Using Felt with Cedar Shakes
Cedar shakes were always installed without felt (1970's thing)
The installation process is different
The shakes need to be triple layered & the offsets between the spaces need to be 3 rows instead of 2 up.
A 24" shake will have a 7 1/2" exposure instead of 10" You will actually require about 33% more material for this installation not the 25% trhat it looks like. The reason is your making up on third of 7 1/2" not 25% of 10"
An 18" shake will have a 5 1/2" exposure instead of 7 1/2" requiring 40% more material
I built an open deck & installed 1 x 4 T & G cedar over the entire deck then strapped it with 1 x 4 hemlock & felted it as usuall. In this application it looks good underneath, has felt installed, takes less cedar & still breathes
DC Roofing Inc