One problem I encountered with a B & Q greenhouse in the garden of the
house we moved in to a few years ago was that the polycarb' sheeting (said
to be ideal for greenhouses) I bought from a supplier was bowed in the wind
to such an extent that it came out of the frame. Obviously a design problem
because the greenhouse windows are approx. 29 inches wide which is much too
wide to allow thin polycarb' sheet's stiffness to prevent it being blown
out.

On the thin polycarb sheets I had used in other positions I braced them with
thin wooden slats to prevent bowing.

I have recently replaced two more glass windows (safety considerations) in
the greenhouse but used polycarb sheets twice as thick and braced them too!
I bought greenhouse spring clips from an ironmonger's and to prevent them
collapsing the polycarb' I stuck short metal strips on the pollycarb'
where the springs would be.

By the way, It's adviseable to seal the top edge of the polycarb uning a
decent waterproof strip. I used an aluminium adhesive backed strip -
expensive but worthwhile for the sunlight does not degrade it.

Geoff