Plastic roof expansion

Hello all,
I'm making a porch roof, it'll be about 3' deep and about 5' - 6' long with a wooden bracket structure to support it. The roof is flat ?polycarbonate sheet from Been and Queued; my worry is that thermal expansion of this sheet may damage the joints of the brackets and supporting longitudinal beams, should I improvise some sort of sliding mounting points? It seems to me that slots for the fixing screws wouldn't be adequately waterproof; I could glue some mounting slots on the underside to engage with hooks on the beams, or I could just not worry.
Your thoughts would be appreciated...
Regards Ant
* ps - Don't let the EU rule Britannia! *
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Just make sure you use neoprene or rubber washers under your fixings and these should allow the sheeting to move enough. I've tried the little cup things they sell for fixing these sheets down, but I find that a copper washer on top and rubber tap washer underneath works better. Works out cheaper to buy as well.
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