I'm guessing from your photo that there are just five 8'x2" ventilation strips. That's 40/6 sq.ft., that's less than 7 sq.ft., barely enough for 7 chickens. For 15 chickens, the strips should be 4.5" high.
The fabric top of a 4-legged umbrella set up on my patio all summer a few years ago got completely shredded in a wind storm. Be prepared to replace the top periodically. A cheaper solution might be to string up a tarp.