Retrofit on a Tight Timeline: Budget Bonuses with Lightweight Roof System
By Mirza Hodzic
Tight deadlines and seasonal challenges factored into roofing decisions last fall as Hudson’s Bay Company aimed to open a new Zellers store at the Place Portobello mall in Brossard, Quebec during the holiday shopping season. Timely installation was important, but so too were quality and cost effectiveness.
Company officials opted to retrofit the existing 128,650-square-foot roof with a non-penetrating mechanically attached EPDM (ethylene-propylene-diene monomer) system.
“Because the roofing work couldn’t begin until mid-October and we wanted to open the store in early December, we were dealing with an extremely short time frame,” says John Watson, Director of Construction and Store Planning for Hudson’s Bay Company. “Plus, we had the added challenge of coordinating schedules around wet weather conditions at that time of year.”
The existing roof on the 1960s-era building was a multi-ply tar and gravel built-up roofing (BUR) system that was about 15 years old. Although it was damaged and leaking in several areas, a thorough review and analysis revealed that the majority of the underlying polyiso insulation and metal decking was in excellent condition, but, at the time, market prices for oil-based products would have made asphalt repair work cost prohibitive.
The project’s general contractor assessed the situation, met with several sub-trades and recommended peeling the BUR system down to the insulation layer and retrofitting the roof assembly with a new membrane. This would provide a better long-term roofing solution over repairs or a time-consuming complete tear-off of the existing system.
SPEEDY, SAFE APPLICATION
Decision makers with Hudson’s Bay Company were presented with three options:
1) installing a similar BUR system;
2) a new thermoplastic polyolefin (TPO) membrane; or
3) an EPDM reinforced mechanically attached (R.M.A.) roofing system. However, the advisors did voice their preference for the chosen RMA option, which would be attached with seam tape under the EPDM membrane to avoid fasteners penetrating the membrane – an approach that minimizes the risk of leaks and provides uninterrupted weatherable thickness.
Company officials and their roofing consultants also weighed how installation of the roofing systems would affect tenants and shoppers in the adjoining Place Portobello mall. A BUR retrofit would have created asphalt fumes and odours and possible safety implications since the hot kettle would have to come into proximity with 2,000 feet of gas lines on top of the roof.
Although EPDM roofing systems are not new to the commercial roofing industry, they continue to gain popularity for new construction and retrofit low-slope roofing applications. Performance attributes include: cyclical membrane fatigue resistance; high resistance to ozone, weathering and abrasion; flexibility in low temperatures; and thermal shock durability.
“We felt the mechanically attached EPDM system offered the best solution, because of its light weight and ability to be installed quickly,” says Ross Commodari, project manager and engineer, with the general contractor, Montreal-based CAL Construction. “We were looking for the most cost-effective way to get a quality roofing system without imposing additional weight on the structure.”
Pete Jefferson, Hudson’s Bay Company’s construction manager for the Zellers project concurs. “We looked at all three options and decided that the EPDM mechanically attached system was not only the most cost effective option, but it also gave us the best opportunity to meet the time challenges involved with getting the store open prior to the holiday shopping season,” he recalls.
COST & LABOUR SAVINGS
To begin, the roofing contractor removed the existing BUR down to the polyiso insulation, replaced damaged or wet insulation boards and then attached half-inch-thick fibreboard over the insulation and into the metal deck for additional protection. Next, 10-inch R.M.A. strips were fastened atop the fibreboard with seam plates.
With the strips in place, the roofing crew positioned 20- and 30-foot-wide by 200-foot-long sheets of 45-mil-EPDM membrane in place and then primed the back of the EPDM membrane and between the tapes on the R.M.A. strip. Large EPDM membrane panels were installed over the strips.
“Installation ease was very important on this project. This system gave us better flexibility to work around the wet weather conditions during the fall,” Commodari notes. The large EPDM panels allow for more rapid installation and also require fewer field seams, which reduces emissions of volatile organic compounds (VOCs).
Labour costs were also about 10 to 15% lower than for a BUR or TPO project of comparable size. “We didn’t have to remove the rooftop mechanical units and vents. The roofing crew was able to install the membrane under or around rooftop units, which eliminated the need for additional carpentry work, sped up the process and saved considerable money. The EPDM membrane was installed over the existing parapets, saving the cost of installing new metal counter flashings,” Commodari explains.
As planned, the new Zellers store opened on December 4, 2008.
“Everyone worked together to develop a good plan for this project and helped make it a success from start to finish,” Jefferson says. “We’re very happy with the outcome and are confident that we made the right choice.”
Mirza Hodzic is Marketing and Communications Coordinator with Firestone Building Products, the manufacturer of the Firestone RubberGardÔ EPDM Reinforced Mechanically Attached (R.M.A.) Roofing System used in the Zellers store roofing retrofit at Place Portobello. For more information, see the web site at www.firestonebpco.ca.