Protected and Modified Protected Roof Systems
Protected and Modified Protected Roof Systems
NOTE TO READER: Where applicable, the content of this page is replicated in the RoofStar Guarantee Standards sections of this Manual
In a “conventional” roof assembly the roof membrane is on top of the insulation where it is subject to many life-shortening elements, and it must function as more than a waterproofing agent. In addition to ultraviolet degradation, thermal cycling, hailstone impact, and other environmental hazards the membrane is exposed to physical abuse and chemical attack. Built-up roof membranes are particularly susceptible to problems associated with conventionally insulated roof assemblies, such as:
- evaporation of the bitumen's volatile oils and accelerated oxidation, leading to embrittlement and alligatoring
- splitting, ridging, and blistering
- slippage and membrane migration
- surface wind erosion and blow-offs
Furthermore, a conventional roof assembly, with the insulation between a vapour retarder and the roof membrane, may produce a problem with vapour trapped within the assembly. This can lead to insulation and roof membrane deterioration and, eventually, roof failure.
The primary distinction of a Protected Membrane Roof Assembly (PMRA) is that the roof insulation is placed on top of the membrane. Although the theory of a PMRA assembly can be adapted many ways, the most common form consists of a roof deck, a directly adhered or loose-laid roof membrane, a loose-laid extruded expanded polystyrene, a loose-laid filter fabric, and a gravel or paver ballast. This assembly has produced a variety of terms such as “an upside-down roof” or “inverted roof system” and in 1968 a patent was issued on the “Insulated Roof Membrane Assembly (IRMA)” developed by J.S. Best of the Dow Chemical Co. Much of the initial research on these systems was conducted by the National Research Council of Canada during the late 1950's and early 1960's.
In a PMR Assembly the membrane functions solely as the waterproofing agent (the requirement for a vapour retarder is eliminated) and the properties of the insulation become more critical. Extruded expanded polystyrene (conforming to CAN / CGSB-51.20-M87, Type 4) is the only commercially produced roof insulation suitable for a PMRA, providing properties such as water resistance (i.e. resistance to water absorption, moisture transfer, and capillary action), resistance to freeze-thaw cycling, and high compressive strength.
Protective surfacing (either gravel, pavers, or composite concrete topping) protects the insulation from ultraviolet degradation, provides a non-combustible surface, prevents wind uplift, and provides ballast against flotation. The filter fabric between the insulation and ballast distributes localized wind uplift and buoyancy forces, permitting lighter ballast requirements (frequently referred to as a “lightweight system”) which may be an important structural (and financial) consideration.
A Modified Protected Membrane Roof Assembly (MPMRA) is similar to a PMRA except that a layer of insulation is installed underneath the membrane as well as on top. This may offer cost savings as only the top layer of insulation requires ballast and the bottom layer (mechanically-fastened or adhered) need not be extruded expanded polystyrene and may be tapered to provide slope. The membrane must be located such that the dew point temperature (for the inside air) occurs above the membrane. As a general rule, two-thirds or more of the total thermal resistance (RSI or R value) should be above the membrane, but in all cases the design authority should perform the required psychrometric calculations before designing a roof system.
PMRA and MPMR Assemblies may offer the following advantages and properties:
- Minimal thermal cycling, as the membrane is closer to the building's heated interior. Conventionally-placed membranes may have temperature variations ranging 55°C (100°F) in one day, where a PMR might only range 5°C (10°F) under the same conditions.
- Thermal stress and membrane contraction splitting caused by extremely cold membrane temperatures may be eliminated.
- Somewhat retarded loss of volatile oils from bituminous membranes may result in prolonged membrane life.
- The membrane is protected from physical damage as soon as the insulation is installed.
- The problems associated with water vapour entrapment and vapour pressure are eliminated.
- Ballast, filter mat, and insulation may be removed and reinstalled to facilitate additional insulation, membrane changes or modifications, or construction of additional storeys.
The possible disadvantages or precautions involved with PMRA or MPMRA's include:
- The heavier ballast requirements may effect the structural costs of the building.
- Leaks are difficult to find, particularly in loose-laid systems.
- The impeded drainage caused by insulation may require more built-in slope; a minimum slope of 1:30 (3/8" in 12") should be considered.
