A modified bitumen membrane (MBS) roof is a system
developed by taking the qualities of a BUR system
(built up roof) along with the qualities of a
single-ply roof. A standard MBS roof system should
have a base sheet or ply, similar to the BUR system,
then a waterproofing sheet similar to single-ply.
A modified bitumen membrane is composed primarily of
polymer-modified bitumen reinforced with one or more
plies of fabric such as polyester, fiberglass, or a
combination of both. Factory surfacing, if applied,
includes mineral granules, slag, aluminum, or
copper.
Modified bitumen's generally use a traditional
waterproofing medium -- asphalt -- modified with
atactic polypropylene (APP), styrene butadiene
styrene (SBS), synthetic rubber, or other agents
that will enhance the asphalt properties. APP and
SBS are the most common bitumen modifiers.
The most common modified bitumen's are:
- SBS (Styrene-Butadiene-Styrene)
modifies the asphalt by forming a polymer network
within the bitumen. SBS gives the bitumen
rubber-like characteristics and improved resistance
to aging and weathering. Most SBS modified bitumen
sheets are either set in hot mopping asphalt,
torch-applied, or adhered with cold-process
adhesives. SBS modified bitumen sheets that do not
have factory applied granule or foil surfacing need
some form of field-applied ultraviolet protective
coating.
- APP (Atactic Polypropylene) is
a thermoplastic polymer, which forms a uniform
matrix within the asphalt. This enhances the
bitumen's performance by increasing its UV
resistance, increasing its flexibility at low
temperatures, and improving its flow resistance at
high temperatures. APP modified bitumen sheets are
generally applied using a propane-fueled torch.
Applicators use the heat to soften the modified
bitumen on the underside of the sheet. The sheet's
bottom surface becomes a molten adhesive, which
flows upon the substrate and then cools to form a
waterproof adhesive bond. Some APP sheets can also
be applied with cold-process adhesives.
Base sheet/shingling of felts
MBS are not considered a built up roof but considered
a single-ply. Still it is encouraged to install a base
sheet/underlayment. Similar to the built up roof
systems, a contractor should apply one of the following:
- Standard glass-fiber felt (2-ply fiberglass
platform (minimum) or a 3-ply base)
- Coated glass-fiber felt
- Coated organic felt
This is to protect the roof system from any water
penetration and protect the insulation from the heat of
the torch. After the plies have been installed then the
MBS roof system can be installed. For the reasons
mentioned above we have classified the MBS roof system
as a built up roof.
Surfacing
- Mineral granules
- Metallic foil
- Asphalt (hot or cold applied)
- Heat-reflective coatings
