Hello

I think you are looking for liquid applied roofing. It is recommended to cover over existing roofs that have reached the end of their life without the expense of removing the existing roof. Do some research and get some quotes. The following are a few links to get you started.

Good luck

Jo

Costs: Liquid roof waterproofing systems - Building

Recommended UK Websites: Liquid Applied Roofing: Liquid Applied Roofing Suppliers

http://www.polyroof.co.uk/store/Home.aspx