It’s a Plastic World
During WWII, natural rubber was in short supply as normal supply lanes were cut off. A highly successful replacement was GR-S (Government Rubber-Styrene), later renamed styrene-butadiene-styrene. This was the birth of the synthetic polymer world in roofing. To a chemist, a polymer with a double bond at the molecular level has a “diene” ending (e.g. isoprene, neoprene, etc). That means polymer chains can be formed from monomers by opening one of the double bonds to form extremely high molecular weights.
By the mid-1960s, polymer manufacturers were seeking new markets for their materials. Butyl rubber (isobutylene isoprene rubber, or IIR) was recognized as having very low permeability, but no natural fire resistance. Chlorobutyl rubber provided fire resistance through chlorine atoms on the polymer chain. Other polymer products were also tried as membrane roofs, considering the virtues of being lightweight, produced in wide rolls, etc.
Polyisobutylene had excellent weather resistance, but without the isoprene, it could not be vulcanized. Under stress, the membrane suffered from cold flow creep. While ethylene could be polymerized into polyethylene, it does not have good weather resistance nor fire resistance, and it is difficult, if not impossible, to adhere anything to it.
Adding chlorine to the polymer chain resulted in chlorinated polyethylene (CPE), polyvinyl chloride (PVC), and chlorosulfonated polyethylene (CSPE). These, along with ethylene-propylene-diene monomer (EPDM) and thermoplastic polyolefin (TPO) polymers are blends or alloys of polypropylene plastic, polypropylene and ethylene propylene rubber (EPR), or EPDM. APP has a higher melt point and is generally torch-applied.
Attachment Rules of Thumb
Full adhesion: bituminous roofing systems, single-ply membranes
Partial attachment: mechanically fastened single ply, ribbons of low-rise foam, bar systems
Loose-laid and ballasted: mainly EPDM rubber
Cover boards: mechanically fastened, followed by fully adhered membranes
It should be obvious that re-roofing and re-cover depend heavily on what you have now. If the deck is solid concrete and you are going to tear everything off down to the deck, you can prime the deck and fully adhere everything from there on up.
For steel decks, mechanically fastened thermal insulation or a combination of insulation and high-density cover boards of different types provides a substrate suitable for all of the above membrane systems.
Chemical compatibility is important with polymer systems. PVC and TPO may use polymer-coated steel flashings intended to receive heat-welded membrane directly. They are, however, incompatible with each other.
Even when we are only patching some punctures, proper techniques are important. A patch of fresh polymer works best when the patch is slid beneath the weathered membrane so that you are welding the patch to the unexposed bottom side of the old membrane.
Winter Roofing Survival Guide
How to maintain your roof before and after a winter storm.
Metal Roof Design for Cold Climates
Keep snow, ice, and moisture from wreaking havoc on your metal panels.
Is Roofing a Wintertime Sport?
Winter preparation tips for roofing professionals.