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When Bay Area Rapid Transit’s (BART) 8.7-mile extension from Colma to Millbrae and into the new international terminal at San Francisco International Airport opens, the Millbrae Intermodal Station could be a glowing addition.


The tensioned-membrane roof’s translucent material is designed to allow natural light in by day, and internal lighting out by night.


"It’s a beautiful roof and acts like a giant skylight," says Molly McArthur, BART manager of communications and government relations for the SFO Airport Extension. "It will bathe the entire concourse area."


Manufactured by Birdair Inc., the fabric is made of woven fiberglass coated with polytetrafluoroethylene, more commonly known as Teflon. Workers this week began unrolling the varying-width strips of the fabric onto steel trusses. A rope has been sewn into the edges, much like sail material. The strips are overlapped 2 - 3 inches, then heat-welded together. After the section pieces are connected, workers stretch the material with mechanical winches, then clamp the rope-encased edge to the steel pavilion-style frame.


The material would be left for several days, then stretched again. The process would continue until the membrane reached its desired level of tension — about two months.


After installation is complete, maintenance would consist mainly of cleaning the gutters with a high-pressure hose and possibly a non-abrasive soap. The membrane is designed to be resistant to ultra-violet rays and tearing.


"The fabric roof should last at least 25 years, if not longer," says Tony Hitchings, BART principal engineer.


And would move with the building itself, in case of an earthquake.


A similar installation on a Burlingame, Calif., Hyatt hotel was in place in 1989’s Loma Prieta earthquake. While the hotel was damaged, the fabric was not, says John Douds, Birdair senior manager of business development.


The material initially would be light tan, and is designed to fade to a light white — which sunlight itself would maintain.


The station’s trusswork is colonial yellow to complement the white roof, says Hitchings, and to try to recreate the look and feel of an older train station.


The $1.5 billion BART-SFO Extension Project is scheduled to open by year-end. The Millbrae Intermodal Station’s design includes five sets of tracks: three serving BART, and two serving Caltrain, San Francisco Bay-area’s commuter rail system.


— Kathi Kube