When it’s time to install a new rubber roof over an existing roof, the most important thing that will make or break the roof, is the flashing.
Flashing is the metal part on a roof that makes the transition between the roof and the wall or skylight. Most of the time the flashing is installed before the siding is installed. The metal flashing has to be under the siding.
Many times when carpenters install siding, they take it too low to the roof which makes it difficult to install a second layer of a rubber roof. Either the siding must be removed to raise the flashing or the existing roofing material must be removed from the flashing to allow enough room for the new roofing material.
When you have a tar and gravel roof, the flashing is sometimes embedded an inch below the surface. That’s when it’s better to install new flashing.
On this particular roof, the previous roofer installed a rubber membrane over an existing hot tar roof. That is perfectly okay. What he did not do was remove the roofing material and clean the flashing to accept the new membrane.
The roof he installed did not last 3 years. What is even worse is he tarred the edges where the flashing is supposed to be. The tar went all the way up against the siding. That was a disaster and it took an entire day just to remove the tar from the edges and siding to prepare it for a new membrane.
It is very important to prepare an existing roof to receive the new membrane. The existing flashing has to be exposed and the drains have to be cleaned from old roofing material and residues so the new rubber membrane can be attached to the drain and flashing.