26 September 2006
reduce somewhat their lifecycle cost. However, the BUR membrane temperature is the
same as the temperature of the surfacing which adversely impacts the service life of the
roof system. Therefore, expected longevity is approximately the same or just slightly
more than a smooth surface BUR system. Mineral surfaced modified bitumen cap
sheets can also be used where a mineral surface cap sheet BUR system is desirable
and will provide longevity over the mineral surface cap sheet.
Aggregate Surface BUR Systems. The most common type of BUR
surfacing is aggregate embedded in a bituminous flood coat. The thickness of 180 kg
(400 lb.) gravel surfacing per square of roofing is 12 to 18 mm (0.5 to 0.75 in.). This
surfacing allows the temperature of the BUR membrane to remain somewhat cooler
than smooth surface or mineral surfaced cap sheet BUR systems. Aggregate surface
BUR systems tend to have a longer expected service life than smooth surface or
mineral surfaced cap sheet BUR systems because of lower membrane temperature and
due to protection of the membrane by the aggregate. Restrict the use of embedded
aggregate in areas with wind speeds of 161 km/hour (100 miles/hr) or greater and near
aircraft flight lines.
Modified Bitumen (MB) Membranes.
MB roofing systems have low maintenance cost, and should be considered when long
service life is required but aggregate surfacing is not practical. The polymers modify the
asphalt bitumen and improve the performance characteristics of the asphalt. The
addition of polymers increases low temperature flexibility and high temperature stability.
MB membranes must have at least two-ply or as specified by the UFGS.
Polymer Modifiers. The two major types of bitumen modifiers used are
an elastomeric Styrene Butadiene Styrene (SBS) polymer and a thermoplastic Atactic
Polypropylene (APP) polymer. SBS systems are usually applied by hot mopping
asphalt, however, torch varieties of SBS membranes are gaining prominence. Some
SBS products are applied with cold adhesive or torching, and torch varieties of SBS
membranes are becoming more common. APP systems are typically applied with
propane torches, which necessitate special requirements for safe handling and storage.
All torch-applied MB systems will utilize non-combustible cant strips at parapets and
curbs. Only non-combustible materials may be used for torch-applied systems. The
UFGS specifies a fire watch is required for each day for a period of time after the last
torch is extinguished.
MB Membrane Surfacings. The three common types of surfacing used
for the top sheet of MB systems are mineral surfaced, metal foil faced, and smooth
Mineral Surface MB Systems. The addition of mineral granules reduces
the temperature effect on MB systems. However, as granules are lost, degradation due
to ultraviolet (UV) radiation will negatively impact performance. Longevity of these
systems on average is not as great as aggregate surfaced BUR systems.