Fixing polythene sheets in a Poly House is very important as they can be easily broken by wind and storms, so it is advised to fix them properly. Drive at least 8-10 iron rods in cement concrete slabs placed on walls along with half an inch protruding out of upper parts towards the roof. Hammer tap head nails on each rod. These will prevent wind pressure from breaking your sheet(s). You may also drive nails all-around upper walls for extra strength.