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Thread: DIY 25ft x 14ft approx polytunnel advice

  1. #1
    finglas is offline Germinator
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    Default DIY 25ft x 14ft approx polytunnel advice

    Hi everyone,

    I have had a search about the forum and found some good information out about DIY polytunnels but just looking for a little more advice.

    My dad has a reasonable sized garden and we would like a polytunnel. We have measured it and are looking for approximately 25ft long by 14ft wide.

    First question would be, am i better trenching the poly into the ground or using a base rail? It seem like using a base rail would be a lot more work and introduce a greater likelihood of tearing etc? Perhaps not though, what do you think is the best option?

    Second question, the garden has a reasonable slope on it but nothing major. We have not measured it yet but maybe from top to bottom its a 2ft difference. Should we cut/fill and level the ground or built the tunnel on the slope? Is a slope good to encourage run-off?

    Third question, our galvanised steel pipes are 48mm outside diameter. 50mm MDPE pipe seems to have a inside diameter of around 41mm so it wouldn't fit over the 48mm steel pipes. The other two options we have considered is 63mm outside diameter pipe which would fit over the steel pipes but would it be too heavy gauge ?
    The other option is I get the 40mm stuff and put it INSIDE the 48mm steel pipe which has a 42mm inside diameter.

    Fourth question, what height is desirable at the inside edge. If on the inside of the polytunnel I have a 3ft bed ( that's another question, what size of beds?) I come in 1ft I'd imagine I would want a reasonable height to accommodate various plants, tomatoes etc. So, what height of verticle steel pipes would be good?

    These are just some of the questions we have ran into but it would be really great if I could get any pointers on these points.

    Thank you all,

    Jamie

  2. #2
    SarrissUK's Avatar
    SarrissUK is offline Cropper
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    Having seen polytunnels put up commercially, I'd say that the plastic is better trenched on the lengths, than fastened with a base rail. That's also a good size poly that. Get the best plastic you can afford, and make sure you get it bigger than you think you need. Trenching swallows a whole lot.
    I'd also use something to hold down the plastic with, at regular lengths, whether that is blue pipe that is held down with screws or brackets.

    Level ground means everything if you want to have tables in there with anything on them such as trays. Otherwise I don't think it's meaningful to level it.

    I'm doubtful that you will get enough stability with plastic piping alone - for the size you are aiming for, I'd always go for a metal frame, then plastic pipe or something similar to hold the plastic in place on the outside.

    In that size tunnel, I'd have one bed along either side, and one down the middle
    finglas likes this.

  3. #3
    finglas is offline Germinator
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    Thank you Sarriss that is great. I hadn't really considered the mdpe pipe not providing sufficient stability but you may well be right. Perhaps we will need to reconsider our plans....

    thank you

    Jamie

  4. #4
    4Shoes's Avatar
    4Shoes is offline Cropper
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    Quote Originally Posted by finglas View Post
    Hi everyone,

    I have had a search about the forum and found some good information out about DIY polytunnels but just looking for a little more advice.

    My dad has a reasonable sized garden and we would like a polytunnel. We have measured it and are looking for approximately 25ft long by 14ft wide.

    First question would be, am i better trenching the poly into the ground or using a base rail? It seem like using a base rail would be a lot more work and introduce a greater likelihood of tearing etc? Perhaps not though, what do you think is the best option?

    Second question, the garden has a reasonable slope on it but nothing major. We have not measured it yet but maybe from top to bottom its a 2ft difference. Should we cut/fill and level the ground or built the tunnel on the slope? Is a slope good to encourage run-off?

    Third question, our galvanised steel pipes are 48mm outside diameter. 50mm MDPE pipe seems to have a inside diameter of around 41mm so it wouldn't fit over the 48mm steel pipes. The other two options we have considered is 63mm outside diameter pipe which would fit over the steel pipes but would it be too heavy gauge ?
    The other option is I get the 40mm stuff and put it INSIDE the 48mm steel pipe which has a 42mm inside diameter.

    Fourth question, what height is desirable at the inside edge. If on the inside of the polytunnel I have a 3ft bed ( that's another question, what size of beds?) I come in 1ft I'd imagine I would want a reasonable height to accommodate various plants, tomatoes etc. So, what height of verticle steel pipes would be good?

    These are just some of the questions we have ran into but it would be really great if I could get any pointers on these points.

    Thank you all,

    Jamie

    1. Easier to tension skin with base rails.
    Base Rail is a 1" x 2" batten. Nail a 1" x 1/2" top edge of base Rail. Hold plastic with you foot and nail 2nd 1" x 1/2" batten below sandwich plastic.
    Option 1 - Ensure you can lift hoop free from base rail by 100mm - Secure by bold / Jubilee Clip
    Option 2 - Pipe /Saddle Clip around pipe with Bolts through into tunnel. Set Base Rail 100mm above floor. Secure Plastic. Stand on Base Rail and secure to tension plastic.



