Someone may propose installing a single-ply membrane as an option. There are risks to these membrane roofing systems that are listed below.
The name single-ply is just that, one layer. They a membrane suspended in air with a water absorbing recovery board between the existing roof and the overlaid single-ply. One screw dropped by an A/C tech and the entire system is compromised, allowing moisture to migrate throughout the entire roofing system. The other is that we live in a high velocity wind zone. Too many times have I seen entire single-ply roofing systems laying in the parking lot.
EPDM single-ply are thermoset membranes that will fail in higher temperature climates at the seams, and can shrink pulling apart the laps. Animal and vegetable oils from restaurants will destroy EPDM membranes because of swelling and distortion. A major point of failure of EPDM membranes is that they cannot withstand ponding water.
TPO manufacturers have revised their proprietary formulations to try and get it to work. This is a thermoplastic membrane and has yet to be proven. It splits along the seam and fasteners with accelerated aging problems along the walkpad and polymer erosion is common. It is susceptible to deterioration from exposure of high heat and UV.
PVC single-ply membranes are also a thermoplastic. Overtime, UV,, heat, and environmental contaminants draw the plastisol to the surface of the membrane and it is washed away by wind and rain. This will reduce flexibility and leave the membrane susceptible to damage from thermal shock, hail, and foot traffic. This will also make the material impossible to repair and maintain.
Once you install a roof over a roof, when it fails you have no other options per the building department. Florida building code will only allow two roofs before a complete tear off will be mandated. Installing a roof over a roof should be the last step in the life cycle of a roof. Installing a waterproofing to the existing roof is the natural next step in the life cycle of a roof.
Roof coatings are fully adhered to the existing membrane so if damage occurs the existing roof you can make a small repair to that area. The existing roof provides support.
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