Rooftop Remedy Gets to the Heart of Attic Mould Problem

Posted On December 10, 2015

by Peter Kenter from the Daily Commercial News

The roof was peeled off this townhouse complex in Mississauga, Ont. to access moulded rafters and joists.

Blasting equipment was used to shave the wood surfaces, exposing clean timber and a HEPAvac system was used to contain the particles for transfer.

When mould infestation reared its ugly head in a townhouse complex in Mississauga, Ont. solving the problem required the direct approach — popping the roofs on every one of the 69 units diagnosed with the problem.

The contract for the rent-controlled units operated by the Region of Peel involved everything from roof and insulation removal, dry ice blasting of all exposed wooden roofing members, replacement of insulation and reconstruction of the roof.

The general contractor on the project was Triumph Roofing, with Wickens Dry Ice Blasting Inc. taking on the remediation sub-contracting role. Stephenson Engineering Ltd. designed the overall project, while Safetech Environmental Limited designed the remediation protocol.

"In the case of this townhouse complex, the plywood sheathing was in particularly bad shape due to exposure to moisture and repeated wet-dry cycles," says Simon Brown, vice-president of contracting with Wickens. "We worked with Safetech to determine that the best approach would simply be to peel everything off the roofing members to allow us the greatest access to the rafters and joists that had been exposed to mould."

Dry ice is formed from liquid carbon dioxide, which is a byproduct of other chemical processes.

"Dry ice blasting gives CO2 a second useful life and adds nothing to the carbon footprint," says Brown.

It's formed into dry ice pellets about the size of a grain of rice at -79 degrees Celsius. The pellets are then transported in insulated plastic drums that contain up to 250 kilograms of the material.

Remediation technicians use a high-volume compressor to force the pellets through a nozzle and eject them at high pressure. However, unlike traditional abrasive media, CO2 pellets provide an extra bang for the buck. After striking the surface, they sublimate — transform from a solid directly to a gas — with explosive force, lifting away contaminants in the process and leaving no secondary waste.

Work on the project was occasionally delayed due to rain, but began in mid-August with a project closing date of Oct. 3.

"We tried to get into a rhythm of completing two to three units per day," says Brown. "Triumph would expose the roof members and we would go inside and blast the joists and timbers."

Standard personal protective equipment used on the job included a Tyvek suit, blue nitrile surgical gloves, steel-toed and shanked roofing boots, hearing protection and full face masks offering P100 particulate filters.

"The attics were separated from the living quarters downstairs and it was determined that it would be safest to proceed without containing the interiors of the units and forcing people to move out of their homes during the project," says Brown. "Mould generally exists very close to the surface of the wood. We went in with the blasting equipment and took off a fraction of an inch of the wood surface exposing clean timber. We then went in with a HEPAvac system and removed the particles which were double-wrapped and shipped to landfill."

Safetech performed a "tape lift" on the exposed wood to determine that remediation was successful.

"It's an effective, high-detail test that involves applying adhesive tape to the wood surface at some random location that's been remediated," says Brown. "You place the tape on a microscope and count up the remaining spores to make sure that the concentration is lower than you would expect in the ambient atmosphere."

Once the wood is cleared of spores, it's treated using an anti-microbial wipe and the surface is coated with a clear encapsulant.

"It took us a day or two to find our rhythm," says Brown. "We brought in six people and eventually we were mixing and matching nicely as the roofing contractor removed the roof and insulation and we moved in to blast both units, then followed up with the HEPAvac team. Eventually roofs were going back on at the beginning of the line as they were coming off down the line. We just kept getting better and faster and brought in the job on time."