Car Dealership Auto Wash Bay Framed in Wood

I’m reviewing an addition to a car dealership. 1/2 of the space is dedicated to a wash bay. Cars come into get fixed and get a wash. Everything is framed in light frame wood (studs & trusses). To my knowledge, there may not even be interior finishes.

How do we feel about durability with this?


The weak point in a system like this is the action of water vapour. The relative humidity in the space is going to be 100% much of the time and the vapour drive to the exterior will be huge. We did one of these about 15 years ago and it has worked pretty well. We used PT plywood sheathing on the interior of the studs and trusses, then covered that with Bituthene. The Bituthene was in turn covered with a FRP panel to protect the Bituthene and shed most of the liquid water. The Bituthene will keep nearly all the water out of the wall. The exterior wall materials need to be permeable so that when water does get into the wall (and it will) it can dry to outside. Work with the ME to control the RH and make sure there’s sufficient insulation in the wall (and roof) to prevent the condensation plane from being too close to interior sheathing. Also, the sheathing needs to be plywood – not OSB.

Above is a snippet.

Should all be PWF material… and special waterproof coatings… wood is great but doesn’t do moisture very well.


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The wash bay in my Dad’s Shell station was cinder block and concrete floor. The cinder block had a coating on it that resisted water very well. I don’t know what it was. I was a teenager. From its appearance, I suspect it was oil based.

Sometimes oil based paints don’t hold up well with alkali material…