If you are shingling your own roof, you will at some point encounter a need to trim or cut the shingles to a certain size or shape. This may entail cutting across the shingle in a straight line to match the line of the roof or it may require more difficult precision cuts such as ones that allow fitting around pipes or vents. No matter the cut required, there are several tools and techniques that can be employed to get the job done.
Under some conditions a circular saw can be used to cut straight lines on a stack of shingles. Typically this will happen when you need to trim an edge off several shingles at once, such as downsizing all the pieces that will go along the edge of the roof. You'll need to use a strong-tipped saw blade such as one designed for cutting through old nails, otherwise it will wear down extremely fast from cutting through the shingles. Use a chalk line to mark where you want the cut to be on a large stack of shingles. Then use the saw as you normally would to cut through the entire stack at once. Work quickly because allowing the blade to get too hot while resting against the shingles as it cuts through can start to melt the tar on the back of them and render them useless.
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For any shingle cutting that doesn't include an entire stack you'll want to skip the saw and opt instead for the common utility blade. These popular tools feature removable razor blades that can be retracted or extended to several lengths before locking in place for use. A standard straight blade used on the smooth side of the shingle will complete the cut, but you will find that you need to replace them every few shingles as they become dull extremely quickly on the rough material. Instead, use a hook blade in the knife to complete the cut more quickly and with better results. The hook blade can be purchased in packs just like a standard straight blade and will typically fit into any utility knife. This blade will allow you to pull it along the cut line and quickly trim pieces off the shingle, but it can still be difficult on more precise cuts that include curving.
Tin snips are very sharp, powerful scissors designed to cut through metal, but they can be equally effective for small, complex cuts in shingles. If you need to trim the shingle to fit around pipes or small vents, then tin snips are your best choice. A utility knife can work as well but you'll soon find it difficult to maneuver the knife around curved cuts without creating an uneven and jagged edge. Snips allow you to cut through the shingle in different curves and directions just as you would use regular scissors to cut through paper. Like with any blade, though, the rough finish on most shingles will quickly dull the blades of the snips. Because the snips are designed to power through heavy materials, even a dull blade can make a good cut but it will require some extra effort on your part.