01 May 1999
b. Discussion of Headers in Tables 2-4 and 2-5.
(1) Initial Cost. This may include materials, labor, and special set-up for construction.
Initial cost may determine if the roof, as designed, is affordable. Perhaps a somewhat less
expensive system should be considered if it does not incur significantly increased maintenance
costs or have a shortened life.
(2) Life Cycle Cost.
LCC considers durability but also presumes that routine
maintenance will be performed to achieve the projected life. Consider whether the building is
temporary or permanent. It would be hard to justify an expensive copper or slate roof on a
building scheduled for demolition in the near future. Also consider the mission of the building.
There are levels of quality in many systems. For example, while 1.1 mm (45 mil) EPDM is the
standard, for little extra cost 2.3 mm (90 mil) material with greater puncture resistance, or
conversion to a PMR system, could be specified for a building with a critical mission.
(3) Construction Difficulty. Some systems require more clearance to accommodate
application methods and equipment. Large prefabricated roof sheets (i.e., 15 m (50 ft.) by 60 m
(200 ft.) may be fine on a large roof with few penetrations, but are impractical on a roof area that
is broken up by many curbs and equipment supports. On multiple penetration surfaces, relatively
narrow sheets (e.g., BUR, MB, thermoplastic single-ply) or sprayed-in-place polyurethane foam
should be considered. Penetrations through standing seam metal roofing need to accommodate
the expected thermal movement of the metal panels. Thermal movement is cumulative,
increasing with distance from the point where the panels are restrained (typically the eaves).
Penetrations in SSSMR panels must pass through the flat portion of the panel, not through the
standing seam. Penetrations wider than a single panel require a diverter to carry water around
the obstruction. Water must flow parallel to the raised seams, never over them.
(4) Periodic Maintenance--(The need for periodic maintenance and difficulty of
inspection or maintenance.) Some roof systems require periodic recoating for weather protection.
Aggregate surfaced roofs are more difficult to inspect and patch than smooth surfaced roofs.
(5) Life Expectancy. A mean life is listed but the actual life is affected by drainage,
maintenance, and extreme use or abuse.
(6) Suitability in Severe Cold. Effects of freeze-thaw, hail, ice scrubbing, and traffic while
cold (i.e., snow removal) is considered. Some materials embrittle dramatically at low
temperatures (i.e., have a relatively high glass transition temperature); others may embrittle as
they weather and lose plasticizer or are degraded by UV or thermal load. H indicates highly
suitable; L indicates less suitable.
(7) Suitability in Extremely Hot or Humid Conditions. Effects of thermal expansion and
algae growth are considered. H indicates more suitable, L indicates less suitable.
(8) Wind Resistance. Roofs are vulnerable to wind scour and blow-off. While arbitrary
ratings are provided here, the resistance is affected by building height, terrain, parapet height and
measures taken to upgrade perimeter and corner attachment. H indicates highly wind resistant
(when properly designed). For membrane roofing, impermeable roof decks such as cast-in-place
concrete are best. Air retarders are needed with loose laid and mechanically fastened single-ply
systems as they may otherwise balloon from interior air leakage. Perimeter wood blocking must
be well anchored to prevent peeling of the membrane or loss of fascia metal. Avoid the use of
small aggregate (e.g., pea gravel) near tarmacs and on skyscrapers due to the damage it can
cause if blown off the roof by high wind. Asphalt shingles may require manual application of tab
adhesive. Interlocking asphalt shingles provide excellent wind resistance. Metal panel systems
are wind resistant only when all components including clips, fasteners, and secondary structural
members are installed as wind-tested. SPF has outstanding resistance to wind and to wind-blown