From NachiWiki

There are many types of single ply roofing materials and roofing systems for installation. You need to know what the material is and how it is attached. EPDM, PVC, Modified Bitumen and others are mechanically fastened (usually in the seam), adhered (with hot asphalt or adhesives) or ballasted. Unlike built-up roofing (more plys than a single ply system), bubbles are usually formed when being installed. There are contact adhesives that are used needing to "cure out" after the sheet meets the substrate, not the deck sheathing, and the proper drying wasn't achieved before mating the materials and there is a release of gas forming the bubble.

Types of Single Ply Roofing

First of all you will run into people that think they have a rubber roof when in reality it is modified bituman. This asphalt product is better than the rolled roofing it is designed to replace because it has been modified with rubber polymers, added to make it more flexible. A rule of thumb, if they're using a torch to apply the roll then it isn't rubber.

Next, all single ply membranes need a suitable substrate, usually a 2"-4" sheet of fiberglass hardboard insulation but may have a high density fiberboard. These are fastened with 3" plates[washers] and self drilling screws. EPDM is installed in several ways, some that are no longer used. Mechanically fastened membrane (usually 7'x100' rolls) uses flat 1" bars or 1" washers that are fastened with self drilling screws, in the seams every foot in the middle of the field and six inches at the perimeter 1-3 laps in depending on the wind uplift in the area. After fastening the bottom edge of the lap, the top section is adhered over the fasteners with a contact cement or most of the manufacturers are using a two sided tape.

Totally adhered is another attaching method using contact cement that is applied to the membrane and the substrate then rolled in. Carlisle also has a fleece backed membrane that can use asphalt to adhere it. These adhereing methods allow the installer to use 10' or 20'x100' rolls cutting down on the weakest point of single ply membranes-the lap splice! The third method is ballasted. This allows the installer to use the largest sheets possible up to 50'x100' rolls. Roof penetrations are the biggest reason to use smaller sheets. New construction is 50sq. at a crack and cut in penetratios afterwards. The insulation is loose laid, rubber rolled out, perimeter termination strips fixed, splice the laps and throw on #2 round double washed gravel and there you are, instant roof covering.

When inspecting these roofs, clear the gravel from around the base of penetrations, inside and outside corners and terminations. Look for fishmouths, cuts, flange metal edges wearing through and repairs made using rubberized roof cement as this will eat through the rubber in short order. Check terminations at walls, top of penetrations, termination bars and counterflashings. If you are dealing with a white membrane, it may be EPDM or it could be PVC. You can tell the difference easily because the PVC uses a far superior heat weld. The field seams are welded using a robot that is adjusted for speed and heat to accomodate weather conditions then welds all seams the same. This is the closest to factory conditions that you can get.

This article was originally based a post by Ralph Smith on the NACHI message board.