Roof Design

[rwalter]

What is the steepest practical roof pitch you can use in a design? I noticed in the 3d Archictect lessons John has on this site that there is one roof with a 18 3/4 by 12 pitch. Its the Providence roof style.


http://www.countryplans.com/3dha/

[Shelley]

Key word in your post, I think, is "pratical".

I'm gravitating toward the lowest that still looks OK....course depends upon where you live and what you need to shed, and how much headroom you need in a possible second story.

We used to always try for a 6:12.  Get much steeper than that and everything becomes more trouble...construction as well as on going.

We have a 7:12 now.  Metal roof.  Every time himself gets up there he ties a rope to the truck and throws it over the peak.  He doesn't like it at all and he's not scared of heights.  Consequently, checking the roof penetrations and checking for backed-out screws doesn't get done in a timely fashion....and I'm not about to get up there.

Surprisingly, the neighbors house, which we also built, is 6:12....and himself walked right up it the other day doing a favor for them.

Now I'm thinking that even 6:12 is too much trouble for old folks with a metal roof.  I'm playing around with various combos of sidewall/roofs trying to get a look that I like.

It all winds up to be the look you like balanced against the difficulty building and maintaining it.

How 'bout that for a non-answer?

[Frank Auer]

Your reply was great.  I really like the astetics  :-/ of a steep roof, but the next twenty years will take me into my upper 70s.  I better watch this, who wants a broken hip.  

[Bart_Cubbins]

I'm planning to have a steep-pitched roof (14 or 15 in 12) because I need someplace high to mount solar panels, and also because I too like the aesthetics.

To deal somewhat with the safety issue, one side will have a shed roof dormer with a comfortably shallow pitch. The panels as well as the chimney will be above the dormer, so the dormer roof will provide a reasonably safe place to stand while working on the panels or cleaning the chimney.  Also above the dormer will be a skylight or hatch of some sort to give easy access.

Bart

[Amanda_931]

I can't speak to taste.

But isn't the most efficient use of Solar Panels on a tracking mount?

Some friends have a non-tracking mount, and one of them goes out and moves the array whenever they think about it--three or five times a day is what they'd like to do, I'd guess it doesn't happen a whole lot.

[Bart_Cubbins]

Hi Amanda, yes a tracking mount gets more power out of your panels, but for the same cost you could just buy more panels. Trackers provide the biggest gain during summer, when the sun is tracing a very wide arc across the sky. Since I'm off-grid, I have to design my system to provide enough power in winter... which will give me more than I can use in summer.

The NREL provides a great calculator (http://rredc.nrel.gov/solar/calculators/PVWATTS/) for comparing tilt angles and trackers. Using the location closest to me, Quillayute WA, it calculated that a single-axis tracker would give at most a 12% gain in December, while a dual-axis tracker would give at most 15% extra. I say at most because the calculator assumes a clear view of the sun from sunrise to sunset. With morning fog over the water to the east and trees to the south-west, I'll be lucky to get three hours of good sun on a winter day. As a result, the panels will always be facing within 25 degrees of the winter sun. If you play with the PVWATTS calculator, you can see that having the array face 20 degrees east or west off due south barely affects the output.

A single-axis tracker costs about $1400 and a dual-axis about $1800. That would pay for another 350-450 W of panel. With a small system of perhaps 2 KW, that's about a 20% gain. And that only considers the cost of the tracker itself. I'd also need a 24 ft pole or tilt-up tower to get the height I need, some concrete, and guy wires and anchors to keep the whole thing from flying away. I'd probably be lucky to get away with less than $4000 just for parts and materials, enough to buy 50% more panels. And the bonus is, the extra panels will of course be a lot more reliable than a mechanical tracker.

Bart

[glenn kangiser]

I have heard up to 30% gain -maybe where I'm at and haven't check out the validity of this statement.

You can make a low cost tracker if you do it yourself - one of the best.  It came from an old M.E.N. magazine.  

You put a 6" high shade each east and  west edge of your panels.  Panels on a Pivot -probably group of 4 or so.  1" copper tube painted black both sides at bottom of shade, called and plumbed for propane.  Pressure line from top to opposite sides of a air or hyd. cyl with a little oil in it for lube.  
fill tubes about 1/2 with propane each.  Pressure goes up on heated side and rotates panels until other tube gets equal sun and tracks all day.  Will return in morning.  Pressure differential can reach a couple hundred lbs.
 Found the article.
http://www.motherearthnews.com/library/1977_November_December/Mother_s_Super_Simple_Solar_Tracker

Read the article to the end to see later mods.  Note that I said to use propane.  You are on your own to decide the  safety of this, but it is a good refrigerant and is a second generation (Duracool) replacement for auto air conditioners and in fact is more efficient than freon.  A retired group of GM people did a study on repairing RV ac units.  In the open air it is going to be very diluted in case of a leak and the quantity is very small  - .  The safety of Freon over propane was more of a political/profit thing years ago when Dupont invented freon and caused the corporate/law/government to set up a profit making business out of it.

[Bart_Cubbins]

Yep, PVWATTS tells me I'll get a 32% gain over the whole year by using a dual-axis tracker. But most of that is during the summer when I don't need it.

Passive trackers apparently don't perform that well in cold and windy weather. Home Power did a review about a year ago. A passive tracker is still facing west first thing in the morning, and HP said it could take an hour to get turned around. With a good cold wind cooling the cylinders, they also become sluggish and don't follow the sun very well.

I think trackers (any kind) make a whole lot of sense in cooling climates where you can use the extra summer output to run an air conditioner, and with grid-tie setups where you can use the summer output to offset winter usage.

Bart

[glenn kangiser]

Thanks for that info, Bart - now I have another excuse not to try it for a while.  Actually we're fairly warm here year round - not much under 32f for a short time usually so it may work pretty decent.