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Attaching twin wall polycarbonate to a cold frame

Hi all, I’m going to top my newly built cold frame with 10mm twin wall polycarbonate, which will be screwed to a wood frame to form the lid.

To attach poly to frame I bought the fancy cushioned button fixings (left of pic 2) but they are expensive monsters at 40mm wide and need wider lid timber than is ideal.

Question is, can I use the smaller button screws (right of pic 2), which are only ever listed for use with corrugated polycarb rather than twin wall? The screws are the same length in either case. I don’t want to compromise the twin wall in future, but with no wind lift/structural integrity issues as it’s a ground-hugging cold frame the smaller style fixings are tempting. Anyone used them with twin wall polycarb for a cold frame or similar successfully?

Thanks, Johnny


Posts

  • BobTheGardenerBobTheGardener Posts: 11,385
    edited February 2022
    Yes, I used the exact same clear type to fix the 10mm polycarb sheeting to roof my potting shed, too, and both frame and shed are still going strong after 30 years.  If you ever have to unscrew them, the caps usually fly off or crack though.  I would say pay more attention to sealing the ends with good quality tape - a breathable tape on one end, and aluminium tape on the other.  I used a very cheap breathable tape, and it deteriorated after a few years, letting in water and insects from that edge.
    A trowel in the hand is worth a thousand lost under a bush.
  • I fixed mine using Galv clout nails (plasterboard nails). Cheap as..
    It's still fine after 2 years but I don't expect it to see 30.
    I'd go with Bob's suggestion if you want longevity

    Just another day at the plant...
  • Yes, I used the exact same clear type to fix the 10mm polycarb sheeting to roof my potting shed, too, and both frame and shed are still going strong after 30 years.  If you ever have to unscrew them, the caps usually fly off or crack though.  I would say pay more attention to sealing the ends with good quality tape - a breathable tape on one end, and aluminium tape on the other.  I used a very cheap breathable tape, and it deteriorated after a few years, letting in water and insects from that edge.

    Thanks Bob. Clearly not all tapes are created equal so I won’t skimp on those. I’ll top the tapes with PVC end closures for an added seal.

    I’m more concerned about the smaller fixings not being as watertight and causing the polycarbonate flutes to discolour with algae etc over time.

    Fingers crossed mine last as long as yours - I didn’t realise polycarbonate had been around that long.
  • Fortunately, I've not noticed any water penetration to the inner from around the caps.  I made sure the seal was only slightly squashed when screwing them in, rather than flattening it, to extend the life as much as poss. (flexible plastic under strain + UV from sunlight will shorten their life), if it helps.  :)
    A trowel in the hand is worth a thousand lost under a bush.
  • Fortunately, I've not noticed any water penetration to the inner from around the caps.  I made sure the seal was only slightly squashed when screwing them in, rather than flattening it, to extend the life as much as poss. (flexible plastic under strain + UV from sunlight will shorten their life), if it helps.  :)

    That’s encouraging. Not too loose, not too tight seems to be the way ahead. Best crack on with it then (hopefully not literally), thanks.
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