The Accidental Smallholder Forum
Smallholding => Buildings & planning => Topic started by: ScribbleUk on December 06, 2013, 05:19:17 pm
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Hi all,
I just wanted to see how much movement (if any) people experience with their polytunnels during winds. Ours is a 14ft wide from First Tunnels using the aluminium base rail and anchor plate kit. It runs parallel to a hedge protecting it from northerly winds to some extent. It is rock solid with winds running the length of the tunnel, but there is quite a bit of movement/rocking when the winds hit it broadside. In fact it can be rocked by hand to some extent. Do other experience similar movement and is it normal? I'm sure some flexing is appropriate but it is hard to know if there is an acceptable limit.
Thanks.
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Our polytunnel is a 16m long by 6m wide with hoops every 2 metres. The hoops are sunk into the ground with the plastic buried into trenches. Essentially there is quite a bit of movement of the plastic in strong winds but otherwise is pretty solid.
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Hmm....might need to think of a way to brace it a bit better.
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My mini one is a goner. It was anchored all round by staples into the ground, with heavy fence posts all round, sheltered on two sides by a 6 foot fence, and two webbing straps over the top also anchored. I left the front door and back window slightly open hoping the wind would go through, but it lifted it clean off the ground, twisted the aluminium frame and threw it against the uprights of my raised beds 40 feet down the garden!
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Was thinking of using some anchors screwed into the ground inside the polytunnel, and then some tie-down straps between the anchor and the storm stays/braces. Would this work or a waste of effort?
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I would expect if you have sufficient plastic buried at the sides and ends it won't go anywhere ScribbleUk. Ours relied on the weight of the soil on the plastic to keep it in place -a 1 foot square trench all round. Think anchors inside may get in the way a bit?
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Ours is 30' x 14' and has been up for over three years. It's a metal tubed one with the sheet dug in about a foot all round. It's not particularly exposed having trees off to one side and a stone wall about 4' high on the south side. I've spent a fair bit of time in there in bad weather and can't say I've ever noticed it moving really.
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I think that may be the difference, in that we are using the supplied anchor plates buried under ground with the polythene attached along a base rail rather than trenched polythene. Although we did tuck the surplus under the ground sheet and now have full length raised beds sitting on the surplus.
I think trenching the polythene probably provides a greater level of stability along the length rather than just anchor plates, although I would imagine trying to recover or retention the polythene that is trenched could be a right pain.
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When you move the polytunnel the trenched plastic is scrap ScribbleUk because digging it out will invariably tear it. But how often will you move it, because the plastic only lasts 5 years (the salesman told me)?
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This might help
http://m.youtube.com/watch?v=XmPpKyOq6S0&desktop_uri=%2Fwatch%3Fv%3DXmPpKyOq6S0 (http://m.youtube.com/watch?v=XmPpKyOq6S0&desktop_uri=%2Fwatch%3Fv%3DXmPpKyOq6S0)
If the link doesn't work, just google polytunnnle in a storm.
My tunnel is from fist tunnels 12 x 20 and because of its location it has every thing, anchor plates, storm brace, crop bars and the long bars that run the length of the tunnel on each side ( can't remember the name of them now) and I have the wooden base rail.
My tunnel coped very well with the storm last week. No movement at all. :relief:
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I remember seeing this vid before I bought ours. I think it was one of the reasons I went for First Tunnels - are you on commission for them - you should be :-)
Did you concrete your anchor plates in or just backfill with soil?
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Well ours is the smallest size commercial tunnel from First Tunnels, put in 7 years ago, still on its first set of polythene... Same set-up - anchor plates in the ground (stabilised with lots of heavy stones buried on top of the plates), wooden base rail, excess plastic buried about 3 -4 inches in the ground. Never moved yet in the wind, but sits in a sheltered position (and therefor frost pocket, but you can't have everything).
We have the extra stabiliser bars at the joints of the hoops, plus all crossbars too.
If it moves I think something is not quite right....
Snow is biggest problem for us, as it is quite a job to brush it off.
