Carport Conversion...

I need a place to store stuff at my grow lot.
I could build a shed,  but I'm planning on having a greenhouse there anyway.
After pricing materials and such,  I've settled on buying a Harbor Freight carport.
10'x20', $94.00 before tax.
Untill the fall I'll use the top that comes with it.
At that point I'll replace it with clear poly.

I'm planning on two modifications to the basic frame:
-Snow  fencing under the roof plastic to keep water from pooling.
-Pallet stemwalls.

What I want inside:
-Twin raised beds down each side, 4' H x 3' W x 20'(?).
  - Perforated black drain pipe in the base and a solar
    powered fan for air-to-soil heat storage.
  - Hot water loops for introducing wood or solar heat.

I'm looking for other recommendations,warnings and suggestions, especially for insulation, foundation or how to implement a double layer of poly.

The failure mode of the several cheap carport shelters and greenhouses that I have seen fail under snow load has been fairly consistent: the walls blow out.

Reinforcement in a rafter-tie sort of arrangment above head height seems to be quite effective at strengthening the structure against this failure mode.

Gee..that plan's missing a lateral support.

My FIL has a carport like this and after I ziptied snow fencing under the (replacement) roof tarps , the snow tended to slough off.
That said,  I like the idea of reinforcement.
2x4s or even 1x4s on edge,  secured to the tubing could work, or maybe I could use conduit or even steel cables, given the direction of the forces.

That diagram is very illustrative of my own reasoning, though I imagined fewer and   longer pieces of lumber.
I tend to use deck screws to put together wooden structures.
I know not to rely heavily on the shear strength of fasteners but I'm not a skilled enough carpenter to match the strength of steel welds.


I'm leaning towards decoupling the beds and infact the entire greenhouse from the ground beneath and around it.
Digging down and insulating is out of the question,  there are way too many rocks in my soil.
200 square feet of water resistant insulation is kind of pricey so I'm thinking of alternatives.
To begin with I think plastic sheeting to keep out rising damp.
Maybe spread it out beyond the edges of the greenhouse in a bid to create an umbrella of dry(er) earth effect.
I considered creating an air gap but I'm not sure it would take any less money than conventional insulation, plus it creates space for pests.
Cardboard treated with boric acid or 20 mule borox would be insulative and pest resistant.

I used harbor freight wire rope as a bottom chord to reinforce the rafter tubes.  It didn't blow out, but it almost caved in from using them as hanging basket holders.  

I have a ShelterLogic high tunnel. It measures around 12 x 24.  I have had it for at least 3 years.  I live in the southern tip of Illinois. That leaves me in the Ohio River Valley and pretty much in the same climate belt as you.  I have never had a snow load problem.  I did make a serious effort to anchor the darned thing.  I am in the process of closing the N end with plywood and installing a storm door for access.