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Serving Alberta's Roofing Industry Since 1961
MB Section 9: Protected Membrane & Combination Designs
A protected membrane roofing design is an
insulated and ballasted roofing system in which the primary
insulation is applied above the modified bituminous membrane and
retained in place by ballast.
A combination roofing assembly is comprised of a
conventional insulated membrane system where the primary modified
bituminous membrane is covered with insulation held in place with
Protected membrane and combination designs do
not require back nailing of the primary membrane plies for roof
slopes 1:8 (1 1/2"/ft.) and greater.
For protected membrane and combination designs
the drainage slope shall be a minimum 1:50 (1/4"/ft.). For
re-roofing projects see Section 10 – Reroofing, Item
The maximum roof slope is restricted to 1:6
When flow control drains are incorporated into
the roof drainage system, membrane level scuppers shall be provided
at the roof perimeter as emergency drainage. Roof drains and
scuppers must have gravel screens installed when gravel ballast is
specified. Drainage openings are not permitted through expansion
MB-9.3 Penetrations and Perimeter
Except for roof drain and pedestal fall
protection roof anchors, all membrane penetrations shall be
contained within or supported by a curb secured to the structure or
Roof parapets, roof/wall junctions and membrane
penetrations including curbs, area dividers, equipment supports and
pedestal fall protection anchors shall extend a minimum distance of
200mm (8") above the surface of the insulation when using gravel
ballast or above the surface of concrete pavers when they form the
Pedestal fall protection roof anchors
shall be waterproofed with an ARCA Warranty Ltd. approved one-piece
manufactured metal specialty flashings. The one-piece metal
specialty flashing shall incorporate a minimum 100mm (4”)
wide integral flange. The flashing flange shall be mechanically
fastened to the structure or decking over the primary membrane
prior to the application of the membrane flashing.
Roof area dividers are not required for
protected membrane designs.
MB-9.4 Membrane Flashing
A minimum of two (2) plies of S.B.S. modified
bituminous membrane flashing shall be installed at internal roof
drains, parapet, curb and wall junctions.
When gravel ballast is specified, the vertical
extension of the membrane flashing shall terminate a minimum
vertical distance of 200mm (8”) above the surface of the
For concrete pavers or concrete topped
insulation systems, the vertical extension of the membrane flashing
shall terminate a minimum distance of 200mm (8”) above the
top of the concrete surface.
The roof insulation shall be Type 4 Extruded
Polystyrene Insulation loose-laid over the primary membrane. The
insulation provides ultraviolet light protection for the membrane,
as well as the thermal resistance for the roofing system.
Roof insulation 100mm (4") thick and less shall
be installed in a single layer.
For multi-layer insulation applications, the
base layer of insulation boards shall not be less than 100mm
(4”) thick. When using more than one insulation layer, the
board joints shall be offset a minimum distance of 300mm
(12”) from the joints of the previous insulation layer.
Poured-in-place concrete topping ballast is not
accepted for Warranty Certificate issuance.
The ballast may be comprised of rock or concrete
Gravel (rock) ballast shall be
relatively free of fines and other foreign matter and shall contain
gravel sizes between 19.0mm (3/4") and 38mm (1 1/2") in diameter
and shall conform to ASTM D7655 "Standard Classification for Size
of Aggregate Used as Ballast for Membrane Roof Systems".
An acceptable filter fabric must be installed
over the insulation when using gravel ballast.
A minimum ballast mass of 50 kg/m² (10
lbs/ft.²) is recommended over the field of the filter fabric
and loose-laid insulation. With the minimum gravel ballast mass,
the filter fabric may be visible. For thick insulation layers,
consult with the insulation manufacturer for ballasting
For concrete paver ballast, the weight of
individual pavers shall not exceed 45 kg (100 lbs.) in mass. When
using a paver ballast system, the pavers shall be elevated above
the insulation to provide an air space between the paver and the
primary insulation. Pedestals or insulation strips shall be
employed to elevate the concrete pavers above the surface of the
Type 4 insulation boards. Paver ballast shall not be placed on a
The design parameters of the roof structure
itself must include the weight of the newly installed ballasted
roofing system. The structure must be capable of supporting the
weight of the roofing system plus the specified ballast without
encroaching on the necessary live load allowance and without
creating or aggravating water ponding problems. A design
professional should be consulted to certify that the structural
system will support these loads.
MB-9.7 Sheet Metal Flashings
Drip edge flashings are not accepted for roof
edge terminations at membrane level.
For a gravel ballast system, the sheet metal
base flashings, when required, shall contain a minimum 50mm
(2”) wide horizontal leg that rests on the top surface of the
With paver ballast, the 50mm (2”)
horizontal base flashing, when required, leg shall rest on the
surface of the paver.
MB-9.8 Roof Terraces
Roof Terraces are to be installed over a minimum
6.4mm (1/4") asphalt impregnated core board membrane protection
The designer must take into
consideration the compressive strength of the underlying materials
to prevent damage to the insulation and roofing membrane from point
loads and concentrated loads in excess of 113kg. (250 lbs.) which
may exceed design limits. It is recommended that an ARCA approved
HD coverboard is installed under the membrane for additional
Roof areas with roof terraces are eligible for
five (5) or ten (10) year Warranty Certificates only.
The cost to remove and replace the roof
terrace material to facilitate access to the roof membrane for
investigation and repair of workmanship related leaks is included
however; it is the responsibility of the building owner to cover
costs for removal and replacement of items that were not installed
by the roofing contractor. It is recommended that the design of the
roof terrace incorporate future service and maintenance
Roof leaks as the result of membrane damage due
to the work and activities of others, maintenance or from
contaminants are not covered under the Warranty Certificate.
© 2016 Alberta Roofing Contractors Association