b'Roof Blisters Blister TypesThere are three types of blisters, and they are not mutually exclusive. You potentially could observe more than one type in the same roof area. The first type is the plate, or pancake, blister. This classic blister shape typi-cally is the size of a dinner plate or larger. A plate blister is one continu-ousairpocket.Someplateblisters canbesohighlypressurizedthey can withstand a person standing on them like an air mattress. Photo 1 shows a plate blister in the field.Thenexttypeofblisteristhe linearblister,whichcommonlyis foundwhererolledmembranesor felts are layered and/or installed with staggeredlaps.Theassemblyand stagger can lead to a systemic issue where a blister forms along the lap of two layers of felt or polymer-mod-ified bitumen cap sheet, base sheet or vapor retarders. Linear blistering was the primary concern in NRCAs 1990s work. Photo 2 shows a linear blister along a lap.Photo 3: This close-up view shows a rash blisterThethirdblisterissmallbut on detailing tape for an EPDM roof system. Theseprevalent: the rash blister. Rash blis-blisters typically are tightly clustered and mostters are small, typicallytoof an common on tapes, laps and liquid coatings and/or membranes but also can occur on asphalt shingles. inch in size. They are tightly packed together in an area and can be found on most types of roofs. Most com-the findings of earlier works and ex- blisterhaspulledapartaseambytion providesadditionalguidanceIn the 1990s, other researchers mademonly they are seen at seam tapes, plorednewvariables.Theresearchabout one-third or more of the seamregardingblisterassessmentandimportant contributions on adhesiondetailingtapesorliquidcoatings/ledNRCAtoissuethefollowingwidth. This effectively set guidancerepair.Themanualrecommendstests for polymer coatings and describedmembranes.Thesecanevenoc-recommendations in an interim re- for when to leave blisters alone andrepairing blisters if they are open,the mechanics of the blister testing incur on asphalt shingles, particularly port:Considerrepairingblisterswhen to repair them. haveexcessivelossofsurfacing,closed-form equations for the interfa- whenanasphaltappliquisused that occur in high traffic areas. Re- TheNRCA Repair Manual forhave exposed reinforcement or havecial fracture energy. This allowed blis- duringthemanufacturingprocess. pair should also be considered if aLow-slopeRoofSystems,2ndEdi- fatigue cracking around them. ters to be explained mathematically. Photo 3 shows a rash blister.14ONTARIO ROOFING NEWSISSUE 3 2024 THE ONLY SOURCE FOR PROFESSIONAL ICI ROOFING CONTRACTORS IN ONTARIO ORN'