Bill
I am going to share with you what I know and believe. Please use what makes sense to you and ignore the things that don't fit your needs. From one tree guy to another I want you to be successful and enjoy a successful experience.
Your growing location is a given and all decisions have to be made with that given. First thing to do is go see some greenhouses in your area and tell them what you are about to do. I promise you that 9 out of 10 of them will help you. Just don't expect them to talk to you when they have a paying customer on the place.
Location of where you place your greenhouse is important - mine gets full sun and I have no worry of anything falling on it. I have electrical and water in my greenhouse. I have some automation in it to manage heat in the summer. I like my size for me 16 wide and 26 long. I raised my grade up after a horrible 10" inch rain right before I sank anything in the ground. I love my gravel floor.
If you think you are going to build a greenhouse in Texas without water and electricity I say stop now and go no further.
When you visit the local greenhouses with 50 miles of your location, look at automation, water, electricity, location, and size. Greenhouses have the flexibility on length - most kits are such that you can add 5 feet to the determined width. The greater the width you select raises your height and structural members overhead.
When you visit the local greenhouses, ask them if they know of a used greenhouse for sale. You can find a deal if you look hard enough. I used electrical PVC because I was scared of bending my own metal.
Are you growing trees, flowers, and / or food?
Do you want benches in your greenhouse?
Visit local greenhouses and look at their structures and their covers.
I like my black 55% shade cloth for my climate. Ask the locals about their use of shade cloth and covers.
Make sure to look at greenhouse on high school campuses. They will be a great resource to see their approach and size of greenhouse.
Got a good man for you to talk to in McMinnville, TN which is touted as the Nursery Capitol of the World. Do I think they are that - probably are. Morton's Horticultural Products is a greenhouse supply company that I used. My local garden center took me over and saved me major bucks. Tony Morton can be reached at 800.473.7753.
Website is found at
www.mortonproducts.com
When you go that website look at Series H. Keep in mind many greenhouse packages are priced two different ways: structure is one price and then you have a cover price to add to that. Shipping is a major cost and erection is another cost. Being retired and able to build myself - I saved some money. I visited sites and spent many hours on YouTube looking at DIY greenhouses.
My hip board is my heat / ventilation feature that I got off YouTube from DIY greenhouse projects. Automated windows were above my financial threshold.
Now I have replied and if I didn't create any questions in your mind I would be shocked. Turkey Creek is a greenhouse man and Lakngulf is a greenhouse man. Turkey makes a living at it and Lak is a hobbyist like me.
A problem avoided is a problem solved! What you figure out now and get right now will not cost you later.
Your problem is heat load - not snow load. You might have a wind issue and you might not. Never underestimate the knowledge of the locals. They are working with their greenhouse 24/7 and know all of the issues.
My greenhouse is about 90 to 95 feet from my backdoor. Love my setup and am having great fun. I hope you do too.