Yes, I think you need more bracing and the purlins need to be closer together so that means adding more. With 5 spaced over 26 ft that leaves about 6ft between them. I don't know the snow load rating of your greenhouse but I know that is not enough. Find out how many inches or ft of snow you get and I will find a site that rates snow load when I get a chance. I think Atlas Greenhouses or maybe Sundance might have something. Or GW's that live up north can help you.

Here's some info.

Here is a link that might be useful: snow loads

Snow load won't be an issue for a gothic arch greenhouse if it is heated...If not heated, I cannot imagine having purlins that far apart.

Hmmmm. I was thinking i was safe seeing that the GH was up and functioning locally for years, but i am not certain when they grew crops. Certainly they did for spring 6-pack market but not sure about the rest of the winter.

I bought this one specifically because i was worried about the snow load. It has a peaked roof rather than the quonset (1/2 round) style that tends to let snow accumulate. I thought the shape would be enough, but after reading that link that described sagging plastic hanging on the purlins and preventing snowslide - it's got me worried.

I wasn't planning to really use the thing this winter. Just wanted to be able to get stuff set up inside, so it won't be heated. Now i am thinking i'll put up the frame but not cover it until spring (or get my butt in gear and get it done!).

20" of wet snow is not unheard of here.

Thanks!

Poppa

Don't jump to spend a bunch of money on something you don't need. Perlins probably are not as much of a factor as the diameter of your arch poles. Regardless, you should get a portable propane heater and plan to run it during heavy snow fall. Also a broom with an extension handle duct taped onto it. Don't plan to travel in the middle of a snowstorm. My biggest problem has always been the pressure that sliding snow puts on the plastic at the sides.

If you feel you want extra perlins your cheapest bet would be to buy the 1.66" galvanized, swagged poles at Lowes and purchase cross-connectors from your nearest greenhouse supply center. The 10' poles will give you better weight distribution strength than the hammered end 4' connecting perlin pipes that come fron many greenhouse suppliers. I install extra perlins so I can position them for my tomato rows (drop string line from perlin to support plants).