Template:All Systems - Protected and Modified Protected Membranes
Template:All Systems - Protected and Modified Protected Membranes
1 General
In a Protected Membrane Roof Assembly (PMRA) 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.
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.
2 Roof Decks and Drainage
All roof decks should have proper drainage of the membrane. The RoofStar Guarantee Program recommends the deck have a minimum slope of 1:50 (¼" 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.
3 Membranes
Membrane flashings to be extended well above the expected high water level. Refer to the Perimeter Flashing: Membranes section in these Standards for details.
Materials used in the roof assembly must be listed as accepted in the Accepted Materials section of this 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.
4 Insulation & Filter Mats
Insulation, which is bonded or mopped directly to the roof membrane, is not acceptable.
On all protected and modified protected membrane roofing projects, a filter mat of approved material is to be properly installed between gravel ballast and insulation. The filter mat is required under pre-cast pavers that are used as ballast or walkways.
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 flashings.
5 Ballast
The building structure must be designed to support the weight of the ballast or surface treatment and other superimposed loads on the roof.
Concrete ballast, poured-in-place concrete or concrete topping as ballast or traffic topping is not permitted on RoofStar-guaranteed roofs. Only ballast, which permits easy or reasonable access to the membrane, is acceptable.
Acceptable ballast includes: properly sized gravel, concrete pavers, concrete-topped insulation, etc.
- Gravel Ballast: Protected Membrane and Ballasted Membrane Roof Assemblies (EPDM, TPO, PVC, etc.) shall use clean, washed, round or crushed stone, falling within the following gradations:
- 35 mm (1 ½") - 100 % Passing 25 mm (1") 70 - 100 % Passing 20 mm (¾") 5 - 20 % Passing 12.5 mm (½") 0 - 6 % Passing 5 mm (3/16") 0 - 2 % Passing
- Any variance to the above must be accepted by the owner or the owner's representative and be confirmed in writing.
- Pedestals: Installation of proprietary (purpose-made) pedestals, with integral spacer ribs for uniform spacing of pavers, or an accepted drainage layer designed to provide vertical separation between pre-cast pavers and substrate or other underlying materials, is mandatory. The use of proprietary drainage boards or pea gravel with filter fabric to provide drainage for unit masonry products (bricks, stone, etc.), is required.
Proprietary (purpose-made) pedestals must have a 3mm (⅛”) integral spacer ribs for uniform spacing between pavers that provide a minimum 12.5 mm (½″) of vertical separation layer for airflow and leveling. The choice of pedestal or drainage layer type is the responsibility of the design authority. Pedestals / drainage layers provide airflow for drying surfaces and assist in leveling. They should not impede the flow of water or air, and should uniformly distribute the dead load of pavers, and other unit masonry products, as well as predicted live loads.
| Extruded Polystyrene Insulation Thickness | Required Weight of Stone Ballast | Approximate Depth of Ballast |
|---|---|---|
| Standard Measurements | ||
| 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" |
| Metric Measurements | ||
| 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.