Haven't put on a new roof in 30 some years. Forgot what little I know. Have a shed with rotten wood shingles, strip sheathed, 10X8. How many wood shingles will I need? Still using tar paper under? Anything else I need to remember?
it all depends on what kind of wood species shingle/shake and the size of the coarses.
80 sq ft divided by 100 times 3 will give you 4 inch courses.
80 sq ft divided by 100 times 2 will give you 6 inch courses.
red cedar can be larger courses (stiffer, less curl)
white cedar courses usually don't exceed 5 inches (curl easy with wider couses)
with wood shake shingles, the supplier short be able to let you know how many bundles of shakes you need. 8x10 ft is 0.8 square (80 square ft) without taking into consideration any pitch that wasn't factored in. with about 7 bundles of 18" shake making a square and 5 bundles of 24" making a square, with waste you are looking at about a little more than a square of shake needed (with shake starter, ridge, and waste). in your situation, I'd probably be looking at the 18" shake, aesthetically, because of the size roof.
as for the underlayment, use regular roofing felt, 15 lb is fine, but use 30lb if it's slope is lower than 4/12. ideally, you would install felt strips that actually slide up to about a couple inches below when the end of the shake is. they slide in between each layer of shakes. for fastening, you have to use galvanized hot-dipped nails.