Silage covers are available in a wide range of thicknesses, but it's impossible to compare the quality a material based simply on its thickness. Most silage covers are made from LDPE or LLDPE polyethylene plastic, or heavy duty, reinforced RPE. A few outlets offer covers made of synthetic rubber (EPDM). Each material has unique strength, durability, and weight characteristics which affect the min/max thickness at which it is offered.
Polyethylene plastics are usually found in thicknesses as little as 5 or 6 mils (1 mil = 0.001 inch). Reinforced polyethylene is typically 9-12 mils which are still remarkably lightweight. EPDM is a much heavier material, but it's only available in thicknesses around 33 mils (equivalent to 0.84 mm).