- For built-up bituminous membranes in PMRA or MPMRA the use of glass ply felts, as opposed to organic (No. 15) felts, is highly recommended. These assemblies tend to hold water and organic felts will deteriorate prematurely if water reaches the felts and “wicks” into the system.
Filter mats specified must meet the insulation / membrane manufacturer's specifications.
The use of fabric filter mat allows for the use of less ballast. This is achieved by using water permeable fabric between the loose laid insulation boards and the stone ballast. The effect of the fabric is to prevent the displacement of individual boards in case of flotation. The fabric will also prevent fines in the stone ballast from entering the board joints. The fabric must be water permeable and have proven long term weather resistance. It should be strong enough to withstand traffic abuse and prevent displacement of the boards under flotation conditions. The fabric is applied unbonded over the installed insulation. Overlap all edges a minimum of 300mm (12"). If a small piece of fabric is to be used, its dimension shall be at least 2.5 m x 2.5 m (8’ x 8’). Slit fabric to fit over roof penetrations, cut out around roof drains and other openings. Extend fabric up roof perimeter cants and roof protrusions and place it loose under the metal counter flashing.
The building structure must be designed to support the weight of the ballast or surface treatment and other superimposed loads on the roof.
Membrane flashing to be extended well above the expected high water level. Refer to the RoofStar Roofing Practices Manual for details.
Materials used in the roof assembly must be listed as accepted in the RoofStar Roofing Practices Manual and conform to applicable material standards CGSB, ULL, CSA, etc.
The roof system must be designed to meet applicable building codes. This may require that 12.7 mm (½") thick gypsum board with siliconzied core and fibreglass facers or equivalent be installed on steel decks. Concrete decks do not require a separate barrier.
All roof decks should have proper drainage of the membrane. The RoofStar Guarantee Program recommends the deck have a minimum slope of 1:50 (1/4" in 12") towards the roof drains. If, however, an existing roof allows ponding, the insulation is to be applied loose with a permeable fabric over the insulation.
Roof drains are to be located at the low points in the roof. Stone ballast must be prevented from entering drains and gutters. Perforated collars and paving stones are common methods used.
|Extruded Polystyrene Insulation Thickness||Required Weight of Stone Ballast||Approximate Depth of Ballast|
|Up to 2"||12 lb./ sq. ft||1 ¾"|
|3"||17 lb./ sq. ft||2 ¼"|
|4"||22 lb./ sq. ft||3"|
|5"||27 lb./ sq. ft||3 ½"|
|6"||32 lb./ sq. ft||4 ¼"|
|7"||37 lb./ sq. ft||5"|
|8"||42 lb./ sq. ft||5 ½"|
|Up to 50 mm||60 kg / m2||40 mm|
|75 mm||84 kg / m2||60 mm|
|100 mm||108 kg / m2||75 mm|
|125 mm||132 kg / m2||90 mm|
|150 mm||156 kg / m2||105 mm|
|175 mm||180 kg / m2||125 mm|
|200 mm||204 kg / m2||140 mm|
For an engineered design approach, consult the DOW Chemical TECH SOLUTIONS 508.2 Ballast Design Guide for PMRA roof systems. The resource is provided for reference purposes only.
Resistance to Wind Loads
Roof corners subject to high winds or gusts may scour the stone ballast. Parapets and / or paving slabs can be used where necessary to prevent scouring. In the case of a lightweight PMR roof, with insulation loose applied over the membrane and covered with 50 kg / m2 (10 lb. / sq. ft.) of ballast, external pressures due to wind are mostly applied to the membrane.
The membrane will withstand the National Building Code (Canada) calculated pressure if it is properly attached to the deck. In the case of a loose applied membrane it is important to prevent any air infiltration underneath the membrane. Indeed, when air infiltration is restricted, any movement of the membrane will create a vacuum which will neutralize the uplift forces and keep the membrane on the deck.
Tunnel tests done at the National Research Council Canada show that the suction applied to the insulation boards is much lower than the calculated pressure applied to the membrane because of the rapid pressure equalization between the top and the bottom surfaces of the boards. This reduced pressure is insufficient to uplift insulation covered with 50 kg / m2 (10 lb. / sq. ft.) of ballast.