Shade House Construction

After getting a load of the prices for high tunnel type frame kits, I decided to go with the design principles used at a number of university ag demonstrations. I settled on 18 by 40 feet as being a manageable size with plenty of headroom and sufficiently vertical sides as to not lose the use of the space nearest the walls. Eleven ribs, each thirty feet long, works fine. Throughout this, remember that you can scale this idea to any size, from a small backyard installation to a 300 foot long high tunnel.

Briefly, the structure is eleven 30-foot long 1-1/2 inch PVC ribs, with a 40-foot center brace of the same pipe material. The rib arc is maintained by rebar stakes, over which the ends of the pipe ribs slide. That’s your first task, to locate the stakes. I used fairly heavy rebar, 3/8 to 1/2 inch diameter. I picked up surplus rebar from a metal recycler, but frugalistas might find it for free wherever concrete is being poured. You need pieces of very roughly two feet, which is probably scrap to the concrete folks. You could also use 1/2 inch iron pipe scraps or any other substantial metal or plastic that would fit inside the PCV.

Begin by laying out 11 stake positions in a straight line. I just laid out a 50 foot tape and drove the stakes in every four feet for forty feet. But think first about how you want the house oriented. Mine is mostly a shade house, so I oriented it close to north-south, keeping the sun passing over the arch. For a greenhouse application, north-south also often works, because winds tend to flow north and south, and the house can be efficiently cooled by opening the ends. I have come to suspect the direction of very strong winds doesn’t make much difference. If very heavy winds are frequent, consider increasing the number of ribs and braces for added strength.

 

Once the stakes are in place for one side, use simple geometry to locate the other side squarely. It is easiest to locate the center (number 6) stake by marking off a distance of 18 feet from the opposite center stake. Then shift the stake position back and forth parallel to the first side, so that it ends up equally distant from the end stakes of the first side. Once that center is driven in, the end stakes of the second side are simply 18 feet from the opposite ends and 20 feet from the center. The others go in every four feet along that line. It isn’t super critical, but put some effort into being close to square.

A word on the stakes and depth. I used roughly cut stakes that ranged from 18 inches long to two feet and drove each in about half way. In my soil, there was no chance that the modest outward pressure of the ribs would dislodge them. In very powdery soil, you might need to go deeper. But always leave at least 8 inches above ground, and more is better. You need not worry about making the stakes lean inward.

The ribs are made from 20-foot sections of belled end 1-1/2 inch Schedule 40 PVC pipe. If you don’t have pipe with belled ends, use PVC joints to join them. I cut six of the 20-foot pieces to make enough 10-foot pieces to extend the 20’s to the needed 30 feet. You could as well use three tens to make up the 30 feet. Glue them with PCV cement. But no matter how you make up the 30-foot ribs, mark the centers before you put them up. You will need to know the 15-foot point later. 

Up to this point, it was a one-person job. Now you need a team of two people to install the ribs. Each person takes an end and forces the rib up into an arch until each person fits their end over the appropriate stake. The rib will flop a bit, but it will stand on its own while you do the rest.

You now need to install a brace to maintain the four-foot distance between ribs. I used a 40-foot piece of the same PVC pipe. I predrilled a hole every four feet, eleven holes for eleven ribs. This brace will be attached to the underside of the ribs along the center line. I chose 3-1/2 inch ¼-20 carriage bolts for that job. Carriage bolts have a rounded head that will not tear the shade material. 3-1/2 inches allows it to pass through both 1-1/2 inch pipes with enough left for a nut. I chose wingnuts for ease of installation and added a washer under each wingnut.

You will need a drill bit long enough to pass through both pipes and slightly larger than the bolt. In my case, for 1/4 inch bolts, a 3/8 inch bit worked. You want some slack, because you’ll will be working over your head on a ladder. You may be tempted to drill the rib centers before putting them up. But, unless you are very lucky, there is little chance you will get the holes in both walls of both pipes to line up. You can see the center marked around the rib in the photo.

