Just my opinion but trying to span a 30' wide greenhouse with 2 x 4's would be a big mistake. First, the weight load of snow would snap them. Second, the length of the rafters would be huge to get a steep enough pitch. And third, you would have to have a massive amount of verticle supports inside the greenhouse. I would suggest you contact someone with experience in building greenhouses in your zone before doing anything further. Just trying to be helpful pass.

This post was edited by dirtguy50 on Sat, Oct 25, 14 at 23:03

I appreciate the opinion. That is a good point but outside of what you pointed out would 2x4s be a bad idea. I have read that the steel tubing allows more light into the greenhouse but 2x4s wouldn't block to much more I wouldn't think. Any thoughts on this or something else I might be missing. Thanks.

2x4 would be reasonable in a narrow greenhouse but not one 30 ft wide. Think more like 10-12ft wide max depending on snow load and pitch. You need a narrow steeply pitch roof in high snow areas.

Aside from that wood warps and rots in a greenhouse. That makes it hard to cover if dimensions keep changing. Steel will last longer. If this might be a short term project or a learning experience wood might be a good idea.

The difference in light won't be as important as the other factors IMO. Also if only for a couple weeks on each end you'll have a hard time ever recouping your investment.

You could consider uncovering in winter so as to avoid the majority of snow load.

I'm building a 20x100 greenhouse next year for my family. My plan is to use chain link top rail and posts in an a-frame shape, connected with 1.5" emt conduit elbows. I cut a 90 degree elbow in half to make two 45s. Tek screws hold it together.

I've designed and built 8 different greenhouses with 2 x 4's ranging from 360 sq.ft. to 1600 sq. ft. My two connected houses now are about 600 sq. ft. My longest rafter span is 12' blocked, but with no center support. Had a 6" snowfall last year that collapsed a friends 35' xx72' steel frame house. I like the snow it insulates my roof. this is in Oklahoma...in Ontario you'd want to spend a lot of time on this issue.
By the way you can make 20' the width and 30' the length Frame it up with everything two feet on center, paint it...Greenhouses do not usual need heavy foundations, I use concrete piers. Orientation toward the south is very overrated by non-growers. My house faces east and I am thrilled with it.

I am finishing up a 12x50 hoop house using 20' rebar for the hoops...just an idea. They rest on a center beam and supported by posts for snow load. We'll see how it holds the snow this year. I like the way it turned out though and it was under $3 a sq ft.

I was wondering if you would have any pictures you could post of your greenhouse sand mueller? I know snow load is a concern for any greenhouse where heavy snowfall comes but I would think that as long as there is enough pitch it's all going to just slide off seeing as it's just slippery plastic on the roof. Is that not the case in your experiences? Thanks.

We have made several high tunnels using PVC and chainlink fencerail.
{{gwi:12743}}

With snowload, I don't take chances. I sweep them off during and after a storm (if the storm warrants it). In my location, if the wind blows, most of the time the snow slides off and doesn't accumulate on the top. There are a few exceptions. 30-45 minutes once or twice at night is well worth it compared to the alternative.

{{gwi:305981}}
Here is a link to some of my plans and other plans I have seen used in person.

Jay

Here is a link that might be useful: High tunnel plans

Here it is last winter

Here is mine...finished.