Roof tear off question
Do you have any leaks? If so, you would want to check out the decking beneath the shingles. Do you have ice and water shield onder the shingles now? you can tell by prying up the first row of shingles and looking underneath. I think the ice and water shield is necessary in our area. Also, any valleys? I use ice and water shield there aso. Is your venting adequate? If not you can add venting, either passive or powered, without tearing the whole roof off. Finally, I had the same condition a few years ago, and I removed all the old shingles, but I just like the way the new shingles lay flatter.