There are roofing companies out there that will do a "lay over" this is when you take new shingles and install them on top of the existing shingles. This is a cheaper alternative stripping the old roof off first. It can be the difference of hundreds to thousands of dollars depending on the project. But the thing they won't tell you is the money you might save in that moment you will end up spending later on. This project shows that, the existing roof had been a layover a few years previous. But when we stripped off the two layers we found massive damage to the decking. The reason for this is the two layers. What happens when you layer asphalt shingles is two fold. One your home is not made to with stand that kind of extra weight load. Two the extra layer heats up the roofing surface far more than just one layer. It also heats up the decking of the roof as well. Aside from the damage to the decking, anytime you have a leak with two layers its almost impossible to find and fix. With that many shingles it can be a real needle in a haystack to try and find the source of the leak. Which this roof did have leaking as well.
Stripping away the two layers and redecking the areas affected. This can make your project take longer than originally planned. But the benefits are outstanding. It will make your home more energy efficient along with having the correct ventilation. It will also make the life of your roof last longer. When you have multiple layers it takes away from the life span of your roof. When you have just a shingle layer the shingles will often last the span of time they were designed too.