    2. Ensure tunnel is level across the hoops to make opening doors easier. Hoops are plumb, you will get away with slop front to back.


    3. Some use rebar hammered into the ground.


    4. 2m to crop bars 2.5m to top of hoop? any higher and you may to run into planning issues. If ground good condition, may consider lower beds. Vertical pipe around 600 to 900mm

    the 50mm plastic pipe will work. Ensure you have centre rail and one either side. You'll be able to screw straight through the pipes into the hoops. Wrap with plenty of tape to protect the cover.


    You'll need diagonals at the end hoops to keep them plumb. Crop bars will add more stability.


    Edit. - more info.
    remembered now - U bolds / Exhaust pipe brackets around the hoops (not saddles)
    Go to First Tunnels and watch the videos on how to build.
    Max 5' between hoops.
    Best to burry (if not trenched in extra plastic around bottom to ensure no air gaps if on windy site.

    If in a very hot area, put in a side rail with Scaffolding mesh below to give extra ventilation. Fixed with u bolts - same way as Base Rail. You can sandwich netting and plastic at side rail height and then roll up plastic to give a roller ventilation system.


    Slide Door easier to use on windy site, but go for single door. No experience of double hinged doors.
    Last edited by 4Shoes; 05-01-2019 at 10:59 PM.
    unclefudgly and finglas like this.

  5. #5
    Aberdeenplotter is offline Gardening Guru
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    My tunnel is on a slope. It works ok. It is 25ft x 10ft. I have base rails, one at ground level and at the other side I have the base rail 2ft from ground level with plastic lights/windows hinged below. I can't remeber what they are called but at the foot of each hoop I have plates sunk into the ground and covered with soil which anchor the thing down. I have two ft wide slabs down the middle of the tunnel and doors at each end. The top door hinges into the tunnel and the bottom one hinges to the outside both the only way they can be to allow clearance.

    I agree with Sarriss about metal.

    When I bought mine, I went for the high sided option and I guess it must be 3 - 4 ft before it starts to curve up.

    I'm on my second skin the first having lasted 10 years and the present one has been there for 5 years.
    Again I agree with Sarriss, buy the best quality plastic and be sure to fit it with the right side out. I know someone who got that wrong.
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  6. #6
    finglas is offline Germinator
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    Quote Originally Posted by 4Shoes View Post
    1. Easier to tension skin with base rails.
    Base Rail is a 1" x 2" batten. Nail a 1" x 1/2" top edge of base Rail. Hold plastic with you foot and nail 2nd 1" x 1/2" batten below sandwich plastic.
    Option 1 - Ensure you can lift hoop free from base rail by 100mm - Secure by bold / Jubilee Clip
    Option 2 - Pipe /Saddle Clip around pipe with Bolts through into tunnel. Set Base Rail 100mm above floor. Secure Plastic. Stand on Base Rail and secure to tension plastic.



    2. Ensure tunnel is level across the hoops to make opening doors easier. Hoops are plumb, you will get away with slop front to back.


    3. Some use rebar hammered into the ground.


    4. 2m to crop bars 2.5m to top of hoop? any higher and you may to run into planning issues. If ground good condition, may consider lower beds. Vertical pipe around 600 to 900mm

    the 50mm plastic pipe will work. Ensure you have centre rail and one either side. You'll be able to screw straight through the pipes into the hoops. Wrap with plenty of tape to protect the cover.


    You'll need diagonals at the end hoops to keep them plumb. Crop bars will add more stability.

    Thanks 4 Shoes, that is great advice.
    Very interesting about crop bars and top of hoop height, I didn't think that planning issues would arise but of course we don't want anything like that!

    Vertical pipe at 900 was around the size we were considering so thats good to hear.

    Thank you

    Jamie

  7. #7
    finglas is offline Germinator
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    Thank you Aberdeenplotter, sounds like great advice. Is that enough ventilation? Doors at both ends? That was what we were planning and they would face the prevailing wind. Seems like putting side panels in with netting etc would be a lot more work....

    Thanks again,

    Jamie

  8. #8
    Aberdeenplotter is offline Gardening Guru
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    The lights/hinged windows below the higher of the two base bars provide extra ventilation when the lights/windows are open. I have three separate ones fitted along the whole length with brass hinges. Steel rusts . If you think that's not going to give enough ventilation then consider having lights/hinged windows at both sides. Up here in the frozen north one side is plenty I should have said with the windows/lights you need another base bar below.

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