Sorry. I can't answer questions other than construction related. The customer ordered it and I install it. School and universities all have there own ideas of what is best. However I am in the office a lot and can ask the guys that keep up with these details.

Plexi gets brittle in winter.

I built a lean to with polycarbonate panels from Griffin Greenhouse and nursery supplies.  Their catalog shows they have a place in Morgantown PA if that helps.

Here are some of the videos I posted on youtube showing the construction.

Thanks for additional suggestions. I'm going to pass on the plexiglass if it gets brittle. Unfortunately, Morgantown Pa is about 4 1/2 hrs away. Going to keep looking. Here is a picture of my progress so far. Roof still needs framed. It's not very big but it should help. Thanks..Tony

Whatever material you choose for the roof,
remember the snow load, make sure the material can handle it.

The only rule I know is build it at least 3 times bigger than you think you want  :)

If I only had the space :) I have a nice sized yard...but I'm city folk.

If you do go for the corrugated roof keep in mind they have 2 different qualities 1 has a 10 year warranty and the other has a 20 year warranty and there is a big difference between the 2 

http://www.cranecomposites.com/industries/bp/sequentia_cor.html

Thanks Dave. I actually just got a price from a local supply company for Crane (Sequentia Weatherglaze) panels. The lower end wasn't any more than Lowes. I'm going to call back and see how much the commercial grade is. The thing I'm wrestling with most is whether to go clear or white? The greenhouse has a great exposure and will get a ton of sun. So maybe white is the way to go. I don't want to bake the plants. Also, Lowes sells a Tuftex translucent white with a 78% light transmission with a limited lifetime warranty. So not sure which brand is better. Thanks..Tony

twin wall to quad wall poly carbonate is very good material, won't break as easily but costs a fortune, will cut down on heating though.

The roof of our sun room is twin wall polycarbonate, same as Tom used and Chivas mentioned. I don't know when it was installed, I have to guestimate maybe 15 years ago. It has handled huge snow load with no problems whatsoever.  In the last two years it has started to degrade and is starting to take damage from moderate sized hail. Panel replacement is on the to-do list for next spring.
In the future, if I were to build a sun room on a new house, I would use the same plan and materials. And as Chivas said, it really does well keeping the heat in.

I'm still wrestling with which roof to pick for the greenhouse.  I had an easier time picking my kids names!  I was all set on the Crane Composite corrugated panel.  Then I found out that an agricultural supply company 5 miles from me, which I never even knew was there, sells Green-Tek ThermaGlas 6ft x 10ft sheets.  That stuff looks good and has an 80% light transmission.  So I think I might go with that and if it's too much sun get a sun shade....until I change my mind again.  Thanks..Tony    

http://www.greenhousecatalog.com/greenhouse-covering

Tony I don't know anything about greenhouses except I had aluminum and glass lean to greenhouse years ago and the first thing I ordered after getting it up was shade cloth to wrap the roof the one that I just built now with the white roof doesn't seem to need any of that I have plenty of light it's always about 10 degrees hotter than the outside temp in summer I am able to keep my plants in there without them burning up in winter it stays around 40 degrees unless it drops outside to 0 or teens then it goes down to about 30's just for the day and warms up,hope some of this information helps  whatever you choose to do I wish you luck with your project. 

Thanks Dave.  Now my head is going to explode.  I guess I'm going to do some more reading.  Good info.