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Raising the roof at Wanapum Dam

What has five layers and is larger than a football field – including end zones? The new main and side roofs on the Wanapum Dam powerhouse.

Work has already begun on the $1.9 million project to replace the roof, which was last replaced in 1986 and was, by all accounts, well-overdue for a rebuild. Work should be finished by late November.

A crew from contractor Axiom Divison 7 of Everett is working now on the powerhouse's smaller "vent house" roofs. Replacement of the 70,000-square-foot main deck is expected to begin next week.

The old roof will be stripped down to its metal sheeting base layer and rebuilt from scratch, five layers in all. The first layer is a drywall-like material called Densdeck, followed by a layer of torch-applied tar paper membrane, 1-to-8 inches of rigid insulation, another layer of Densdeck and a top layer of a rubber-like roofing membrane.

The new roof will be better insulated than the old and rated for 20 years and winds of up to 120 mph. The dam area can experience winds of more than 100 mph, project manager Dave Dempsey says.

The company last spring installed a tough sheet of flexible plastic-like material on the underside of the roof, inside the powerhouse, similar to a "ceiling." Since the roof's metal base will not be removed, no danger exists from old roofing material falling onto coworkers and generators inside the powerhouse, Dave said.

— Christine Pratt, Public Affairs