I'm in the middle of having my roof (cedar with additional layer of regular shingles) replaced. This requires removing the old roof and placing 5/8" plywood over the firring strips before adding the new shingles. The old roof rafters are true 2 x 4 construction and are about 24" apart. The roof has many valleys and is very steep. Is there any weight problem with this? (The house was built in 1912).