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Deck and Drains Present Roof Replacement Challenges Lightweight Insulating Concrete Offers Solution
March, 2007
By Zen Szewczyk
After performing thousands of roof condition reports and preparing the same amount of roof design drawings and specifications, it's rare to come across a situation that presents a unique problem. However, an evaluation of the roof assembly on a 30+-year-old school building uncovered some particular challenges.
The roof was the original four-ply asphalt BUR. It was not experiencing many leaks, but, due to the age of the roof, the owners thought it might be prudent to perform some preventive maintenance. They were looking for ways to extend the roof life, and were also considering a retrofit to add insulation and improve thermal resistance.
Contractors arrived on site to find the roof completely under water, so the initial investigation was limited to some wading around and interior investigation. The relatively few number of roof drains was noted.
The existing deck varied from area to area, and consisted of poured-in-place concrete, metal roof deck, wood roof deck and concrete slab. The underside was plastered ceiling. There was little water staining, and it was limited to areas around openings.
When the roof surface was dry, the evaluation was completed. It seems the continuity of the decks and a good vapour barrier had been hiding a secret for some time. Infrared scans, core-cuts and probes revealed total moisture saturation above all decks.
Roof replacement should be an easy remove-and-replace exercise, but this case was more complicated. Although drainage calculations showed that the size of the rainwater leaders was adequate to get all the water off the roof, they were too few and far apart.
The distance between drains was as much as 125 feet. The potential for adding additional drains was thwarted by an inaccessible ceiling space, firewalls,and a fire resistant mineral fibre no longer used in building construction.
Random core samples confirmed that the existing poured-in-place concrete deck was very uneven, with deviations up to two inches. Introducing rigid tapered insulation would have yielded an inconsistent surface with unsatisfactory results. Furthermore, in order to achieve success, the insulation would require a slope of at least 1/4-inch per foot so the insulation thickness would have varied from three inches at the drain to 19 inches at the high point.
Projected costs were rising. It didn't take long to conclude that the optimum solution was lightweight insulating concrete.
The existing roof was removed to the deck. After deck repairs, a new vapour barrier was mopped to the deck to act as a temporary roof.
Lightweight insulating concrete was pumped onto the roof surface where it would find its level. The stair-stepped polystyrene would be installed at a 1/16-inch per foot slope. The topcoat was installed and finished smooth. After the pour reached adequate pull strength (4-7 days), the venting sheet and base sheet were installed.
After the base sheet was installed, a 24-hour rainstorm arrived. To everyone's delight, the water was draining off the roof like never before. Even after the cap sheet was installed, the thickness of the overlaps had little effect on the ability of the roof to shed water.
The project was completed to the satisfaction of the manufacturer, contractor, consultant, and the building owner and all within budget.
Zen Szewczyk is Director of Marketing & Sales with IRC Building Sciences Group Inc., the contractor for the roof replacement project. For more information, see the web site at www.ircgroup.com.
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