SBS FLAT ROOF SYSTEM
Modified Bitumen Roofing Membranes are composite sheets consisting of bitumen, modifiers and reinforcements. The term “modified bitumen” encompasses a broad range of materials, with each specific material differing from the others with respect to the modifiers and reinforcements used. Modified bitumen membranes exhibit the thermoplastic quality of being softened by heat. They are typically bonded to substrates by torch application or asphalt.
Modified bitumens (primarily asphalt) have found many uses in practical roofing applications. SBS (sequenced butadiene-styrene) and APP (atactic polypropylene) polymer modifiers are relating to roll roofing. However, there is significant history with respect to polymer blending as a performance enhancement in cold roofing mastics in the late 1960 and early 1970. A few manufacturers were blending SBS polymers with traditional roofing mastics to impart elastomeric characteristics for use at those roofing joints that demanded higher performance to maintain watertight integrity.Especially, pitch pocket penetrations, edge fascia metal joints were a constant challenge. The coefficient of expansion and contraction of these details exceeded the limits of the typical fiber reinforced mastic, that exhibit cracks and splits that accelerated after a nominal amount of field exposure. As roofing chemistry became more sophisticated, various formulations and viscosity ranges were developed for specific uses, such as elastomeric sealants/mastics for high movement joints and terminations. This improvements for bituminous material’s durability and extensibility (especially at lower temperatures) was done by adding polymers (rubber and plastics). Reinforcements have evolved from the true rag felts to pulp, glass, and more recently, to heat-stabilized polyester blends and combinations of glass and polyester.
Modified products were developed for different reasons. It was very hazardous to work with hot bitumens in confined areas, and to achieve the desired bond to vertical surfaces. Polyurethane based on application are provided in horizontal and vertical viscosities. Most products have a low VOC that are compliant with air quality regulations and worker safety concerns. These materials are usually applied with a notched trowel to achieve the appropriate thickness for waterproofing integrity, and when cured, offer excellent MVT resistance and crack spanning capabilities.
Hot Applied Water Proofing
To meet the needs of extensive lightweight slab construction, hot-applied modified asphalts appeared in the construction market in the mid 1970s. Unlike roofing asphalts, these materials were blends of non-blown asphalts and elastomeric polymers, primarily SBS. The advantage over conventional waterproofing configurations for on-grade waterproofing of plaza decks is that multiple layers of reinforcing felts are not needed. Unlike roof deck constructions, the concrete pours are stable. Typically, the hot-applied modified waterproofing systems are reinforced with a single layer of compatible, heat-stabilized, polyester fabric. Some Department of Transportation divisions accept hot modified bitumen materials for bridge deck repair as well as other concrete joint remediation.
The current generation of asphalt hot-melts is derived from the success of the modified product developments in the waterproofing industry. However, unlike the strict heating parameters required (oil-jacketed kettles) for hot-melt waterproofing bitumens, current hot-melt roofing adhesives are not as sensitive to heating as their predecessors. The modified bitumen adhesive can be installed with traditional means and methods, such as a roofing mop or felt-laying machine. The key to temperature flexibility is the result of the use of SBS polymers.
The unique benefit of using compatible hot-melt modified bitumens with MB felts is that all components of the system are elastomeric. Although the commonly accepted recommendation for MB ply installation is to use Type III or IV asphalt, system elongation, cold bend flex temperature, and strain energy are superior with the use of hot-melt adhesives and SBS modified plies and surfacing sheets.
Cold Process Roofing
Cold process roofing had its beginnings in the mid 1970s with asphalt cutbacks employed as the adhesive between coated base sheets. The application was developed for those projects where kettle locations were restricted or the fumes were highly objectionable. The materials are applied at ambient temperature or slightly heated to facilitate application through spraying. Rarely does the application temperature reach 100 F. A cold asphalt cutback is modified with calcium carbonate, a were highly objec-variety of reinforcing fibers and other thixotropes. When a typical roof cut is examined, the core appears to be homogenous. It is difficult to discern the ply lines.
Evolution of Modified Piles
It is quite interesting to know that cold process applications became the differentiating factor between the coated base sheets. Often soft and mushy asphalt was used to coat the rolls by the manufacturers. This used to be quite tricky as it required prolonged intervals on the roof. The industrial glues especially meant for cold-processes were used for this. Manufacturers needed to take special care as they were quite a delicate system to bear the weight of mechanical or even human impact. Roof could bleed heavily and would not be ready for weeks to withstand even marginal traffic. In case of aluminum or acrylic coating, manufacturers suggested rather a month long cure along with some extra efforts to wash off the deposits of contaminants sitting on the surface. Usually, the whole process ends with flooding the sheets with adhesives after broadcasting the granules. Times have changed. Now the sheets are engineered. The sheets that are available today have coats of polyester fabric and glass fibers which enable quick installation. Even the asphalt used as coats is modified to make them compatible with the adhesives used. The glues that are used for cold process are supplied in tanker trucks, refill vessels or large barrels. Next, with the help of hydraulic or pneumatic sprayers, this viscose fluid is pumped on to the roof. For projects with space constraints, it is supplied in containers of 5 gallons capacity and applied with brushes. For projects still in production phase, heat-exchange units powered by propane are recommended by the contractors for the approved coatings called Torch Grade SBS membranes.
The consistency of these materials is thicker than the hot-applied membranes which makes reinforcing fabric resistant to the potential damage caused by heat while installing the same. Torch applications cause damage to the polyester reinforcements and the ‘Torch Grade SBS membranes” have reinforcements of glass. In most of the cases, modified APP is used which softens when exposed to high temperature. There are more restrictions pertaining to the naked flames, the smell, roof areas with crunched space. As result, the cold-applied systems are gaining more popularity. Even the manufacturers are coming up with modified bitumen roll adhesives formulated especially for this purpose. The traditional cold process glues were solvent based mechanism while SBS cold process systems use no or negligible solvent based adhesives used for joining the plies of modified bitumen to themselves as well the substrates.
Modified Bitumen Roofing System
A 2-ply SBS roof system, often known as Torch Down Modified Bitumen roof is gaining it’s foothold in the industry at quite a rapid pace. In this system, fiberglass or polyester material is pressed in the middle of two rubberized asphalt layers. This system is long-lasting and easy to install. The tradition roofing system of Modified Bitumen has five main components that can be layered in any combination. These components are:
• Surfacing- It can be field or factory applied. It gives protection against harmful ultra violet radiation and different climatic conditions.
• Adhesives- This gives waterproofing to the structure and is the sticking agent which is applied either hot or cold.
• SBS or APP Modified Bitumen Membrane- A co-polymer bitumen sheet which is usually reinforced with fiberglass or polyester.
• Insulation- Used for R-values and give stability to roof system with substrates.
• Flashing- Makes the roof perimeter, projection and the equipment waterproof.
To install a SBS flat roof sytem, proper training and experience is necessary to properly install Torch Down roofing system. If untrained workers are hired for such structures, the roof may fall after two years of installation. To avoid unwanted expense in a roof replacement, it is important to use applicators of highest standard. A 2-ply SBS roof system can be different from another and cost is not always the only parameter to judge the quality.