Calculating roof angles - is this accurate?

Hi everyone,

I just joined the forums and this is my first post! I'm designing a bioshelter for my PDC, but I want to make sure that I'm calculating the slope of my southern roof correctly.

I live in Toronto, which has a latitude of 43.7°N.

The following website gave me the precise sun altitudes at 12:00 pm, around the time of the solstices and equinoxes:
http://www.nrc-cnrc.gc.ca/eng/services/sunrise/advanced.html

Mar 21: 45.9°
Jun 21: 69.3°
Sep 21: 47.1°
Dec 21: 22.7°

I read that if I want to optimize my greenhouse for maximum sun exposure during the winter, I should calculate the sun angle by taking my latitude and adding 20 degrees. Conversely, subtracting 20 degrees from my latitude would optimize the building for maximum summer exposure. The goal is for the sun to be perpendicular to the roof, when the sun is at it's highest point in the sky, correct?

Is the above method meant to be used as an approximate guess, when you don't know the precise sun altitudes? Because I found that subtracting the sun's altitude from 90° yielded better results. Does this look correct to you?

Thanks!

If the goal is to be perfectly perpendicular to the sun, then yes, your calculations are correct. I'm not sure what the optimum for greenhouses is, although following your stated numbers of 44 degrees latitude with the 20 plus or minus values gives you 64 degrees for winter and 24 degrees for summer. Same approximate angles as your 90 degree calculation model.

I would also think about what angle it takes for the roof to be "self cleaning" so the snow will slide off in a timely matter.

I have seen some recommend setting the angle to optimize for 10 hours of sun (check here for your latitude) and let plants go dormant or die or use artificial lighting for the shortest days.

Mark, your calculations look correct but you haven't said whether you want to optimize for summer or winter. I vote for winter! There is more than enough sun in the summer. In fact, if you have lots of glass on your greenhouse, you may need to shade (and definitely vent) in summer to prevent overheating. Designing for winter also means deciding if you want to shed snow or keep it as an insulator on the roof.

Jerry