Protected Membrane Roofs

May 6, 2004
Not All Roofs Are Created the Same
As long as there have been buildings, there have been roofs – and a whole range of different types of roofing assemblies. In institutional, commercial, and industrial (ICI) construction, typically two different types of roofing assemblies are specified: Protected Membrane Roofs (PMRs) and conventional roofs. In a Protected Membrane Roof (or “upside-down” roof), a closed-cell, extruded polystyrene insulation is placed on top of the waterproofing membrane; in a conventional roof, the insulation is placed under the waterproofing membrane. The placement of the insulation relative to the membrane affects the membrane’s performance. For example, in a PMR, by protecting the membrane with an insulation that can withstand traffic and a moist environment, the membrane’s effectiveness is improved, damage from repeated freeze/thaw cycling is avoided, and ultra-violet degradation is reduced. Conversely, in a conventional roof, the exposed membrane is vulnerable to physical abuse, temperature extremes, ultraviolet degradation, and damaging freeze-thaw cycling. Exposure to all these elements may compromise the integrity of the membrane – and once the underlying insulation becomes wet, thermal performance is reduced. Building a PMR roof from the bottom up.Starting from the deck upward, the roof deck must be properly sloped. The roofing industry generally recognizes a minimum slope-to-drain of 1:50 (1/4 inch in 12 inches) toward the roof drains for bituminous membranes. Over the deck, a membrane is installed. Depending on applicable codes, a thermal barrier may be required over steel decks; concrete decks do not require a separate thermal barrier. Protecting the membrane, a durable, closed-cell, water-resistant insulation, such as Dow Chemical’s STYROFOAM™ extruded polystyrene, is laid in a staggered fashion without adhering it to the membrane. A water-permeable fabric can be placed over top of the loose-laid insulation, depending on the design requirements. Using a fabric reduces the amount of stone ballast required and prevents small fines in the stone ballast from entering between the board joints. Finally, the stone ballast is laid to prevent wind displacement and to provide a Class A fire-resistance rating. When using an approved fabric, regardless of the insulation thickness, 10 lb./sq. ft. of ballast should be installed, with additional ballast around the perimeters and around drains. Designers can also specify pavers, plaza decks, and even sod and other green roof applications. Looking for long-term performance?Today, building owners and designers are more focused on the life-cycle cost of the roofing assembly – a “cradle-to-grave” approach. Remember that the initial up-front construction costs account for just five to 10 percent of the building’s cost over its service lifetime; operating and maintenance costs account for 60 to 80 percent. Using a PMR can reduce annual maintenance and utility costs and increase the life of the roof. Just deferring a roof replacement by 10 years – which is often the incremental difference between a PMR and conventional roof life – can save hundreds of thousands of dollars. In addition, using a PMR results in reduced maintenance costs because the membrane is protected from physical and environmental abuse. This adds to the bottom line. Throw in the reusability of the insulation and ballast when doing an expansion or re-roof, and a PMR makes the life-cycle costs very attractive.Specify PMR with extruded polystyrene insulation in your next roof – a proven roofing solution.Cecile Mutton is a professional engineer and technical writer based in Toronto, ON, Canada.

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