ThermA Ash Gives a West Seattle Rooftop Deck a Fresh Start

When a small roof leak revealed larger problems with an existing rooftop deck, a West Seattle homeowner needed more than a replacement. The new deck had to manage water properly, stay within weight limits, withstand the Pacific Northwest climate, and require minimal maintenance.

Rooftop decks are common in the Puget Sound area. While large backyards are ideal, Seattle-area housing is more compact and dense, particularly within the city limits. For this West Seattle home, the rooftop deck offers western views across Puget Sound, with the Olympic Mountains in the distance. It's the kind of setting that makes a rooftop deck worth having, especially for the spectacular sunsets the Northwest is known for.

But beneath the old deck, a festering issue remained.

“We remodeled the deck for a couple of reasons,” said homeowner George McCarroll. "First, we discovered a small leak in the flat roof. Second, the existing deck trapped water and debris instead of allowing proper drainage."

As a result, and after further inspection, the existing deck construction was found to have been improperly installed. Without a replacement, it could lead to a more costly repair later.

A neighboring tree dropped leaves and debris onto the roof, and with nowhere for that material to go, it was trapped beneath and around the existing decking, causing additional issues.

Replacing the deck was an opportunity to solve the underlying problems—and rethink the material underfoot.

Rooftop Decking Challenges

Adjustable pedestal systems let you install a level deck surface above a sloped roof while leaving space below for water to move toward roof drains or scuppers. Open joints between the decking allow water to pass through to the drainage plane below. For the McCarrolls, the drainage capability wasn't just a benefit — it was one of the primary reasons for rebuilding the deck.

The homeowners considered a few material options, including natural wood, composite, PVC, and deck tiles. They wanted a material that could meet the rooftop's structural requirements while delivering long-term performance.

“We wanted something that would last as long as we own the house and require as little upkeep as possible,” said McCarroll.

Just as important was what happened below the decking.

The 420-square-foot deck replacement was designed as a pedestal rooftop deck, creating space beneath the boards so water can move freely across the roof surface.

“We specifically wanted a pedestal rooftop deck to improve water drainage,” said McCarroll. "With the pedestal system and 1/4-inch spacing between the deck boards, rainwater, leaves, and debris can pass through, drain, and wash away properly."

Finding the Right Material

With the pedestal deck system decided, the homeowners selected ThermA Ash from Westwood Millworks.

Thermal modification uses heat and steam to enhance wood's stability and durability without compromising the natural character. For this project, those characteristics came into play, but the homeowners were also drawn to two benefits they hadn't anticipated: a cooler surface and much less sound transmission.

"We chose thermally modified ash because it's durable, stays cooler on sunny days, and provides better sound insulation," added McCarroll.

The ThermA Ash was installed with a natural finish, and the McCarrolls then applied a coat of Cabot's Australian Timber Oil, which brought out richer, warmer tones and the grain's natural variation.

The Unexpected Benefits

The rooftop deck was completed in early July 2026, just as the Puget Sound summer was getting into full swing, so that they could use the deck for several months leading into winter.  

For a rooftop deck exposed to the strong afternoon summer sun, a cooler surface is an added benefit. Reduced sound transmission makes it ideal not only for those above the deck but also
for those below.

As you can see, the rooftop deck is simple, yet beautiful. The wide deck boards allow the wood's natural grain to stand out, while the pedestal system does the quiet work underneath.

"We'd definitely recommend Westwood Millworks ThermA Ash decking as an option for homeowners considering real wood use here in the Pacific Northwest," added McCarroll.