How strong are your prevailing winds and what direction will you be running your length of your greenhouse ? And mostly is the snow a dry or wet snow,will depend on how much will stick to your greenhouse covering.

RE: Your second concern- Ventilation

Many passively” cooled” greenhouses need about one air exchange per minute to keep them somewhat closer to outside temperatures in the summer. The 20% of floor area roof vent, and 10% bottom vent area should give you the needed air exchange. What are your summer high temperatures? With your design, on days over about 80 F., it will be fairly hot in the GH. The side vents are good, and the large doors will help if you have mostly breezy days, but the 6 in. ridge vent is probably not large enough to get much air flow. Your design should allow about 4620 cubic ft. / min. of air exchange.
Manually opening and closing the side vents, doors, and windows every day, sometimes twice if it is cloudy, would take some will power and an everyday effort. I would use solar powered vent openers, i.e. Univent or Bayless or similar quality on the vents. If you could design the end wall vents to also auto-open, as well as be as large as the entire upper triangle you have on your diagram above the doors, it might just work, depending on your summer high temperatures. A solar-electric powered exhaust fan, placed high on a greenhouse end wall, would also help, especially since you have already designed a large bottom side wall vent area.

dbrya1 - thanks for the follow-up. Prevailing winds are relatively mild - I've attributed this to the location being in a clearing surrounded by forest - but it may be for other reasons as well.

Regarding orientation, lengthwise runs North / South so the end wall (door) is facing South.

Regarding load, since I am not an engineer and I am using pipe for structural purposes, its hard to find any data to estimate the strength of this structure. I am mainly going off of standards for greenhouses being sold online. Most use 1 1/4" tubing with either 4' or 5' spacing and only 3 perlins. I am hoping that going 12:12 and using cross braces (essentially making trusses) is enough to offset reducing the conduit size from 1 1/4" to 1". Wet snow will stick - I suppose for any structure, snow build-up has to be monitored. I just want to make sure I am in the right ballpark and not fooling myself.

This post was edited by mseifert on Tue, Mar 4, 14 at 2:44

cuestaroble- I very much appreciate the feedback. 90 degree weather is not that uncommon in the summer and it will sometimes get over 100. Clearly I need to take your advice and add a solar fan and auto vents.

I've spent the evening researching fans. The only two that seem worth while are the 1500 cfm 16" snap-fan (nice but expensive) and the 1000cfm Ventamatic. We are cloudy enough of the time that I wonder if I need not only the solar panel but batteries and a charge controller as well. If so, this adds up. I suppose I could start without the batteries and see how it goes.

I like the Bayless auto vent. I will design the side vents and end wall vents to auto open with them.

I suppose the good news is that I am glad to dispense with the 6 in. ridge vent and the effort (and expense) can go into the above projects.

It is the sun that heats the greenhouse inside temps to cooking highs. No sun, no extra heat. I run my fans directly from my solar panels. When the sun is out, the fans, water pumps etc run. I do have a battery for night time lighting.

Sounds to me your design and fans will work fine,but you say clearing in a forest,makes wonder just how close you might be to falling tress? I use my greenhouse only for overwintering tropical plants and palms,but I do have 2,1500 ventimatic fans on the west side of my greenhouse,which the strong daily winds turn the blades most of the time,and a 24 inch exhaust fan on the east side of the greenhouse,these are controlled by thermostats,my greenhouse is a 2ox30x12 lean too off the back of my home. I take my plants out of the greenhouse late spring,but it does get very warm until I can get them out,so I have a misting system set up to keep it fairly cool. I live west of Amarillo Texas,where it is really dry,very low dew points and humidity,but have brisk winds about everyday.I wish I could give you more advice,but we have different greenhouse needs and growing conditions,but I think your plans are good for what your needs will be.

A couple of thought:

First, having just gone thru designing and building an engineered GH, I doubt your conduit on 4' centers will hold 52#/sqft load. My place is also 15' wide. I use 14ga 2" sq steel tubing on 4' centers, and that would not meet our 48# load. I ended up needing a center support (a small I beam) running the length at the midpoint of the span, with posts to ground to take the extra load. So I doubt the smaller, thinner conduit will be able to do it without some mid span supports.

The steep slope of your roof might help (I don't recall the amount of load shedding that gives), but if you really want the structure to be able to hold 52# per sq ft, I don't think this will do it. You could look into building the "arches" into trusses (more so than you are already doing), by adding strategic cross members. That would certainly up the load they can handle but I can't say that alone would get you to 52#.

Of course, if you are willing to go out there and brush the snow off after every big storm, it may not matter.

BTW, does your 52# load include wind loading?

I am curious why you plan to run the length of the GH N-S vs E-W. Typically for maximum solar gain in fal/spring/winter one aligns them E-W.

I'd agree with the other comments about vent size. You might consider vents on the ends close to the peaks. Not entirely as good as peak roof venting, but much easier to build, and if the building is aligned with the prevailing breezes, can work OK. You might even consider removing the ends for the summer.

steve333 - thanks for posting your experience.

You've confirmed what I pretty much knew - that I would have to add the I-beam center support or something similar. It's a good idea to add crosses to enhance the "trusses," but the special connectors for pipe are expensive! The I-beam is a simpler (cheaper) solution. If it turns out that it isn't enough, then I can retrofit the crosses or, as you say, clear the roof.

For load, I used the formula RSL = 0.7 * (Ground Snow Load) * (Exposure Factor) * (Thermal Factor) * (Slope Factor). I used an Exposure Factor = 1 (partially covered or sheltered from the wind by trees). So, in one sense, you could say it doesn't really include wind loading - and I am hoping not to have both huge snow load and an unusual wind load at the same time. I don't know if this is a generally accepted formula. The was all I could find on the web. If you have any resources which could educate me better on this, please post a link.

Regarding the orientation, I have read advice that says to orient the GH N/S if not growing crops over the winter. Actually I've read advice in favor of doing it either way. In my case, N/S is also a better fit for the location. What I haven't found is any empirical data on how much it matters.

For the reasons you've stated, I've settled on the idea of self-opening vents on the ends close to the peaks. If I can afford it, I will add a 16" solar powered fan to help out. The cost of this project is quickly growing. Those are going to be expensive tomatoes.

Speaking totally subjectively here I've worked in greenhouses from Chicago to Santa Fe to Baja and now in Oklahoma over a 40 year horticulture career and I never felt that the orientation of a greenhouse (excepting north sided lean-to's) made any significant difference. I'm growing great crops now facing 3/4 east.