When installing a ballasted roof system, the membrane is loose-laid, meaning it is not traditionally fastened in place with membrane screws and plates. Instead, sheets of roofing membrane are laid over the roof and sealed only at the seams. It is successfully held in place by the weight of the rock ballast, typically about an inch and a half in diameter. These rocks are layered to create a weight of 10-25 lbs a square foot, securing the membrane below. This rock layer is additionally helpful in preventing the sun from beating down on the membrane below, as well as preventing punctures in the membrane, thereby lengthening the life of the roof.
When reroofing a ballasted roof, it is not a good idea to leave the membrane intact but rather it should either be relief cut, or completely removed (as described in the next paragraph.) This is because the original or bottom membrane will shrink and pull away from penetrations such as skylights, pipes and joints. Also, if the new or top layer of membrane did happen to develop a leak, the water could run between the two membranes for hundreds of feet before it finds a way through the original membrane below. This makes it nearly impossible to locate and resolve leaks.
Instead, we are faced with two options when reroofing a ballast system. First, we can make relief cuts in the original membrane in 10 X 10 ft grids. This way, if the membrane shrinks (which it inevitably will), it does not cause problems to the remaining roof. Also, if a mechanical repairman walks across the roof and causes a puncture penetration, we only have a small 10 ft radius to search for a leak source, as moisture will find their way through at the cuts.
However, if we make these cuts to the membrane below, we might as well throw the membrane away. The remaining membrane does nothing to strengthen the roof. PVC membrane will actually curl at the edges and become brittle, something we refer to as ‘potato chipping’. This can make the new membrane layer bumpy and uneven, inhibiting the proper flow of water.
Instead, it is much more effective to remove the original membrane and install new membrane over the top. This creates a clean, efficient new roof. By removing the original membrane, we eliminate the possibility of it bunching or causing uneven bumps on the roof.
This can easily be done using a technique called ‘wind rowing’. Instead of hauling all of the existing rock and gravel from the roof, which could cost thousands of dollars, a roofing contractor can group the rocks into rows along the roof. We take about ten feet from each side and pile it in the middle to form a row. This exposes 10 ft increments of EPDM spaced 10 ft apart.
The EPDM can then be removed and a layer of TPO (or other membrane) can be put down. In between these two rows of TPO, we would still have an entirely built up rock level resting on an EPDM layer. These rocks are then moved to lay on top of the TPO and expose the EPDM. This allows us to remove the EPDM and lay down another strip of TPO. Your contractor should measure before they begin the project, so they know exactly where to lay down each row to make it perfectly straight and aligned as is necessary. All we need to do at the end of this process is seal the TPO seams from the middle row to the outside rows, and lay the rock back on top of the new membrane.
This simple process creates 30 ft of membrane, quickly and effectively installed, without needing fasteners in the seams. Fasteners are still required through the new membrane around the perimeter, at angle changes, and around roof penetrations such as ventilation pipes and HVAC curbs etc. These fasteners are vital to keep the membrane from bridging or pulling away at the edges and corners as the roof system ages.
Some companies will bid incredibly low on these reroofs, so be sure that they are not cutting corners on your roof. Are they going to tear off the old membrane? Will they relief cut it? It will cost much more money if your new roof leaks and the water travels across your roof before finding a way through the old membrane below. Building owners have been known to spend large amounts of money on simply locating leaks in ballasted roofing systems.