I just finished the greenhouse again. I use double poly and have to have
it weather tight and have a "good bubble" when it is inflated or I have
real problems with snow load in the winter.

Some things I have learned.

1) Spread the whole thing out and get both layers of plastic matched up
prior to pulling it over the frame.

2) Duct tape over bolt heads and other sharp edges prior to pulling the
plastic over the frame.

3) Get at least 1 person for 10 feet of greenhouse length to help pull,
it prevents stretch and strain in the plastic, make sure the people use
2 hands

4) Position ladders or other climbing aids if the green house is short
enough to help get the plastic over the peak

5) Get it square on the frame, choose one long edge as the reference for
doing that - do not lock it in place (i normally leave about 6 inches of
slack on the ground, beyond my wire lock.

6) Start on one end hoop and lock down the plastic - for the outer sheet
allow one inch of slack for every foot of the hoop (in other words put
13 inches of plastic on 12 inches of frame on the outer sheet. Lock down
one hoop - remember to keep the inner sheet should be 12 inches per 12
inches of hoop.

7) Leave approximately 1 foot of slack in the long side for every 20
feet of run for the outer sheet of plastic and then repeat step (6) for
the other end.

8) Install the inflation fan inlet (I normally use patch tape on both
sides of the inner sheet of plastic, before I cut the hole for the fan)
Start the inflation fan.

8) Start at the inflation fan end and on the side that in inlet is on
locking down the plastic. Remember to use some of the slack that you
left in the outer sheet of plastic as you move along locking the plastic
down. The fan will make it easier to slide the inner and outer sheets
against each other and to help get the bubble good. Lock one full side
down from one end to the other prior to switching to the final long side.

9) repeat from the end the fan is on. Allow the plastic to inflate as
you go, and let it naturally lay in the locking location, take your
time, if you goof, you will see wrinkles in the plastic.

10) Check the whole thing for problems when you are done. If you have
measured out the slack carefully and evenly you should have a bubble on
a 50 foot greenhouse that is about 18 inches high and an inner sheet of
plastic with about 3 inches of penetration into the frame.

11) Turn off the fan and adjust the inner plastic to be even on the
frame - remember that if the whole air bubble goes out it gets harder,
but you need to have about 1/2 out to make any real adjustments. Move
the plastic by laying your whole hand on the sheet and sliding your
hands along. (I use clean cotton gloves, they seem to make it easier to
move the plastic without creating "white spots" (these occur when you
stretch the plastic).

Hope this helps someone.

cheers

Doug