SBS Modified Bitumen - Page 4

7A.8.8 The recommended quantity of asphalt has been indicated on each specification in this section. It is important that the asphalt be uniformly spread, without voids, so that felt does not touch felt, so there is complete adhesion between all plies of the system.

7A.8.9 JM considers a ±25% deviation from the specified asphalt quantity to be acceptable.

7A.8.10 The use of the proper asphalt, as called for in the various JM specifications, is critical to the performance of the roofing system. A contractor shall not deviate from the asphalt requirements of the roofing system specified, unless the deviation is approved in advance by a JM Technical Service Specialist.

7A.9 Gravel or Slag Surfacings

7A.9.1 Some of the SBS modified bitumen systems are designed to be surfaced with gravel or slag. Gravel or slag must be dry before using. Wet gravel or slag will cause foaming of the asphalt and prevent proper adhesion of the surfacing. In cold weather, if difficulty is experienced in obtaining proper embedment in the asphalt, the gravel or slag should be heated prior to application.

7A.9.2 JM will approve the use of clean slag or gravel meeting ASTM Specification D 1863, which covers aggregates specified both for use in road construction and roofing. Aggregates meeting ASTM D 1863 are generally available commercially throughout the country.

7A.9.3 Other surfacing material used in place of gravel or slag should be fairly cubical in shape, non-water-absorbent, hard and opaque, and of such size and nature as to result in firm embedment in the asphalt.

7A.9.4 Do not use transparent or translucent stones, dolomite, or crushed masonry.

7A.9.5 Gravel should be spread at the rate of approximately 400 lbs. per square (19.5 kg/m²), and slag, because of its lower density, at the rate of 300 lbs. per square (14.6 kg/m²). Full coverage of the underlying asphalt is required.

7A.10 Application of SBS Modified Bitumens

7A.10.1 JM SBS products are installed using hot asphalt or special cold adhesives. Only certain SBS products may be installed using heat weld application techniques, and these products are designated as such. JM does not recommend the use of typical solvent-based adhesives to bond the membrane to the roof. Traditional solvent-based roofing cements can be used to strip-in laps and other terminations, provided that the mastic is not placed under the modified bitumen sheet. Roofing cements used in this manner will allow the solvent in the adhesive to flash off rapidly and not degrade the modified bitumen blend.

7A.10.2 SBS materials are frequently installed over traditional asphaltic base and ply felts which are applied using typical built-up roofing techniques. However, the use of SBS modified bitumen base felts is becoming more common and these products should be installed using the modified bitumen application techniques described in this section.

7A.11 Hot Asphalt Application

7A.11.1 The installation techniques for SBS modified bitumens in hot asphalt are similar to those used for built-up roofing. The sheets must be firmly and uniformly placed in a full mopping of hot asphalt, without voids, and with all edges well sealed.

7A.11.2 There are, however, a few unique conditions that the applicator must be aware of when installing these materials.

7A.11.3 The temperature of the asphalt at the point of application is very important, and differs from the requirements for built-up roofing. The asphalt must have a minimum temperature of 400°F (204°C) or be at the EVT, whichever is higher, when the sheet is set. This will cause the back coating to remelt or "flux", assuring proper bonding to the substrate.

7A.12 Mop Applied Asphalt

7A.12.1 There are several application techniques that can be used when the asphalt is installed by mopping. The modified bitumen sheet can be rolled, scrolled or flopped into the asphalt. Regardless of the application technique employed, the crucial factor is that the modified bitumen sheet make complete contact and embed in the hot asphalt. This can be accomplished by lightly brooming the modified bitumen sheet immediately after it has been installed. It is also good roofing practice to "scuff in" the side and end laps to assure that they are completely sealed.

7A.12.2 When rolling the modified bitumen sheet into the asphalt, the mechanic should mop no more than 6 feet (1.83 m) in front of the roll to assure that the temperature of the asphalt does not cool and fall below the temperature necessary for good embedment. If the asphalt is allowed to cool too much, an inadequate bond may result. In addition, the viscosity of the asphalt increases, which can result in a wavy appearance or excessive quantities of asphalt.  Excessive asphalt can increase the potential for slippage of the membrane.

7A.12.3 When using this application technique, brooming of the modified bitumen sheet is especially important at the end of the sheet where there may not be sufficient weight from the roll to provide the necessary pressure to embed the sheet into the asphalt.

7A.12.4  The scrolling technique is also used by many mechanics. This technique was originally used to allow the modified bitumen sheet to relax, to reduce some of the wrinkling and shrinkage associated with the early modified bitumen products. Although this is not required with fiber glass and fiber glass/polyester composite reinforced products, this method is occasionally used. The modified bitumen roll is completely unwound, usually turned upside down, and allowed to "relax". After the sheet has warmed, it is then turned right-side-up, placed on the roof in the area where it is to be installed and re-rolled or scrolled from both ends. The product is then mopped into place using the same mopping techniques and precautions described for rolling the product into place.