These corrosion-proof 18" PVC Rain Caps are for the construction of exhaust ventilation systems. PVC material compounds used in the manufacture of these duct fittings conform to PVC 1120, ASTM D-1784, Type 1, Grade 1, Cell Class 12454-B. Style B rain caps are designed to provide zero pressure loss. Basically there is nothing in the way to obstruct the flow of air thru the cap while still preventing rain and snow from entering the system. Overall dimensions are 20" diameter and 84.5" long. Socket depth is 2.5". NOTES: Fittings are triple bead welded. Pictures are to represent style of fittings only; color is gray.