I was talking to a roofer about what to do with our ~25 year old cedar
shake roof and one option he offered was a "restoration" consisting of
pressure washing the existing shakes, replacing all damaged shakes and
felt, and spray-on treatment with a derivative of guanidine phosphate,
which is supposed to be UV protective, and decay and fire retardant.
They offer a 12 year warranty on the treatment.
Are these chemical treatments effective for older roofs? The quoted
price seemed potentially attractive compared to a new shake roof, but
not exactly cheap ($6K for 2700 sq ft). Maybe most of the value of
the "restoration" comes from the repair work?