Design and Build Your Own GH?
 

I am trying to design and build a 12x12 greenhouse with a pressure treated wood frame and twin or triple wall polycarbonate glazing. I would greatly appreciate any knowhow from someone who has real experience.

Hello HD,
I have built a very inexpensive greenhouse using what materials I could find and afford at the time. My materials were: corrugated fiberglass panels, 2x2s, plastic sheeting, home insulation, and 50% ? saran cloth.

Also from 1968-1971 I worked in a commercial orchid farm and was the primary grower of all the non-cymbidium orchids and am familiar with that set up at that time.

On advising you there are specific issues you need to address before even considering construction:
1. Foundation and flooring
2. Orientation
3. Partitioning with designated growing
zones.
4. Height
5. Lower wall: glazed or covered
6. Benching: orientation, height, sub-bench areas
7. Included potting/storage area
8. Exterior environment
9. Blue print of greenhouse

I think that covers the basic considerations. OK. What comments do you have in these regards.

My Plans
 

I've answered almost all of those questions. My main concerns now are three:

1. Warping of pressure treated studs and rafters leading to leaks around polycarbonate panels. Are kiln-dried after treating ACQ 2x4's enough better with respect to warping to warrant the extra cost? Are there other approaches to minimizing the warping?

2. Door and Window Vents I'd like to have a door as wide as possible between 4x4 posts on 4' centers; what is the cheapest mostly glass or polycarbonate door that will be weather tight and durable? Likewise, I'd like to put two roof vents between 2x4 rafters on 4' centers; what is the cheapest glass or polycarbonate awning window that will be weather tight and durable? Can a good carpenter make these with polycarbonate glazing?

3. Drawings or Blueprints I have made pretty good scale drawings (gable ends, side elevation, and plan) of the framing. Can I expect a contractor to be able to use these or do I need blueprints with all measurements specified? What is an inexpensive way to have blueprints made from my drawings?

Have you considered synthetic or composite 2x4s? For instance Trex or equiv.?

#3. On the Blueprints. Many copy services have blueprinting services available. These may be less expensive than a commercial service.

#1: will have to check this out on the net.
#2: ditto. Have you considered a double venting system to bolster weather tightness?

HD,
Here is a site I found regarding warping/shrinkage. I think it answers your question.
click here> UFP: Products: ProWood ACQ Pressure-Treated Lumber: Dry

OK. On the vents. I think your best bet is to purchase a pre-built.
Here is one site that show what is available:Greenhouse Exhaust Fans, Circulation Fans, Shutters, and other ventilation supplies from ACF Greenhouses
But because most heat loss occurs through the vents and the door I would frame the vent so that you can mount a internal vent cover that will completely seal off the vent "shaft" from the inside.

The door can be built to suit your needs.
I would go with the carbonate instead of glass. Also what is important is how the door is framed and the placement of insulating foam so that the contact of the door to the mantle is up against the foam.
My preference is to have a door close up against the foam rather than upon the foam as this latter way will degrade the structure of the insulation and render it useless in time. Think L <--- |door where L is the mantle the door closes into and where foam is placed there on L <--.
This would mean hinging would be inset and the door opens outward.

One thing to look at is that treated lumber leaches fumes and contact with the treated lumber or water that has made contact with the lumber can cause damage to if not even kill orchids

I don't know that the ACQ lumber would leach enough of anything to kill orchids... Possible though. I would go ahead and paint the entire interior (before any plants go in) with a high quality white exterior paint (alkyd or oil based), and go ahead and add a generous amount of the anti-mildew additive (you can buy it in little packets).

If you want to over-engineer (I usually do), to prevent warping you can double a 2x4. I use lag bolts (or lag screws, I suppose). It is cheaper than engineered lumber (or at least easier to get!). Heavy though. I used to use doubled 2x4s for building plant stands (and dorm room lofts), and you could have landed a plane on them.

You might want to consider looking around for metal greenhouse parts (hoops and purlins). They aren't that expensive, really, and a lot easier to use. I know that lots of people are giving away old greenhouses, look around. If you were near me, I'd be able to hook you up in a heartbeat.