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' l l 1 1 1 1 <br /> Relevant design considerations include.- TEAR-OFF OR RE-COVER <br /> —Use of air-impermeable deck construction,such as The decision to tear-off or to re-cover an existing roofing system is <br /> poured-in-place concrete or insulating cellular concrete over a not always clear. <br /> steel pan; Although not an exhaustive list,the following additional design <br /> —Alternatively,installation of an air barrier,such as polyethylene elements typically require consideration for any reroofing project: <br /> sheeting,at deck level beneath mechanically attached <br /> insulation with attachment sufficient to balance positive —Replacement of damaged roof decking or structural <br /> pressure; components; <br /> —Attention to an air-tight seal between roof and side walls/ —Improvement of roof access; <br /> penetrations. —Removal of unused rooftop equipment and associated <br /> equipment mountings; <br /> Rooftop Traffic/Physical Abuse —Remounting of rooftop equipment to allow proper roofing and <br /> Roofing installations that can be expected to experience a high flashing technique; <br /> degree of roof traffic due to equipment maintenance,vandalism or —Matching of architectural elements such as special perimeter <br /> other unauthorized access,frequent hailstorms or high winds,and metalwork; <br /> prolonged periods of temperature extremes or rapid fluctuations in —Repair of deteriorated parapet and penthouse walls, <br /> temperature will require a more durable roofing system. —Protection of roofing membrane by means of concrete paver <br /> Relevant design considerations include: overlay or walkway pad system. <br /> —Use of thicker membrane or multiple-ply system; <br /> —Use of a higher compressive strength insulation substrate; <br /> Tear-off/Replace <br /> —Application of a concrete paver or insulated paver overlay Factors that support the tear-off approach include. <br /> for extreme conditions. <br /> —Two or more existing roofs(building code restriction); <br /> Contamination —Structural weight limitation; <br /> Many roofing installations are exposed to oil,grease,and chemical —More than 25%of existing roof area is wet; <br /> contamination in excess of normal airborne contaminants. —Flashing height limitations; <br /> These conditions are most often associated with restaurants,food —Need to maximize long-term performance. <br /> processing plants,chemical and pharmaceutical plants,refineries, <br /> machining and manufacturing facilities,and airports.Most roofing The basis for any tear-off project is to provide a sound substrate for <br /> materials are degraded by certain families of contaminants,and the installation of a new roofing system and minimize potential <br /> will become brittle,swell and soften,or dissolve,depending on the damage from tear-off activities.At a minimum,attention to the <br /> material formulation and contaminant type. following considerations is recommended. <br /> Long-term exposure,i.e.,28-day immersion testing of roofing —Thoroughly inspect decking,flashing substrates,and wood <br /> material and specific contaminant,remains the preferred method nailers before installing new materials; <br /> of determining material resistance.Even then,unforeseen <br /> combinations of contaminants,environmental exposure effects, —Plan a tear-off strategy so that roof drainage patterns are never <br /> and variation in contaminant concentration prevent an absolute blocked,and so that construction traffic is directed away from <br /> prediction of resistance to contamination in all but the most new roof areas; <br /> common situations. —Protect new roof areas adjacent to tear-off areas from dirt, <br /> Relevant design/maintenance considerations include. debris,and damage. <br /> —Isolation of contaminated roof area with expectation of more <br /> frequent roof membrane replacement, Re-cover <br /> —Periodic power washing of roofing membrane with moderate Factors that support the re-cover approach include.- <br /> pressure; <br /> —Limitation of rooftop spillag%xhaust of contaminating —Need to minimize cost; <br /> materials,i.e.,grease traps. —Disposal restrictions; <br /> Refer to www.gaf.com for chemical—resistance information.GAF —Difficult access to the roof; <br /> guarantees on any GAF membrane,including TPO and PVC,do not —Only one roofing system in place. <br /> cover damage due to chemical contamination. <br /> The requirement for any re-cover project is to eliminate defects in <br /> the existing roof assembly so that their effect on the new roofing <br /> system is minimized.At a minimum,attention to the following <br /> considerations is recommended.• <br /> —Raise all perimeter flashings,penetrations,and equipment to <br /> provide required flashing heights; <br /> 11 <br />