As reported in a previous column, about two-thirds of current low slope roofing activity is in re-cover and reroofing. Looking backwards, those roof systems of the 1950s (or even earlier) generally were built-up roofs, consisting of asphalt or coal tar pitch as the waterproofing agent, reinforced with bitumen saturated organic or asbestos felts. By the 1970s, asbestos felts had been replaced with glass fiber mats meeting ASTM D2178.
Modified Bitumens: Past to Present
Organic feltsgradually shifted to recycled waste paper from rag felts, and perhaps you still have some of these durable, organic felt-based roofs in place. Systems of that generation might have used four or five plies of organic felt hot mopped in place, followed by a flood coat of bitumen and gravel or slag surfacing. Asphalt-coated base sheets were introduced for nailable decks, replacing two of the lower-nailed roofing plies, reducing both labor and materials.
A new version of glass fiber mats meeting ASTM D-2178 Type IV (called wet-process mat) proved to be both economical and stronger than earlier materials and is still in use today. Since the years of the oil embargo (1973-1974), most new roof specifications called for increased layers of thermal insulation, so coated base sheets (which are intended for nailable decks) were eliminated, with three plies of glass fiber mat installed in bitumen directly onto the thermal insulation, a procedure now considered to be state of the art.
Polymer-modified bitumens (MB) were appearing in common-market countries, as the cost of petroleum was becoming prohibitive. Two generic categories of MB were offered: elastomeric extenders (modifiers) are called SBS (for sequenced butadiene-styrene) and thermoplastic extenders (modifiers) are called APP (for atactic polypropylene).
APP vs. SBS
APP-modified membranes were first reported in Italy by Romolo Gorgato in 1967 and were available in Holland and Great Britain by the mid-1970s. By the mid-1980s, MB had nearly 90% of the market in Italy and 60% of France and Norway.
SBS-modified membranes are more flexible at low temperatures than APP, advanced through the development of thermoplastic elastomers. Production of SBS membranes is more complicated than APP, relying upon compatibility of the bitumen with the elastomer.
SBS membranes can be installed by mopping with field with hot asphalt, while APP membranes require much higher torching temperatures to succeed.
Internal fiber-mat reinforcements are needed during the manufacture of both types, with glass fiber and polyester fiber being the most common reinforcements. In addition, surfacing is generally applied in the factory, most often using mineral granules or foil films. Roofing membranes result in two-ply MB systems (a base sheet and a mineral surfaced cap sheet), again reducing both material and labor. Substituting factory-applied mineral granules for field-applied gravel further reduces weight of the system, moves more of the quality control from the field to the factory, and makes inspection and repair far easier.
Rubber modifiers were discussed as early as 1843 for use in roofing, sealants, and lacquer coatings, according to Bituminous Materials: Asphalts, Tars and Pitches by Robert E. Krieger Publishing. Elastomer modification of bituminous materials is conveyed in the following ways:
1) Softening temperature is increased
2) Cold flow is reduced
3) Change in penetration with increased temperature is reduced
4) Brittleness temperature is lowered
5) Elastic recovery is imparted
6) Resistance to deformation under stress is increased markedly, particularly at ambient temperature, resulting in improved toughness and tenacity
7) Ductility is increased, particularly at low temperatures
Versatility of MB Systems as Repair Materials
When your existing asphalt-based roof membranes (especially flashings) are starting to show wear and tear, MB membranes make suitable repairs. For punctures, broken blisters, and the like, the existing surfacing can be wire-brushed (removing mineral aggregate, if present) and a torched-applied patch can be installed. Use of the torch provides heat flux to evaporate trapped moisture and in effect, self-prime the substrate to be repaired.
Chapter 3 of the ARMA/NRCA/SPRI repair manual (Repair Manual for Low-Slope Membrane Roof Systems) is devoted to MB membranes and can be useful in training in-house maintenance staff. Cold repairs can be made using rubberized asphalt mastics, and, of course, BUR repairs can be made with fibrated, non-modified mastics as well.
If you choose torch-applied repairs, you should have your key personnel attend a Certified Torch Applicator Training program, offered either by the National Roofing Contractors Association (NRCA) or the Midwest Roofing Contractors Association (MRCA).
At the very least, plan to use an inexpensive, non-contact infrared thermometer to check the temperature of the repaired area to assure that a smoldering hot-spot below, or in the vicinity of, the torch-applied repair will not later erupt into a damaging fire when least expected.
While most of these MB systems originated in Europe, they are readily available now from virtually all the major North American roofing material producers. Choices for what is best depends upon intangibles such as weather conditions as well as previous successes with certain types.
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