My approach was to attach the center of the brace to the center of the center (number 6) rib first. By attaching to number 6 first, I was lifting the center of the brace into place with it balances end to end and not straining to keep it level from one end. Hold the predrilled brace hole under the center mark of the rib and drill through the existing hole and through the rib. The carriage bolt will then slip through from the top with little trouble and can be bolted in place. Now, when you can do the same for the end ribs, and you will no longer be lifting any weight for the intermediate rib connections.  It is easiest with a tall A-frame ladder and a helper to hand up tools and wingnuts.

There are other ways to brace the ribs. Some have installed braces five or six feet up both sides. For those who intend to make it a greenhouse, braces, either of wood  or PVC, on the outside, at about five feet up, can also be the permanent connection point for the greenhouse film and point at which rolled up sides are stored during warm weather. Think about how you will use the house, and design accordingly.

No matter how you brace, the ribs are now locked at four feet from each other. The structure will still rock back and forth lengthwise in the wind. I added nylon cord ties to complete the bracing. About 3mm woven nylon cord is sufficient. There are four cords. Each runs from the top center of the house, the center of rib number 6, to the bottom of an end rib. To attach the cord to the bottom of a rib, wrap a few times and tie, and wrap the cord with duct tape to keep it from slipping up.

Once these cords are in place, the whole house will be stable. Ribs may deform a bit under heavy wind, but the house will remain fairly stable. I feel that the flexibility of the plastic pipe and nylon cord makes it very resilient.

One last adjustment.  If you use the cord braces as I did, you will find that they are somewhat in the way of a person working around the ends of the house near the wall. The simple cure is to move to the third rib and push the cord directly over to that rib and tie it there, as shown in the photo above, with another piece of nylon, wrapping that, too, with duct tape. The cord will be out of the way, and the whole thing had enough give that pulling it over seems to tighten the whole structure. 

You’re done, and you can then move on to adding any additions required for various cover material. Here’s a shopping/scavenging list. My cost for the frame was about $250 with rebar bought from the recycler.

 

Frame Materials:

• Approx. 44 feet of heavy rebar, 3/8 or ½ inch diameter. (I got mine from a metals recycler.) Or use 1/2 inch iron pipe or any other metal stake you can drive in your soil.  
• 19 pieces 20-foot 1-1/2 inch belled end PVC pipe (Six will be cut into two 10-foot pieces to make the 30-foot ribs.)  Or 37 ten-foot pieces. Or any other combination, including shorter scraps, but remember the joints if you will be piecing together scraps or using non-belled pipe.
• 11 carriage bolts. At least 3-1/2 inches long, so they will pass through both PVC pipes. Any reasonable diameter bolt will work. (I used ¼ inch)
• 11 Wing nuts for above.
• 11 Washers for above.
• Approx. 3mm nylon cord. 250 feet to be safe.
• Duct tape.

 Tools:

• Tape measure.  (50 foot is nice. Two or three tapes is also nice when you’re squaring up that opposite center stake.)
• Drill bit, slightly larger than the carriage bolts and at least 3-1/2 inches long.
• Cordless drill. (Or predrill all ribs and long pipe elsewhere.)
• Heavy hammer to drive rebar stakes.
• Marker to mark centers and measurement points.

A 20' by 48' steel cold frame can typically be had commercially for just about $2,000. There's no question about the steel frame being more permanent and all-around more durable than my PVC frame. But if I were to scrounge a bit harder, I could put up ten of my frames for the cost of one steel. And I fully expect to get at least five years service from mine, and because the stakes are already in place, I could have a new one to replace it in a couple of hours. 

For my purposes, this will primarily be a shade house. I purchased a 20-foot by forty-eight foot 30% knitted shade cloth with grommets every two feet for $160. We worked the cloth was worked over the ribs and tied every other grommet to the corresponding rib at about the five foot point above ground. Again, the connection was made with nylon cord, wrapped and duct taped. I had about four feet of excess at each end. I simply tied these across to ribs on the opposite sides.

Since the cover is 20 feet wide, the bottom five feet are open. The early morning and evening sun is not a problem, and the open area allows access to the outside rows.

I wondered about PVC exposed to Texas summer sun, but a little research revealed that PVC is very stable and not prone to sun damage. If that is a worry, simply paint the pipe ribs and brace before final assembly. Total time was about two hours, not counting sawing rebar and making up pipe ribs. 

If you're wondering about the odd installation down the center of the floor of the house, well, that's the subject of the next post.