We've been in our home for 1 1/2 years. Approx 10 yrs ago, the cedar shake was replaced with shingles directly onto the horizontal 1x6 planks. Roof is now failing due to poor installation. Have gotten conflicting opinions as to the necessity of laying down plywood sheating, or installing directly to the 1x6 planks. One contractor says that is OK, if done right, and I trust they would do it right, but would love to get an unbiased opinion if plywood sheathing is necessary. Obviously, the cost difference is significant. Thank you!