This Aluminum Wall Strap with 4" Inner Diameter from the Round Gas Vent series is a Type B gas component and consists of the duralock system for a tight seal. Use to provide support for offsets and horizontal runs. Recommended every 4' to support vertical common vent. Use also with DuraConnect or DuraConnect II. 10" Type B gas vent (11" OD) uses a 7" DuraPlus Chimney Wall Strap. Wall Strap legs can be rotated 360 to conceal screw.