Roof Lasagna
It used to be common practice to just add another layer every time the roof needed to be replaced. This is not the best practice and below you'll find out why.
Weight
Adding extra layers adds extra weight to the roof that the rafters were not designed to hold. This leads to sagging and can eventually lead to collapse.
Heat
Extra layers of roofing traps extra heat. Shingles get really hot on their own and roofs require proper ventilation even with just a single layer in order to function properly. With extra layers the roof is not as efficient at shedding heat. Extra heat causes the shingles to wear out at a faster rate.
Hail
When another layer of roofing is added on top of an existing layer, it creates a softer roof that is prone to hail damage. There roof now has air pockets and is softer in general which means that when a hail stone impacts the roof it will cause more damage because it can penetrate the shingle more than it could if the shingle was on a flat hard surface.
Wind
The new layer of shingles will never completely seal to an existing layer of shingles because of the irregular surface. This means that the new layer of shingles will have loose corners and pieces, which will be prone to wind damage.
