6 Mil Polyethylene Sheet - Very wide
It is highly desirable to avoid having to join polyethylene sheets because it is a lot of effort and tape to keep the water out. It is much better to obtain a polyethylene sheet that is big enough to not need joins.
It is good to find a sheet that is resistant to tears and punctures.
It is good to have some elasticity so that it stretches a bit and keeps providing a water proof membrane if there is movement, rather than tearing.
It is desirable to have a polyethylene sheet that is marked in a checkerboard pattern so that you can keep it aligned as it goes into the footing trenches.
6 mil is the minimum thickness, but thicker eg 10 mil would be even better if it's affordable.
http://www.americover.com/6_mil_reinforced_clear_683_prd1.htm
60 foot wide $450 for 100' roll. String reinforced, so it will stretch a little without tearing.
They can do custom widths wider than 60'. I need about 75 foot wide (13+15+32+15=75). The length I need is 120 feet (13+15+60+15+12=115) (Need to recalculate these dimensions prior to ordering.)
PSR6CC-78’x120’. $636.48 . Weight is 270 lbs. About $640 + $140 shipping (from near San Diego).
Spec: http://www.americover.com/images_templ/link/Americover_PSR_6CC.pdf