These clips are used to attach PVC to flat and other unique surfaces. The clips are designed to hold 1/2" PVC securely and like all of our other clips, has a relief to pass up to 18AWG 3 core wire through. The clips are pre-drilled and countersunk for #6 wood screws. I prefer the use SPAX branded screws which can be found at most home centers. The screw show in the picture is 1.25" in length. The clips are CNC cut out of 1/2" Marine Grade HDPE which is both UV stable as well as temperature resistant. The number of clips needed is solely dependent on the application. Horizontal installations will require more clips to prevent sagging.