Harcon FW1 Felt Weir To fit any vent it is necessary to cut the felt. If the felt is cut carefully in line with recommended fixing instructions moisture leakage through the cut should not occur. It is however good practice to fit a felt weir above any opening in the roof felt. The felt weir features ridges which guide any condensation or rainwater away from the opening in the felt. It helps to ensure that no moisture will penetrate the opening. THE FELT WEIR IS VERY SIMPLE TO INSTALL 1. Cut a slit in the felt, approximately 50mm above the batten. 2. Slide the weir under the batten directly above the opening in the roofing membrane, inserting top of weir into slit above the batten. 3. Nail the weir to the batten. 4. Nail membrane over the batten in the normal way.