I need some advice regarding polycarbonate corrugated sheeting.
I'm building a long green house, I'm going to use corrugated poly as the roof mounted to flat aluminium hoops (see sketch). My 2 questions are:
1) when mounting/screwing to the aluminium hoops do I still need to leave a 10mm expansion hole in the poly? obviously it'll never be walked on and will expansion be a problem with just 2 sheets
2) I want to join some sheets end to end, so effectively turning two 2m lengths in to one 3.9m length (10cm overlap), so what's the best way to join them? nuts and bolts with wide washers? pop-rivits? plastic rivits? self tappers? glue/silicone? other? again will expansion holes be required?
thanks for any help
Polycarbonates expand and contract significantly when they heat up and cool down. The expansion holes recommended when using polycarbonate roofing sheets are there to stop them buckling up. In your design, it might not be necessary to include them and any expansion and contraction could potentially be a non-issue. Even if the sheeting does buckle up on your greenhouse it might not be of concern, yet you can imagine what someone would think if their roof was doing the same thing.
Silicones are not recommended with polycarbonate sheeting as it fixes them in position and doesn't allow them to move.
I would suggest with mounting to the aluminium and in all joins that you put the 10mm hole in the sheeting and then use pop rivets with washers on either side. You'll need to use washers that have a small internal hole to suit the pop rivets grip but also large enough to cover the 10mm hole. These Zenith 1/4 Zinc Plated Mudguard Washer 12 Pack could be suitable for the task.
Please let me know if you need further assistance or have any questions.
thanks Mitchell for your comprehensive reply, all good stuff. I wonder at the joins if I only need the 10mm hole in the lower of the 2 pieces being joined? Could look neater.
As long as you did the expansion holes where the polycarbonate is fixed to aluminium, then you should be fine just fixing the two sheets together at the join with rivets and no expansion hole. This would lock the two sheets together effectively creating one large sheet which would expand and contract together. I'd still be tempted to use washers so the rivets don't pull through the sheeting.
OK so does that mean silicone is now an option afterall? I see 4 benefits of silicone 1) quick and easy 2-3 blobs and its done 2)invisble 3) no panel drilling so less stress on the panels at the holes 4)easy-ish to separate if a panel needs to be replaced
with 2 downsides 1) will 2 or 3 10cent sized blobs per 80cm width be strong enough 2) will the blobs discolour after a few years?
Since silicone isn't recommended for use on polycarbonate I don't have access to any specific information on its effectiveness. I would certainly suggest you use a non-acetic cure silicone similar to Selleys 260g All Clear Multipurpose Sealant as otherwise, it might react with the polycarbonate. As long as the aluminium is holding the sheets securely in the tunnel shape then some blobs of silicone should be sufficient.
thanks Mitchell, I'm going to try and make any joins close to the aluminium hoops, and have the join facing away from prevailing weather, so I hope silicone blobs will be sufficient. just got to do more research as to the silicone discolouring, but I guess even if it does is it any more visually intrusive than fasteners? probably not
thanks for all your help today