Those of you who do use polythene already will know it gives you a bit more time for your project because it prevents the water from soaking into the dry sub-base. Some people will wet the area down with a hosepipe, but that never totally saturates the sub-base and consequently will never be as efficient as putting down a layer of polythene. Also, wetting the base down will inevitably produce a base that is uneven in terms of wetness and dryness and consequently will produce a variable base onto which the concrete is laid, which will in turn affect the concrete surface too.

Other benefits of laying on polythene are reducing the chance of cracking in the short term (by reducing water loss and/or uneven water loss) and in the long term as the polythene acts as a ‘slip membrane’. Since water is unable to soak into the sub-base, there is more water available to keep the surface moist and reduce the chance of opening fissures resulting from the printing process.

Dr Concrete