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Old 13-04-2003, 06:44 AM
audgen
 
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Default [IBC] Poly Tunnel Covering

Hello again,
I hope everyone is doing good.
I was wondering if anyone might have some tips on building a poly tunnel. What thickness of poly should I use? I need a place for recent repots, a place for some Ficus benjamina that can't quite take the full on sun all day, and a place for everything else
that just needs a little protection from the elements. Will these different applications call for different thicknesses of poly or any other differences in construction?
Sorry to complicate something that's probably very simple.
Thanks for any help!
Audgen (zone 7).


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Old 13-04-2003, 07:32 PM
Brent Walston
 
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Default [IBC] Poly Tunnel Covering

At 12:24 AM 4/13/03 -0700, audgen wrote:
Hello again,
I hope everyone is doing good.
I was wondering if anyone might have some tips on building a poly tunnel.
What thickness of poly should I use? I need a place for recent repots, a
place for some Ficus benjamina that can't quite take the full on sun all
day, and a place for everything else that just needs a little protection
from the elements. Will these different applications call for different
thicknesses of poly or any other differences in construction?
Sorry to complicate something that's probably very simple.
Thanks for any help!


Audgen

You don't say where you are, so that complicates things somewhat. A poly
tunnel in a mild climate is definitely an easier proposition than one in a
hot climate. The actual construction is pretty much a breeze, and there are
many ways of doing it.

First of all, the size- This is very important because the larger you make
it, the more complicated it becomes, mostly because of ventilation
needs. 15 to 20 feet long is about as big as you can make one that is film
covered without positive ventilation (exhaust fans, or roof vents). It gets
very hot in these tunnels even when covered with translucent poly or clear
film covered with shadecloth. It needs to be tall enough so you can walk
into it without bending over.

After several aborted designs for poly or shade tunnels, I have come up
with a strong inexpensive method that is easy to construct. It is made of
hoops of PVC pipe positioned by wires and covered with either shadecloth,
film, or both. 1 1/4 inch is the smallest pipe you can use and still have
the strength you need. There is also a quantum leap in price in sizes
larger than 1 1/4 inch. Be sure to get Schedule 40 PVC water pipe in 20
feet lengths. Don't buy a lesser grade or you will have problems.

If you take a 20 foot piece of 1 1/4 inch PVC and bend it over, it will
span about 13 1/2 to 14 feet in an arc that is a little over six foot
high. My latest generation of hoop houses are 13 1/2 feet wide with the
ends of the PVC slipped over 1 inch EMT conduit (also very inexpensive).
The conduit is cut into lengths and pounded into the ground. A baseboard of
treated 2x4 lumber is attached to the conduit (on the outside) by using
pipe straps. The conduit should stick up above the 2x4 about 6 to 8 inches.
This will give you a nice flattened arc with plenty of standup space. The
ends of the PVC rest on top of the 2x4 edge, no other attachment needed.
You really have to work to get the PVC over the conduit so make sure the
conduit is really firmly anchored into the ground or the PVC will push the
conduit and the 2x4 outward and mess up the arc. Space the PVC hoops every
4 feet. So 5 hoops would give you a 16 foot house, about as big as you can
make with film, but without positive ventilation.

There are any number of ways to connect the hoops so they can't move
horizontally. Most commercial methods involve using a lateral strip or pipe
called a pylon. My houses are 96 feet long, so I have developed a more
economical and practical method, but it is not as practical for small
houses. I use 12 gauge high tensile vineyard wire (available at farm
supplies). This is incredible stuff and really cheap considering that you
get about 2 miles of it in a roll, but you can only buy it by the roll and
it's about $50. It is a huge pain in the butt to unspool without an
unwinding contraption, which I made for the nursery (I use this stuff
everywhere). Sometimes you can rent one. Anyhow, I stretch the wire in two
lengths along the hoops of the house and fasten the ends at steel T posts,
then run it down to the ground at a 45 degree angle and attach it to steel
earth anchors (another story). I use these fabulous connectors called
gripplers that allow you to stretch the wire with a tool (yet another
story). You can get about 600 lbs of force on the wire with this system, so
it is very strong. The wire is attached to the hoops with screws which will
easily go into the PVC.

Now that I read what I just wrote, go with pipe pylons or wood, it will be
much simpler. You will still need some sort of a diagonal brace.

Wooden end frames are probably the easiest, although not strictly necessary
depending on whether you want to enclose the ends in winter or if you are
only going to use shadecloth instead of film. Attaching the film is by far
the hardest job. Attaching it along the length is a breeze. You slide it
over the house and allow a two foot overlap along each side. Cover the film
with earth. That's it, no staples, battens, any of that stuff. The earth is
so heavy that no wind will ever pull it loose or tear it.

The ends are as hard as the sides are easy. The best way is to pull the
film over the end and attach it to a 2x4 frame contructed inside the arc.
Nearly all commercial structures are built this way. A few companies sell
these plastic thingies that slip over the PVC and the plastic film to hold
it in place. These work great until the wind blows. The wind flaps the
plastic continually until the edges of the retainers begin to cut through
the plastic and in short order the ends of the film are loose and you are
screwed because now the plastic is too short. End frames allow you to pull
the plastic over the PVC and gather it and staple it. Then you can batten
over the staples. This is a must or the wind will pull the staples loose or
tear the plastic around them. You can get fancy and make doors if you want
to, depends on how much you want to do with this structure. You can make a
full blown greenhouse if you want.

The plastic you use should only be 6 mil greenhouse plastic with UV
inhibitors. In larger sizes it doesn't cost any more than regular poly and
it will last four years instead of less than one. It is generally only
available at nursery and greenhouse supply outlets so you will have to hunt
for this. Online greenhouse suppliers carry it, but you may have to pay
premium prices and it is quite heavy for shipping (Mellingers.com,
CharleysGreenhouse.com). Do NOT use the regular stuff you find at hardware
stores unless you want to replace the plastic every single season.

For your purposes, you may not even want to use film, I don't. My hoop
houses are covered with shadecloth only. This simplifies construction a bit
because you can attach the shadecloth quickly and easily with drive screws
in the 2x4 base and directly into the end hoops, no other frame necessary.
I even enclosed the ends with shadecloth and made shadecloth flap doors
that just hang from the top of the last hoops. Shadecloth gives you wind
and sun protection, keeps critters out, raises humidity slightly and still
allows excellent air circulation. Perfect for seedlings, transplants and
shade loving plants. It does not however give you any freeze protection,
well only a degree or two, because the shadecloth does slow the rate of
radiation at night.

I love these shade houses, leave them up all year, and have been using them
for ten years. Of course I live in zone 8, so heavier protection is not
usually necessary in winter for most woody plants. I am currently building
one with a heated earth floor with an active solar system and frozen
covering protection, but that is another WHOLE story.

Brent in Northern California
Evergreen Gardenworks USDA Zone 8 Sunset Zone 14

http://www.EvergreenGardenworks.com

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++++Sponsored, in part, by Evergreen Gardenworks++++
************************************************** ******************************
-- The IBC HOME PAGE & FAQ: http://www.internetbonsaiclub.org/ --

+++++ Questions? Help? e-mail +++++
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