We just tore off a cedar shake roof (2/4" to 5/4" rough cedar shake)
about two weeks ago. I haven't replaced on here in South Texas in
about 5 years.
Here, when properly installed they last a long time. But the shakes
and technicians to install them are in short supply. When cedar shake
shingles were all the rage here in the 70s, we had them everywhere,
especially on the mid to upper priced homes.
Two things happened. We found down here that most cedar roofs
wouldn't stand the 100+ degree summers we have here for a usable
service life. The game changer for us was the fact that insurance
companies wrote exclusions or refused to insure homes with cedar shake
roofs as they burned so quickly.
So we change them to a dimensional shingle. You take off all the
shakes like you normally do for a roof replacement. Then deck the
roof over the skip sheathing with plywood or OSB. Felt it, tin cap
it, and shingle. This is also an excellent opportunity to install
ridge venting on your roof system.
I am sure in you area of the world you would need to follow the ice
and water shield details for shingle, and in some instances it may be
a requirement (if not a local code, a requirement for insurability) to
install "ice barriers" under the shingles.
For low slope applications, for "dead" (flat) valleys, under
complicated tile roof details, etc., we always use ice and water
shield. It is cheap insurance for our end product performance.
The last cedar shake roof I put on was about 25 years ago. At that
time, the cost to replace with a vastly inferior 0/4" - 2/4" #1
perfection grade split cedar shingle was about the same price as
converting to asphalt shingles, even with the decking.
You might weigh your options. If you convert, the next time you need
a roof, asphalt shingles are much easier (cheaper) to replace than
wood shakes. And if you have "heavies" or the large wood shakes, it
is <much> cheaper to replace.
Just my 0.02.