide course of underlayment should be laid down at the roof edge and then the first course of shingles should be laid on top of that followed by a narrower course of underlayment that covers the top 4 inches, or so, of the shingles. Courses of shingle and underlayment are subsequently alternated in this manner up to the ridge of the roof. What is the correct way to apply cedar shingles (not shakes) for a roof with respect to the use of #15 underlayment (tar paper)? Your help appreciated. Dave, British Columbia, Canada
For an unheated tool shed, you don't need the 3ft wide course of underlayment. Over a heated house, that 3ft course should be a rubbery waterproof ice barrier that seals around nails, so if an ice dam forms, water will not back up and leak through. Just run one course of the narrow paper, sticking out so that it will hang into the gutter a bit, then a starter course of shingles. If your shingles are 18" and exposure is 7", then run your next strip of paper 7.5" to 8" up from the bottom, then add your 2nd layer of shingles for the double starter course, then paper/shingles/paper/shingles to the top, ensuring that the paper never shows. That would be 14.5 to 15" up from the butt of each shingle course.
This article covers the subject quite well, and I even learned the term "interlayment" for those 18" strips of tar paper:
(I'm in Vancouver, and just re-did my tool shed roof the same way.)
We always treated the two layers as separate waterproofing systems.
Drip edge, if any (if none, leave felts out an inch or so for a crude one, or make one from asphalt shingles)
double layer of 3 foot felt,
then 3 foot felt overlapped 19" (so there is always a double layer - tripled for an inch at the ends of overlaps) - there should be a guideline for this on the felt.
Battens at shingle exposure spacing. If you are not going to do this, skip the felt entirely, or use non-wood shingles (some of which mimic wood pretty well, these days.)
Double layer of shingles extending a bit past the drip edge or felts.
Shingles overlapped normally
Economizing on metal drip edge worked fine for us on shed-class buildings, but we never went to wide exposure / small overlap of felts (resulting in areas of single layer.)
This process also has the benefit that the roof is dry as soon as you put the felt on; if you are only felting as you shingle you are subject to rain until the whole job is done.