How do I orient a building to the south?
Tristan Roberts
| Posted in Mechanicals on
I’m installing a PV array on a shed that I am about to build. I want to orient the shed to true south to best harvest the energy. What’s a relatively simple way — not using surveying tools that I don’t have — to orient to the south? I have a compass and know about declination, and I think I’ve got it pretty close, but with all the advice out there on orienting to the sun, I am surprised that I have not been able to find a simple step-by-step method for this. Thanks. (Note: finding true south is one thing — orienting the long axis of my building at a right angle to it is another.)
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Replies
Tristan
I'm with you on this. I never trust my compass or my ability to get the declination right. I usually pull a survey and set the building on the lot on paper so it is facing due south and then draw a line representing the east-west axis through the center of the building out to the lot line and calculate a triangle based on stakes driven into the lot line and then transfer it to the site that way.
So on a recent project we came down the lot likne from the corner stake 100' and then on another 85' or so from that and pulled in to where one tape read 60' from the first stake (a piece of "designer re-bar festooned with colorful surveyors tape) and another tape read about 80'. A string pulled from this intersection to the first stake represented the east-west axis of the garage which had a 30x10 roof section for a PV array at 8/12 pitch which works for our latitude. Once I had this line I simply arranged the building footprint in reference to it as well as to avoid the trees we were trying to save and to work well with the drainage and the other existing site elements. I check square by pulling diagonals off a pre-determined reference rectangle and diagonal measurement calculated in the margins of the foundation plan.
On another project I used the foundation survey from the bank that confirms no violation of setbacks and pulled a triangle off the building corners. and strung back to one of those corners.
No transit required, the whole thing works off two 100' tapes, a ball of string, and some designer re-bars.
Tristan,
There are plenty of ways to do this. My favorite is to drive a stake in the ground, about 4 to 6 feet tall. A 2x2 works fine. Plumb the stake; whack it with a sledge if necessary until it's plumb and secure. On a sunny day, plan a picnic near your stake. A blanket, a fresh loaf of bread, a good French cheese. Mark the top of the stake's shadow in the dirt from 11:00 am to 1:00 pm. Put a smooth white rock at the shortest shadow. A straight line connecting the white rock and the base of the stake points to true south.
A recent letter to Home Power explains a different way: "Most media in most locales publish the hour and minute of sunrise and sunset. Subtract the former from the latter, divide by two, and add the result to the former to find the hour and minute of local high noon. Select the appropriate corner of your house (any vertical object that will cast a shadow at the location of interest) and, precisely at the hour and minute of high noon, mark a convenient spot up the shadow line from a comparable point on the shadow line at the bottom end. Voila—you have the line of true north."
Damn that's elegant! Bravo!
One caveat about marking the shadow from 11 am to 1 pm: Make sure you're using solar time. Depending on location and time of year, solar noon could be outside that range of clock time.
Allison,
You're right, of course. Assuming that solar noon occurs in the middle of the time zone — which is "kind of" true — noon can occur from roughly 11:30 to 12:30, plus or minus an amount of time corresponding to the inaccuracies of the time-zone lines. During the summer, when many locations have daylight savings time, this window moves later, becoming 12:30 to 1:30. So, anyone unfamiliar with the question of whether daylight savings time is in force, who needs to be super careful, should have a long picnic, from 11:00 to 2:00. But common sense will tell you whether you have seen the shortest shadow. Once the shadow begins to lengthen, your picnic is over and you're done.
Martin,
Does it have to be a smooth white rock? What happens if it's not?
Robert,
You gotta follow the rules. The picnic includes French cheese, not Wisconsin. The little pebble has to be smooth and white. If you don't pay attention, you'll never pass the quiz.
Tristan,
Since I don't like picnics, I always use a compass that's adjustable for declination. If your compass is not adjustable, then use this mnemonic: East coast is right and West coast is left.
Turn the degree ring of your compass so North (or South) is to the right of the direction-of-travel arrow on the base plate by the amount of your declination if Easterly, or to the left if Westerly.
Put a stake in the ground for one corner of your building, stand at the stake and hold the compass straight out in front of you at a little below eye level and rotate your entire body until the needle is housed in the N-S box with the red to North. Have someone else move a second stake until it's aligned with your compass arrow. This can be quite accurate. Then use two more stakes and 3-4-5 triangles to complete the rectangle.
Just make sure you're using a current declination, as it changes annually. Go to http://www.ngdc.noaa.gov/geomagmodels/struts/calcDeclination to get declination from your zip code.