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accommodate rainfall build-up and snowpack.
Rooftop detention systems, however, may
require additional structural supports depending
upon the depth of water being stored and
equipment (HVAC and cell phone sites. for
example) located on the roof. At times, the
existing roof may not be capable of supporting
an additional load.
During a fire, the live load on a roof is increased
by firefighters operating on it. Also included are
tools they are carrying and equipment they bring
up. Water from hoselines may be adding to the
amount of weight being retained on a blue roof.
Moreover, fire may be weakening structural
components to the point where they unable to
support a “fully loaded” blue roof. A blue roof,
therefore, must be designed with a practical limit
on the amount of water that can be stored
commensurate with construction type and the
load-bearing capacity of its structural members.
May 2017
roofing material is employed. A clay-tiled roof,
for example, may have a dead load of 27
pounds per square foot. The typical residential
flat roof has a live load range requirement of
15-20 pounds per square foot.
Over time, roof structural members can suffer
from corrosion (metal) and rot (wood).
Furthermore, years of externally-applied forces
(wind) can loosen fasteners and connectors
reducing a roof’s ability to resist the extra load of
a blue roof system. Buildings with lightweight
roof supporting members, in general, provide
less protection from overload than roofs built
with heavyweight supports. Code officials,
architects, engineers and builders should seek
out fire department input regarding collapse
concerns prior to installation. To ensure safety, a
structural analysis is necessary to verify that the
roof and its load-bearing system are capable of
supporting a blue roof.
Note: The dead load of the common asphalt-
shingled, wooden roof is about 15 pounds per
square foot. The load increases when heavier
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