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Name: Byron B.
Status: Other
Age: 60s
Location: N/A
Country: N/A
Date: March 2004

I live in Dallas, Texas. I am constructing a pergula (patio with a wooden grid on the top). I would like to angle 1 x 6's on the top so that sunlight will come through during Jan 1 through Mar 15 but thereafter will be blocked or shaded during the summer months when the sun angle moves more directly overhead.

I assume that I can orient the boards on a true east-west line on their respective edges, but at what angle should I "tip" them to accomplish what I am trying to do? And what spacing should I use to that they effectively overlap during the "hot" months?

Your latitude is 32 degrees, and you want to accept as much winter sun as you can. At winter solstice, the sun is over a latitude of 23 degrees south, and you want to pass as much sunlight as you can, so you should orient your boards at 32+23=55 degrees from vertical (leaning south, of course). At spring equinox in Dallas, the sun will be over the equator, its rays making a 23-degree angle with your boards, and you want each board to just barely shade the board below it, so the free space between boards, s, should satisfy

s/w = tan(23 degrees) = .4245 where w is the width of the boards. For w = 5.5 in., s = 2.335 in., or roughly 2 3/8 in.

Tim Mooney
Advanced Photon Source

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