I use the
Ventline V2049-01 for my 3 roof vents. To secure the cap to the flange I used a #6 x 1 inch stainless steel sheet metal screw with a #6 flat cut stainless steel washer under the screw's head. I like this vent a whole lot better than other vent designs.
http://ventline.com/i/u/6148923/f/RV_Lit/Plumbing_Caps_Flashing_11-11.pdfDurable Polyethylene with Chimassorb for Ultraviolet Ray Protection
Two-piece Construction
Fits 1¼" and 1½" Pipes
Accommodates Maximum 6/12 Pitch Roofs
Flange has Mounting Holes and a Wipe Seal Feature
Removable Cap Snaps On and Securely Fastens to Flange with Screw (not included)
Available Colors: White (fits 1¼" and 1½" pipe)
Colonial White (fits 1½" pipe)