12x 20kg cromapol
A high performance acrylic based roof coating containing fibres which can be used without a primer and has the ability to be applied in damp conditions on most surfaces.
Cromapol will waterproof instantly, even while wet, and will not wash off while curing.
Will withstand ponding and covers in only one coat which normally needs no primer

£50 each. Cheapest you'll find anywhere usually between £80-£100