Although it may seem advantageous to install new shingles on top of your old roof because it’s cheaper, faster, and requires less labor, you could be setting yourself up for future costs (and headaches). Unless your current roofing system is completely problem free and only features a single layer of functioning shingles, your new roofing system may fail way too soon.
Many roofing contractors may be willing to simply nail new shingles on top of old shingles, but we always advises stripping the old materials complete off, all the way down to the deck, and then adding the new materials. Any defects, vulnerabilities, and potential problems in the plywood deck or truss system will go unnoticed unless you de-nail & re-nail.
Here are a few situations where you will need to remove the old roof before completing a reroofing service:
1- Shingles are missing, curled, split, or mossy.
2- The roof decking is sagging due to rot or deterioration.
3- The new shingles are lighter than your current roofing shingles. (This will cause all of the old bumps & ridges to show through the new roofing materials.)
4- Your current roofing system has more than one layer of shingles.