Could I get an opinion on this please? (Cheap polytunnel fixing)

[gardening_gal]

Hi everyone, I'm hoping someone could run their eye over these plans to see if there is anything I haven't thought of.

I'm being given a new and unused 6m x 3m polytunnel. It's only the green cover kind from eBay but I'm very grateful to be given it as I would never be able to afford to buy one for myself. It seems fairly strong compared to other cheap polytunnels but I do realise it wont be as good as a proper tunnel.

The area where the polytunnel will be going is covered in a rough and uneven tarmac. I have no choice about position, it's here or nowhere. I will have to use deep raised beds sitting on the tarmac to grow things. The property is rented and I'm not allowed to do much to it so digging anything up is out.

I've been pondering over the best way to secure both the frame and the cover so that there is no chance of it being blown into next door's garden.

As the ground is so uneven, I'm concerned about bending the bottom rail trying to fasten it down, so I thought it might be best to lay out a timber base frame first then sit the tunnel on top and fasten the bottom rail of the tunnel to the timber by using galvanised saddle clips like these -25mm Galvanised Spacer Bar Saddles - Pk 10 | Metal Conduit and Acc | Screwfix.com.

I could then fix the timber to the ground using something like these -10 CARAVAN AWNING HARD GROUND SCREW IN PEGS 13mm HEAD | eBay

The bottom of the cover could get tied down by putting a batten over it and screwing it to the timber base frame.

I hope all that makes sense, I can see it so clearly in my head.

Does it seem ok or is there a better way of securing it given the constraints?

[Potstubsdustbins]

Sounds like a good plan to use a timber base.

Three questions, what size of timber were you thinking of and what thickness of asphalt cover do you have and finally is the asphalt sat on hard-core.

Potty

[gardening_gal]

Was thinking about 95mm wide x 45mm thick timber? It would have to be wide enough to allow fastening of the saddle clips and also to enable me to fix it to the ground, screwing at an angle. Wasn't too sure what thickness to go for though.

No idea about the ground make up I'm afraid. I suspect it was done on the cheap hence the rough awful finish so I wouldn't bank on there being a proper hard core sub base.

The ground round here is heavy clay if it makes any difference.

Just had another thought, the area was used for parking so perhaps there is a decent sub base.

[binley100]

I'm thinking you may find it better growing in pots rather than trying to make raised beds on tarmac...

[gardening_gal]

I already have a number of raised beds that I grew things in successfully previously on this surface so I'm not too worried about that although admittedly I did need to buy deep pots for carrots and leeks.

[binley100]

Does it cause issues with drainage being on tarmac?

[gardening_gal]

No, no issues at all. I expect that's down to the surface being so rough and uneven, it's not like standing a pot on a solid slab for example.

The ground also slopes a bit so that probably helps too.

[Bigmallly]

Not sure of the cost implications but could you not hold the cover down with breeze blocks all the way round the base (inside or out), this in turn would secure the frame.

[gardening_gal]

Thanks Bigmally, I came across someone doing just that when I was searching the net for ideas. His were on the outside and a couple of them lifted with a bad wind. They would be more static inside I would think.

I also came across Spirafix ground anchors which someone used with their tunnel and they look really good. They are pricey mind, but by the time I bought other bits I might be as well forking out for these. Will need to give it more thought and price things up properly.

[Mikey]

From experience tarmac is a relatively soft material made softer with heat. As you don't know what is below this surface you might be lucky and get a good fixing or you might be unlucky and it be too loose. You could drill a hole and fill with resin or wet concrete mix before dropping an anchor bolt in. let this go off before trying to secure the frame.

Personally I would look at the weight rule, if you intend to put raised beds inside they will be filled with earth, could you not secure the timber frame to these beds as the weight of these is unlikely to see the frame blow away. A cubic metre of earth is over a tonne in weight, you could get away with much lighter screw fixings.

Out of curiousity do you line your beds with a membrane to stop the soil touching the tarmac?

[gardening_gal]

Good idea about utilising the beds, I never thought about that. Only issue is they are black plastic modular things rather than wood so screwing into them isn't workable. However I could easily fasten a strap of some kind to the wood frame and run it underneath the beds. I could use a small wooden batten in front of each bed to fix the strap to, sort of like a toggle effect.

Yes, I use membrane inside the beds.

My set up isn't what I would choose if I wasn't presently renting, I need to be able to set things up and dismantle fairly easily. One day I will have a huge garden of my own AND an allotment and I will be in plant heaven.