I am replacing my composite roof with a new composite roof shingle because we have had so much traffic related to various chimney repair and maintenance, accessory vent installs and modifications, etc... that it is showing signs of definite wear and I don't want to wait for the leaks to start. So, recently I was told by one of the many sales associates providing an estimate that I will need to have the existing shiplap roof boards covered with 1/2" ply before applying the felt and composition shingle or the shingle manufacturer will not warranty the job. To support their position, they read an excerpt from an installers guidelines book that was provided by the shingle manufacturer that made it sound like it needs to be done for a proper installation and said that the warranty would be voided if not done that way. The case the sales rep was making was that the shiplap shrinks and grows at far more different rates than the ply and without using ply overlay increases the likelihood that the roofing job would not last for the guaranteed lifetime period. The sales associate said that every shingle manufacturer requires that ply be installed over shiplap or the warranty will be voided and that the competition is taking a risk to our detriment that a fully warrantied and quality job will be the outcome. Has anyone else heard of this, or am I being misled?