“A Lightweight Solution to a Heavyweight Challenge”
By Albert Duwyn, RRC, FRCI, President of IRC Building Sciences Group Inc.

When MMC International Architects was asked to design the re-development of Yorkdale Shopping Centre, they proposed a barrel vaulted atrium of laminated glass with an overall length of 294 feet and a width of 56 feet as part of an effort to create a sense of ‘being outside’ inside a building.”

The barrel vault would have a gentle arching radius of approximately 65 feet. It would be the largest under hung (steel support arches above the glazing units) structural glazed skylight in North America. The glazing units are constructed of 10 mm clear, fully-tempered, heat-soaked glass, a 16 mm air space, and a 17 mm interior laminate. Each panel of glass is seven-feet-long by four-foot-six-inches-wide and joined with structure silicone sealant. Several sub-consultants would be called upon to lend collective experience to ensure a successful installation.

At the first consultants, IRC was presented with requirements and challenges of the skylight manufacturer. The manufacturer had concerns over the presence of any loose ballast materials typically used in conventional roof assemblies in the vicinity of the skylight because of the chance that seagulls could pick up the stones and drop them onto the glazing unit causing potential chips or fractures. The other concern was snow and ice cascading off the skylight accumulating at the base and creating additional loading of unpredictable weight. The first concern was easily dealt with by the specification of a smooth surfaced membrane however the second presented more of a challenge.

The architect designed an apron around the entire skylight that would be used to collect the frozen precipitation, melt it and allow it to enter the storm sewer. The apron would need to be durable enough to withstand periodic foot traffic in the event of glass cleaning and/or maintenance. One of the consultants proposed a pour in place concrete slab with heating tubes within, installed over the roof membrane around the entire skylight. Large diameter drains would be placed at regular intervals to collect the melted precipitation and carry to the storm sewers. The concern with that was the weight and size of the slab, and its tendency to crack and spall would cause continual problems with the heating lines, drains and waterproofing.

IRC proposed a roof assembly that would provide thermal insulation, a durable medium to carry the heating lines and a smooth surface membrane that would conduct the heat from the cables below and provide an easily cleaned, reflective surface for easy maintenance. The assembly, commonly known as NVS(2) (non-vented substrate) consisted of a vapour retarder, lightweight concrete, expanded polystyrene insulation, top pour of lightweight concrete (with the heating lines embedded 1-1/2-inches below the surface), venting sheet and two-ply modified bituminous membrane.

The lightweight concrete is an engineered mixture of water, Portland cement and vermiculite aggregate which is mixed, pumped and placed. Slotted EPS insulation is then placed into the primary pour in varying thicknesses to achieve a stair-step slope. A top pour of lightweight concrete is poured in place over the stepped insulation and floated to achieve a constant smooth slope.

After several days of curing, a mechanically-fastened venting sheet and two-ply modified bitumen membrane with an aluminum foil faced cap sheet, is installed. The R=20 assembly, weighing less than eight pounds per square foot, has excellent compressive and pull-out strength, and remains in place for the life of the structure.

At the end of its service life, only the membrane needs to be repaired or replaced leaving the balance of the assembly in place. This, coupled with a 25 year warranty, provides security and low life cycle costing. The NVS assembly is suitable for single-ply, conventional built-up and non-conventional built-up roofing. Owner preference dictated the membrane selection on this project.

The phased application assembly on this project was an added bonus to the general contractor. With the vapour barrier installed over the concrete apron acting as a temporary roof, the structure was immediately made watertight and allowed work of other trades to commence above and below the roof. Workers installed the skylight, mechanical work, welded, hammered and dropped tools and accessories on the temporary roof with little or no damage and without loss of waterproofing. The remainder of the assembly was not installed until almost all other work was completed in order to protect it from unnecessary traffic and abuse, and to ensure adequate curing times.

The project was completed in the Spring of 2005, on time, within budget and to the satisfaction of all parties. Monitoring over the past two winters has yielded positive reports on all counts. As a result of the success of this project, a similar design on a skylight twice the size is being implemented by the same architect and IRC on a project in Calgary, Alta.

For more information on this project, contact the writer at