Corrugated fiberglass or PVC roofing, which can be translucent or opaque, works well for greenhouses, sheds and porch additions. It comes in 4- by 8- or 10-foot sheets, and you install it by screwing it to wooden or plastic braces. The braces are sawn to match the corrugations and attached to the roof rafters. You can cut corrugated plastic roofing with a circular saw and a carbide blade, but you must secure it well before cutting. The vibrations of the saw make it wobble, and the wobbling can bind the saw or force it off the cut line.
Place the piece of corrugated roofing you're going to cut on a flat surface, and measure the length or width you need to cut with a tape measure. Make marks on both sides or ends of the sheet with a felt marker, and draw a line between them, using a straightedge.
Place the sheet over the edge of the flat surface so that the line you marked is about an inch from the edge. If more than about 3 feet of material are hanging on the off-cut side, get a helper to support it while you cut. Both you and your helper should be wearing goggles.
Set the blade of a circular saw to its full cutting depth. The blade should be a general-purpose, carbide-tipped one. Hold the blade guard open with one hand, start the saw and begin cutting along the line.
Release the guard as soon as the saw has gone far enough for the guard to rest on top of the material you're cutting. Use your free hand to press the roofing against the flat surface while you continue the cut. This minimizes vibrations, which can cause chipping, binding and wandering.
Cut slowly and steadily until you reach the other end of the line. Continue cutting past the end of the sheet and let the off-cut fall freely. If someone is supporting the off-cut, that person should simply allow the cut edge to fall before pulling the material away.