Internal corner trims are used to hide the 10mm expansion gap that should be left between two boards at the corner of the eaves. Internal corners are usually found at the foot of a valley on a roof.To enable the system to expand and contract correctly in a different range of temperatures, one side of any joint or corner on any PVC fascia installation should be glued to the board and the other side should be sealed with low modulus silicon, which allows movement within the joint. Internal corners can be cut to size with a standard hacksaw or fine toothed saw.