How to build a greenhouse
STEP 8. The greenhouse cover
Cover the greenhouse with a UV-resistant polythene (there are other options also).
Hold the covering in place by laying thin battens (strips of wood) over the polythene (ensuring the polythene is taut) and nailing the battens to the greenhouse studs, roof rafters or any other solid member.
Cover the doors and windows also.
Most hardware merchants or garden suppliers only stock the standard plastic polythene that is not UV resistant but they should be able to advise you where to get the polythene required for the greenhouse covering.
There are also other options such as acrylic sheet.