About two-thirds of low slope roofing activity today has to do with reroofing or re-cover of an existing roof system. Recent emphasis on upgraded thermal insulation means that the new roof system may have to meet the latest revision of a building code.
To cope with the phenomenon of urban heat islands, in a hotter climate, a reflective roof will generally be selected (usually white). Codes also require that the roof not pond water, which may necessitate additional roof drains, tapered insulation, or both. Carbon footprints are also an issue, as global warming, and freak storms are becoming a fact of life.
Options for repair and re-cover are plentiful. While checking the roofing files on your buildings (I hope they exist), see if they include some information on age, type of membrane in place, and whether warranties are still valid.
What’s Out There?
Traditional hot-applied bituminous roofs were surfaced with aggregate embedded in hot bitumen. For a re-cover, it is possible to degravel, remove areas of wet insulation, and apply a new bituminous or MB roof system. Venting base sheets can be adhered to the degraveled surface by spot adhesion, or a re-cover board can be attached with mechanical fasteners. A new membrane can be based on asphalt, coal tar pitch, or MB systems of APP or SBS modified asphalt.
Another approach on an aggregate-surfaced bituminous roof is to degravel the membrane using a high-pressure, low-volume vacuum system. This is followed by applying sprayed-in-place polyurethane foam (SPF) followed by a surface coating.
The Current Generation
Currently, PVC and TPO dominate the single-ply marketplace. The chlorosulfonated polyethylene (CSPE) can be heat-seamed while fresh, but cross-links into a thermoset upon weather exposure. Most CSPE is applied on the West Coast.
Once an EPDM membrane is vulcanized (cross-linked), it is thermoset and is unaffected by heat or cold. To make field seams, cold adhesives or tapes are employed.
PVC and TPO are thermoplastic and can be seamed by application of heat. Polymer-modified bitumens also hold an important segment of the market. Two polymer types are used for modification: SBS can be applied using hot bitumen or cold adhesives, while atactic polypropylene (APP) has a higher melt point and is generally torch-applied.
Back History and Knowledge
Prior to World War II, most commercial buildings were multiple levels, and because steel was in short supply, roof decks were timber, gypsum, or concrete. Insulation was rarely used, though when steel returned to civilian usages, a minimum layer of insulation was used to bridge the ribs (flutes) of the decking.
The roof membranes consisted of three to five layers of organic roofing felt, embedded in asphalt or coal tar pitch. Surfacings were generally aggregate embedded in a flood-coat of bitumen, although mineral-surfaced cap sheets were used for folded-plate or barrel-shaped decks.
Most of these roofs have been replaced several times over. Current versions of bituminous roofs use glass-fiber felts replacing organic or asbestos felts. The introduction of polymer-modified bitumen roofs (MB) generally are compatible with the older BUR products, and most successfully avoid aggregate surfacing, thus reducing labor.
The earlier versions of thermal insulation were three-quarters to one inch of asphalt-treated fiberboard with a thermal resistance (R-value) of 2.78 per inch. Perlite roof insulation was a fire-resistant version, as was a glass-fiber, high-density board with a Kraft paper surfacing. Today, cellular insulation such as iso-boards and expanded polystyrene are used to meet energy requirements, with specified R-values of 15-25. Iso-boards can be laid using hot asphalt, while the styrenes are generally restricted to single ply roof systems with proper thermal protection against melting. PageBreak
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.
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