How to Cut Asphalt Shingles Cleanly and Safely with a Hook Blade
Cut from the back, cut straight, and keep the knife away from your leg. A few habits that separate clean edges from ragged ones.
- Beginner
- Part of every shingle job
- Updated
The short answer
Cut asphalt shingles with a hook blade utility knife, face down so the blade cuts the back of the shingle and the granules do not dull it. Mark the line with a straightedge or chalk, score through the mat, and snap or finish the cut. Cut warm shingles easily, cold ones carefully. Pull the knife toward a safe line, never toward your leg.
Every roof needs cuts: at rakes, hips, valleys, walls and around penetrations. A roofer who cuts clean, straight edges produces a roof that looks crisp and sheds water properly. One who tears and saws produces ragged edges, wasted shingles and, sooner or later, a blood-stained roof.
The tool is a hook blade utility knife. The hooked blade cuts on the pull stroke, which is why shingle cutting feels different to cutting cardboard. Used properly, it slices through the asphalt and mat without dulling quickly on the granules. Used badly, it skids, wanders off the line or comes back at your thigh.
This guide covers the technique, including cold and hot shingles, and the most common cuts. For the blade itself and how to set it up, see how to use a hook blade knife. The cuts are used when laying courses, at the rake and in valleys.
Tools
- Hook blade utility knife with spare blades
- Straightedge or a shingle as a guide
- Chalk line
- Tape measure
- Cut resistant gloves
- Tin snips for trimming around metal, where needed
Materials
- Scrap shingles for practice
- Spare hook blades
- Asphalt roof cement for sealing cut ends where the detail requires
Safety first
- A hook blade cuts toward you. Keep it pointed away from your legs, your other hand and your safety lanyard, and never cut on your knee.
- Change blades often. A dull blade takes more force and slips, and most cuts on roofs come from a dull blade.
- Keep the cut on a flat, stable surface, away from the roof edge, and stay tied off while you do it.
Step by Step
Step 1: Fit a sharp hook blade and set the depth
A new blade cuts cleanly and needs little force. Extend it just past the thickness of the shingle. Too much blade cuts into the course below or the underlayment, and cut underlayment leaks.
Keep spare blades in a pocket. Change when the cut starts to drag or tear. Laminated shingles are thick and dull blades quickly.
Step 2: Mark the cut line
Measure twice and mark the shingle on the back. For a rake trim, snap a chalk line along the roof. For a hip or valley, mark from the line on the roof, not from the adjacent shingle.
Use a straightedge for any cut that will show. A shingle laid on the roof and used as a guide gives a straight line without measuring.
Step 3: Cut from the back of the shingle
The back of the shingle is smooth and the blade slides. The face is covered in granules that blunt the edge fast. Cutting from the back also gives a cleaner top edge, because the granules are not torn out.
Lay the shingle on a flat surface, such as the deck or a bundle, and use the hook blade on the line. On a slope, put the shingle on the bundle or a board to keep it flat.
Step 4: Score and pull, or cut through in two passes
Pull the knife toward you along the straightedge, in a steady stroke, keeping your free hand back from the line. For laminated shingles, cut through in two passes, because the double layer is thick. For 3-tab shingles, one firm pass is usually enough.
You can also score deeply and snap the shingle over an edge. A clean score leaves a neat break. A shallow score tears and leaves a ragged edge.
Watch out: Do not cut toward your body or across your knee. If the blade slips, the hooked point goes straight into the leg.
Step 5: Adjust for warm and cold shingles
In summer heat, shingles are soft and the blade drags, so use a sharp blade and take care not to stretch the edge. In winter, shingles are brittle and can crack along the cut or snap at the wrong place. Warm the bundles in a heated space before cutting, and score slowly.
See how to shingle in cold weather and how to shingle in hot weather for more on how temperature changes the work.
Step 6: Make the common cuts: rake, valley, hip and penetrations
At the rake, leave the shingle long and trim after nailing, or cut the shingle to length before laying it. At a valley, cut the shingle along a chalk line a set distance from the valley centre, and cut the top corner off to keep water from running under. At a hip, cut along the hip line so the cap covers the cut.
Around a pipe or vent, cut a notch and fit the shingle over the flashing, so it laps correctly. See how to install a pipe boot. For shingles that meet a hip, see how to shingle a hip roof.
Step 7: Reuse the offcuts
The piece cut off the start of a course is often the right size for the start of the next one, so stack offcuts near where you work. Do not use narrow slivers at the edge of a course. A piece under about 4 inches wide is hard to nail and is prone to lifting.
Keep cut pieces flat and face up. Wipe off stray granules, which can be slippery underfoot.
Knife Safety Habits
Most knife injuries on a roof are avoidable. Keep these habits.
- Keep the blade retracted when it is not cutting, and in a holder when you are walking.
- Cut away from yourself and from your lanyard and rope.
- Wear cut resistant gloves on the hand that holds the shingle.
- Do not leave an exposed knife on the slope, where it can slide and cut someone below.
- Dispose of blades in a hard container, not loose in a bin.
Common Mistakes
- Cutting from the face. The granules dull the blade, and the edge tears and looks rough.
- Using a dull blade. It needs more force, wanders off the line and is how most cuts happen.
- Cutting too deep. A blade that cuts through the shingle into the underlayment below creates a leak path.
- Cutting cold shingles fast. They crack or snap off the line.
- Cutting on the knee or toward the body. The most common shingle knife injury.
- Using slivers. Narrow pieces at the rake or ends do not hold a nail and lift in wind.
When to Call a Professional
Cutting shingles is an easy skill and a fair one to learn on a garage or shed. If you are working at height with a knife and a steep slope, or the job has many valleys, hips and dormers, bring in a roofer. Cuts at those details are where most of the skill is.
If you cut yourself on a roof, stop work, get down safely and get the wound seen. A deep cut from a dirty blade needs proper care.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Replacement service or the towns we cover.
How to Cut Asphalt Shingles Cleanly: Questions
What is the best tool to cut asphalt shingles?
A utility knife with a hook blade is the standard tool. The hooked blade cuts on the pull stroke and tracks through the asphalt and mat well. Use a fresh blade, and change it often, because laminated shingles dull blades quickly. Tin snips are for the metal flashing, not for shingles.
Should you cut shingles from the front or back?
Cut from the back. Turn the shingle face down and cut along the line. The smooth back lets the blade slide, and the granules on the face do not dull it. You also get a cleaner top edge, since the granules are not pulled loose as the blade passes.
Can you cut shingles in cold weather?
Yes, but carefully. Cold shingles are stiff and can crack or break off the line. Keep bundles warm until needed, use a fresh blade and score slowly in two passes. If shingles still crack, stop and wait for warmer conditions, or use a heated space to condition the bundles.
How do you cut shingles straight?
Mark a line with a pencil or chalk, lay a straightedge on it, and run the hook blade along the edge from the back of the shingle. For long cuts, such as a rake, snap a chalk line on the roof and cut along it. Steady pressure in a single stroke gives a straighter cut than short, hesitant ones.
How do you cut shingles around a pipe?
Fit the pipe boot first, then lay the shingle over it and mark the pipe position on the back. Cut a notch or hole slightly larger than the pipe, so the shingle lies flat over the flange. The upper half of the boot should be covered by shingles, and the lower half should lap over them.
Related Guides
All Asphalt Shingles guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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