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Our tunnel is 14m x 7m, Northern Polytunnels with extra hefty tube thickness and diameter, and has been sitting on a very windy hilltop for the past 15 years or thereabouts. It has base tubes set in hefty concrete dollies, every stabilising storm saver inside, such as triangular corner braces and braces to the ridge pole, plus crop bars at every hoop. We have the thickest polythene cover available too, wooden base rails, wooden and corrugated poly-whatever-it-is doors and louvres and a 2m windbreak fence set back about 10m all the way round (which forms my veggie patch.
About 9 months after we put it up the cover tore at the upwind side in an 80mph gale. My OH swung from the crop bars and slit the polythene right down the middle to release it before it could destroy the whole tunnel. The force of the wind on so much sheeting is enormous - think sailing vessels. We found that ours had twisted the coachbolts holding the base rail to the hoops to about 30 or 40 degrees. the problem was that we had wound the polythene the wrong way around the slats used to attach the cover to the base rails so this made it vulnerable to a southerly gale which hit it longside on (the correct way seems counter intuitive to me).
Since then we have had no problems with the cover and have had to change it only once in all that time due to old age not tearing.
However, in every side-on gale (and we have a few, even up to 100mph once) the whole structure flexes dramatically, from side to side. My greenhouse is inside the tunnel and the apex comes to within a foot or so of the polythene. One year the cover was torn by rubbing against the sharp corner of the greenhouse roof. So that is how much flexion there can be at the height of the storm with no major damage. Our main precaution is to check every bolt holding the structure together both before and after a storm - in the early days the bolt heads would get shaken and start to undo but that happens less now they have bedded in.
I don't think a dug-in cover would last long up here, but I think the metal parts of the tunnel structure are designed to flex. The wind pressure from the sides can apparently cause the flimsier structures to crumple in windy areas, hence our 'overkill' precautions.
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That is interesting to hear that you seem to experience similar movement in the frame as I do. Will be interesting to see how it performs today. We are forecast to get 50+ mph gusts from the south which means it will be getting hit broadside.
I usually do a precautionary bolt check every couple of months as well, it is surprising how much they can be tightened up, although I suspect this may be as much to do with the expansion and contraction of metal due to the temperature changes as it is to do with frame wobble in the wind.
Any reason for keeping a greenhouse inside the polytunnel?
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Any reason for keeping a greenhouse inside the polytunnel?
It wouldn't last 5 minutes outside (it's just a cheapy), nor would it be warm. The tunnel raises the temp by about one degree C compared to outside, in the winter, then the greenhouse, unheated, raises it another one degree. I use the greenhouse for attempting to overwinter a few plants but they mostly get frosted anyway, then in the summer I fill it with chillies. I'm thinking of taking it down actually as the chillies grow happily in the tunnel soil and the terriers have undermined the sleepers the greenhouse sits on, so it's getting a bit higgledy-piggledy and would need to be redone anyway. It can be unbelievably hot in the summer, well over 100 F.
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Understand now. Had visions of a proper greenhouse inside it! I was planning on making a couple of cold frame covers to go over some of the raised beds to help keep things a bit warmer in the tunnel.
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Oh it is a proper greenhouse - 8'x6', aluminium and glass, just not the hyper toff type Alan Titchmarsh has :roflanim: :garden: .
In fact, does anyone want to buy it, S Lanarkshire?
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a real green house - inside a polytunnel???
that's genius , I have 2 flimsy greenhouses that I wasn't sure what to do with. does it not catch the roof in the gales?
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a real green house - inside a polytunnel???
that's genius , I have 2 flimsy greenhouses that I wasn't sure what to do with. does it not catch the roof in the gales?
It does ;D - I've described it in my long message above, if you can plough through it. The one time it did it poked quite a big hole in the polythene but it mended well with repair tape - fortunately poly cover tears don't seem to 'run'. The greenhouse would be better being further away from the sides. My polytunnel is quite tall, 10' I think so it doesn't hit the crop bars or the middle of the polythene. If I'd realised in advance how much the tunnel structure flexes, and how much the polythene stretches in the heat, I would have put the greenhouse more centrally, or facing the same way as the tunnel - at the moment it sits across the tunnel and one apex comes to within about a foot of the south side wall of the tunnel.
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We have a first tunnels 60' x 20' tunnel and it is rock solid. They build their tunnels with closer hoop spacing than other makes specifically for this climate, it shouldn't rock at all. Have you contacted them for advice scribble?