FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

Volume II Designing the builDing envelope 11
work. Te natural hazards presenting the greatest challenges to low-sloped roofs in the coastal environment
are high winds (see Section 11.5.8.1), earthquakes (see Section 11.5.8.2), and hail (see Section 11.5.8.3).
11.5.8.1 High Winds
Roof membrane blow-of is typically caused by lifting and peeling of metal edge fashings (gravel stops)
or copings, which serve to clamp down the membrane at the roof edge. In hurricane-prone regions, roof
membranes are also often punctured by wind-borne debris.
Following the criteria prescribed in the IBC will typically result
in roof systems that possess adequate wind uplift resistance if
properly installed. IBC references ANSI/SPRI ES-1 for edge
fashings and copings. ANSI/SPRI ES-1 does not specify a
minimum safety factor. Accordingly, a safety factor of 2.0 is
recommended for residences.
A roof system that is compliant with IBC (and the FBC) is
susceptible to interior leakage if the roof membrane is punctured by wind-borne debris. If a roof system is
desired that will avoid interior leakage if struck by debris, refer
to the recommendations in FEMA P-424,
Design Guide for Improving School Safety in Earthquakes, Floods
and High Winds (FEMA 2010a). Section
6.3.3.7 also provides other recommendations for enhancing wind performance.
11.5.8.2 Seismic
If a ballasted roof system is specifed, its weight should be considered during seismic load analysis of the
structure. Also, a parapet should extend above the top of the ballast to restrain the ballast from falling over
the roof edge during a seismic event.
11.5.8.3 Hail
It is recommended that a system that has passed the Factory Mutual Research Corporation’s severe hail test be
specifed. Enhanced hail protection can be provided by a heavyweight concrete-paver-ballasted roof system.
If the pavers are installed over a single-ply membrane, it is recommended that a layer of extruded polystyrene
intended for protected membrane roof systems be specifed over the membrane to provide protection if the
pavers break. Alternatively, a stone protection mat intended for use with aggregate-ballasted systems can be
specifed.
11.6 Attic Vents
COASTAL CONSTRUCTION MANUAL
note
The 2009 edition of the IBC
prohibits the use of aggregate
roof surfacing in hurricane-prone
regions.
High winds can drive large amounts of water through attic ventilation openings, which can lead to collapse
of ceilings. Fact Sheet 7.5, Minimizing Water Intrusion Trough Roof Vents in High-Wind Regions, in FEMA
P-499 provides design and application guidance to minimize water intrusion through new and existing attic
ventilation systems. Fact Sheet 7.5 also contains a discussion of unventilated attics.
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