Monday, April 20, 2009

Hoophouse construction, part 2 (of 5)

This is day 2 of the hoophouse construction project. Click on pictures for bigger versions. I changed the design a little to raise the hoophouse by 3', and used 8' posts because of the soft dirt we're putting it up on. I'm concerned about the pipes being pushed down by snow load. Longer pipe means more friction to resist that push.

That's Jose and Carlos working on the hoophouse. It's taken 16 hours of labor to assemble and tweak all of the trusses so that they're identical. For this particular hoophouse there's 13 trusses and to get the best structural strength out of them they want to be identical and oriented properly, which in this case means that we level the post that goes into the ground, and then make sure that the members are attached in an identical place on each side of the hoop. I'm talking about the distance between the top of the stake and the horizontal element at the top of every other hoop.

These are pretty big structures; here's my brother Bryan standing on a 10' tall ladder for scale and perspective.

Three reasons for the side rail design:

When first constructed this house had a 2x6 along the bottom at the outside. I've changed that to be a double row of 2x4s, one on the inside and one on the outside, and a 2x4 on the inside. You can see the start of that rail on the right in this picture. The wood inside isn't necessary for the greenhouse, but it's there so that I can run a 3' tall chicken wire on the inside to prevent poultry from pecking holes in the plastic if I decide to house poultry in this hoophouse. They like pecking at water droplets inside the plastic and poke holes in it that look like buckshot.

The pictures above is of the damage turkeys have done to another hoophouse that I'm housing them in.

Having a rail like that also allows you to build benches into the greenhouse if you wish, or even do both -- make a 3' tall bench, use the space below to house chickens, and above to grow plants.

The final reason for the side rail is insurance in case any of the bows sink. The side rail prevents the bows from being pushed down too much, and allows the load to be spread evenly along a side. Since there is effectively no bottom under my soil -- it's at least 160' of river bottom mud here -- I have to think about everything sinking. My barns have giant footings under them, big flat plates, the hoophouse has a side rail, etc.

This hoophouse is assembled with 5/16" bolts, most of which are 1.5" long, some are 3" and some are 4". The air compressor and an airgun makes the assembly much shorter and quicker.

Labor costs are higher because we've spent a lot of time fiddling with the trusses to make sure that they're exactly right and identical. Once the plastic goes on you can't adjust anything, and for maximum strength you want it perfect.
We use the duct tape to cover all bolt heads and sharp edges on each frame member. The top of the ground stake, the bolts for the 2x4 side rails, the hangers used to hold the three length-wise pipes to each bow, and so on. No pointy edges, no rough surfaces. Only smooth pipe and duct tape against the plastic.

So the costs to erect this are as follows:

Hoophouse frame: $1,000

Hoophouse plastic: $350

Hoophouse channel: $350

2x4 railing material: 75 @ 1.72 each, $129

4" 5/16" bolts: 50 @ 1.10 each $55

Replacement nuts and bolts and fasteners: $70

10 rolls of duct tape: $35

50 8' x 1.75" steel posts (chain link common post) , $705

Labor: $600

Total to "plastic ready": $2994

Tools used:

Front loader from tractor to drive posts into ground

T-post fence driver for fine adjustment of post

Spool of bailing twine for keeping things straight and checking diagonals

100' tape measure

Air compressor and airgun for spinning nuts on and off, plus a set of deep sockets so that you can spin a nut on a longer bolt.


5/16" drill bits and electric drill (too many holes for cordless)

Laser and regular level

6" pipe cutter

1 8' length of galvanized pipe for those holes that were tough to punch; we'd push this in, drag it back out and put in a weaker pipe.

10' A frame ladder

Lumber crayons to mark pipe


Anonymous said...

My Excel spreadsheet calculates $3294, $300 more than your calculation. Or did you deliberately leave out $300 somewhere?

Bruce King said...

You're right. I suck at math.