Protected Membrane Roofing

Building owners and tenants continue to demand commercial facilities and office space that boast green assemblies, long-term energy efficiency and preserve natural resources. These goals can also challenge budgets. For instance, specifications sometimes call for additional, premium materials to achieve sustainability goals.

According to engineers and architects, the solution to achieving energy efficiency while staying within budget is to look beyond wall and ceiling assemblies, and focus more on the roof.

History
By the mid-20th Century, commercial structures had grown large enough that they yielded roof areas that could not be effectively covered with conventional steep roofs. The water-shedding slope was gone and roofs required a new type of waterproofing. To meet this requirement, the roofing industry began to mop asphalt membrane directly to the roof deck. As heating and cooling costs rose further, energy efficiency became a priority, and designers countered energy loss by adding roof insulation.

Early insulating materials were not water-resistant, so it was traditionally placed underneath the waterproofing membrane. This system protected the insulation. However, the membrane was separated from the roof deck and subject to a new set of conditions and stresses: extreme temperatures, ultraviolet rays and abuse from foot traffic during routine maintenance. All of these can weaken the integrity of the membrane and lead to loss in waterproofing qualities of the membrane and its thermal efficiency.

Today, fuel costs are higher than ever, and the roof-to-wall ratios of today's commercial designs are quickly making the roof one of the most important building components for thermal efficiency.

Protected Membrane Roofing
Insulation technology has evolved over the decades, and some types, like rigid extruded polystyrene foam (XPS) is highly water resistant. This insulation can be placed above the roof membrane, protecting the membrane from elements while continuing to deliver thermal efficiency.
This approach is commonly referred to as a Insulated Roof Membrane Assembly (IRMA) or Protected Membrane Roof (PMR). By simply reversing the insulation and the waterproofing membrane – insulation on top of the membrane rather than beneath – the cause of many roofing problems can be eliminated.
The membrane's temperature range and rate of temperature change are also drastically reduced. The insulation protects the membrane, versus requiring the membrane to protect the insulation. Ballast is placed on top of the insulation, offering further protection of the membrane from harsh elements. It also expands the design options available

Advantages
PMR assemblies require additional insulation and ballast. However, they boast long-term cost savings, efficiency during construction, greater design flexibility and remarkable energy efficiency over the life of the roof.
Cost Savings: In a traditional assembly, membranes can be easily damaged by construction traffic or from maintenance servicing rooftop HVAC units, or natural elements. PMR assemblies require more materials; however, because the insulation and ballast layer lay on top of the membrane, they extend the life of the roof, yielding fewer tearoffs and less patching, cleaning and rejuvenating.

Efficient Construction: In a PMR assembly, the insulation is laid after the membrane and can be handled in any weather conditions. The membrane can also be inspected or flood-tested before insulating and ballasting. This process allows a building to be weatherproofed sooner, allowing interior trades to start work more quickly and accelerate the entire construction process. A PMR can help reduce downstream waste during maintenance and demolition phases as well, because the insulation, fabric and ballast are loose-laid and thus are often able to be reused. This reduces landfill costs as well as environmental costs.

Design flexibility: One of the advantages of using a protected membrane roof assembly is that once the membrane is installed, owners and designers can take advantage of various ballast options (e.g. gravel, pavers, and even vegetated roofs) to suit a specific project's needs. In fact, since a membrane is keeping the structure watertight and monolithically applied directly to the deck, design professionals can combine ballast/assemblies with confidence knowing that drainage will occur at the deck level regardless of the transition between assemblies.

Stone ballast typically requires crushed, graded stone to be placed over a stone filter sheet which has been rolled out over the insulation. In the field of the roof, a minimum of 10 lbs. per sq. ft. of ballast is required, with 20 lbs./sq. ft. needed at perimeters and at large penetrations.   The extra weight protects against wind uplift.

Pavers are ideal for installations where greater wind resistance is required (at building corners, perimeters or the whole roof) or when roof top traffic requires a more durable and walkable surface.  Numerous paver sizes, colors and finishes allow for great design flexibility and can turn a roof into an amenity for tenants. 
Vegetated roofs allow PMRs to reduce the urban heat island effect as well as stormwater peaks. Landscape architects are also sourcing vegetation grown specifically for local climates to deliver aesthetic appeal to any commercial roof.

Dennis Yanez is the national marketing manager at American Hydrotech. He can be contacted through the company website www.hydrotechusa.com.

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