This is a walkthrough in how to cut asphalt roof shingles. Whenever you are installing or repairing asphalt roof shingles, you will probably be cutting them to fit. There are other methods, but this page is dedicated to cutting with a utility knife. With a straight blade or a hooked blade, you can make your cuts like a pro. Remember, the warmer the shingles, the easier they cut. This can be a good thing, or a bad thing.
How to Cut Asphalt Roof Shingles with a Straight Blade
There are times when a straight blade is used to cut asphalt roof shingles. When the shingle is not nailed, and you can turn the shingle upside down to cut it. You cannot cut on the granule or top side of the shingle with a straight blade.
Take the shingle turn it upside down by placing the side you want to cut over to the side you are butting into the other shingle. So your only flipping the shingle from right to left or left to right, not also flipping up or down. Now line up the side of the shingle you don’t want to cut with the point of the edge that you want to cut to. Now back at the shingle that you are butting up against, cut at that point. You can use a square or tape measure to mark if needed. You may want to give a little or take a little depending on where you want your cut to end up. Now put the side that you just cut over to the edge. It should now line up with the edge. You can trim more off, but you can’t add to it, so take your time and make a few cuts before you get in a hurry.
How to Cut Asphalt Roof Shingles with a Hooked Blade
With a hooked blade you can cut on top or the granule side of asphalt roof shingles. There are two ways to do this. One is to make a cut a row at a time, the other is to lay several rows and then pop a line and cut to the line. Laying one row at a time and cutting can sometimes leave you with a crooked rake or valley. If you have dripedge on the rake, then you have a mark to cut to each time. If not, then popping a line will do the trick. W-valley can go either way, cut each one or pop a line. If you are holding your shingles out of the valley or doing a snow valley (tapered) then popping a line is a must. Half-lace or California weave valley, you can pop a line after your first layer goes in, or wait until you lay the second layer and pop a line and cut after.
Blue chalk will wash away. Red chalk will not wash away. I have seen red chalk lines on an asphalt shingle roof 20 years after.
Either way, a hooked blade works a little differently than a straight blade. A hooked blade you hook at the starting point and pull towards you. You don’t push down like a straight blade. Also, you don’t want to cut into the layer of shingles underneath, especially in a valley. Architectural or Dimensional shingles have two layers at the bottom and you can usually only cut one layer at a time (unless it is real hot).
When dealing with Architectural or Dimensional shingles, the tar strip is on the bottom of the shingles. Don’t let the shingles lay there and get hot. Cut them immediately. If it is too long in between, pop a line and cut what you have laid. The shingles will stick down and leave tar strip residue on the finished roof below. You could put something down, like shingle wrappers to insulate and protect your finished roof.