How to Choose and Use Tin Snips for Flashing and Roofing Metal
Red cuts left, green cuts right, yellow cuts straight. Match the snip to the cut and the metal, and keep the blades sharp.
- Beginner
- Learn in under an hour
- Updated
The short answer
Aviation-style tin snips are colour coded: red handles cut left, green handles cut right and yellow handles cut straight. Choose the snip that keeps the waste curling away from the cut, open the jaws wide, cut in long strokes and never close the jaws completely, so the edge stays smooth without notches. Match the snip to the metal's thickness.
Tin snips are the most used cutting tool in metal roofing and flashing. A good pair cuts prefinished steel and aluminum cleanly with little effort, and a pair in poor shape tears the metal, chips the paint and leaves a wavy edge that shows on the roof. Most roofers carry several types, because one pair cannot do everything.
The most common type is the aviation snip, which has compound leverage and handles that are colour coded for the direction they cut. This guide explains the colours, how to choose the right snip, how to make straight and curved cuts, and how to keep the blades sharp. For long cuts in roofing panels, see how to cut metal roofing with shears and nibblers and how to cut metal roof panels. To bend what you have cut, see how to use hand seamers.
Tools
- Left cut (red) aviation snips
- Right cut (green) aviation snips
- Straight cut (yellow) aviation snips
- Offset snips
- Compound action snips for heavier metal
- Cut-resistant gloves and safety glasses
- Permanent marker and square
Materials
- Prefinished steel, aluminum or galvanized flashing
- Scrap metal for practice cuts
- Light oil for pivots and blades
- Fine sharpening stone or file
Safety first
- Cut edges of sheet metal are razor sharp, and the waste strips can cut as deeply as a blade. Wear cut-resistant gloves and safety glasses, and handle offcuts with care.
- Do not use snips on metal that is too thick for them, or on hardened materials such as roofing nails, screws or wire. That damages the blades and can chip metal into your eyes.
- Dispose of offcuts in a container, not loose on the roof. Slivers of metal cut feet and rust into stains.
- Keep fingers clear of the blades and do not hold the workpiece near the cut with your bare hand.
Step by Step
Step 1: Learn the handle colours
The colour tells you which way the snip curves the waste. A left cut (red) snip cuts a straight line or a curve to the left, and pushes the waste to the right of the blade, out of the way. A right cut (green) snip does the opposite. A straight cut (yellow) snip cuts straight lines and gentle curves in either direction, but has less room for the waste.
If you cut with the wrong snip, the waste strip presses against the blade, and the cut binds. That is when you get a ragged edge.
Step 2: Match the snip to the metal
Aviation snips suit sheet up to about the thickness of standard roofing flashing. Thicker material, such as heavy gauge steel, stainless or copper, needs heavy-duty snips or shears. Ribbed or corrugated panels are better cut with a power shear or a nibbler, see how to cut metal roofing with shears and nibblers. Check the snip's label for its maximum capacity.
Offset snips, with handles bent away from the blade, keep your hand above the metal so your knuckles do not scrape the surface on long straight cuts.
Step 3: Mark the cut line
A clear line makes a straight cut. Use a long straightedge or a chalk line for long cuts, and a square for cuts across the width. Mark the waste side so you can see which part to discard. On prefinished metal, mark on the back side, or use a soft marker that wipes off.
Step 4: Open the jaws and start the cut
Put the metal deep in the jaws, close to the pivot, where the leverage is greatest. Keep the lower blade flat against the metal and cut on the line, not on the waste side of it. The waste should curl away from the cut as it goes.
Hold the sheet firmly or clamp it. A sheet that flops as you cut moves the blades and tears the metal.
Step 5: Cut with the back two thirds of the blade, and do not close it fully
Closing the tips of the blades all the way creates a small nick in the edge of the cut, and these nicks can start cracks and leave burrs. Stop with the tips still slightly apart, slide the snips forward, and start the next stroke. A rhythm of long, steady strokes gives an even edge and is easier on your hands.
Step 6: Cut curves and corners
For a curve to the left, use a red snip. For a curve to the right, use green. Make a series of short cuts in the waste area first, called relief cuts, to free the metal so the blades can follow the curve without binding. For an inside corner, drill a small hole at the corner before cutting so the stress does not crack the metal. Round the outside corners slightly to avoid sharp points.
On pipe flashing or other details where you cut a hole, use a hole punch or a drill then snips to work out from it.
Step 7: Deburr and inspect the edge
A burr is a tiny sharp lip left by the blades. On prefinished metal, it can also remove the paint and start rust. Dress the edge lightly with a fine file, and touch up bare metal with compatible paint if the manufacturer allows it. Replace any piece with a cracked or badly wavy edge.
Step 8: Keep the blades sharp and the pivot oiled
Dull blades crumple the metal instead of cutting it. Sharpen along the factory bevel with a fine stone, taking off as little metal as possible. Adjust the pivot bolt so the blades meet with light contact and no play. Never cut wire, screws or nails with snips made for sheet metal, since those nick the blades and make every later cut worse.
The Main Types of Snips
- Aviation snips: compound leverage, colour coded, good for most flashing and trim.
- Offset snips: bent handles that keep your hand off the metal, good for long straight cuts.
- Straight snips (tin snips): simple, strong blades for heavier material, but with little leverage.
- Duckbill snips: wide blades for straight cuts and curves in thin metal.
- Compound action snips: extra leverage for heavier gauge metal.
Cutting Prefinished Metal
Prefinished steel and aluminum have a paint film that chips at a rough cut edge. Use sharp snips and smooth strokes, cut from the back if possible, and avoid dragging the snips along the surface. Abrasive cut-off wheels are not recommended because the heat and sparks damage the paint and leave bare steel that rusts.
Common Mistakes
- Using the wrong colour. The waste jams against the blade and tears the cut.
- Closing the blades completely at each stroke, which leaves nicks in the edge.
- Cutting on the waste side of the line so the piece comes out too small.
- Using snips on wire or screws. It chips the blades.
- Working with dull blades. They crumple the metal and tire your hand.
- Leaving offcuts on the roof where they cut and rust.
When to Call a Professional
If you are cutting more than a few feet of flashing, working with heavy gauge metal, or cutting ribbed or standing seam panels, a power tool or a sheet metal shop will do it faster and cleaner. A crooked or wavy cut is very obvious on a finished roof.
Any time a piece has to fit precisely around a chimney, valley or wall, have the cut made by someone with experience. A mistake in prefinished metal is costly.
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 Choose and Use Tin Snips: Questions
What do the red, green and yellow tin snips mean?
The handle colour shows the direction the snip cuts. Red cuts to the left, green cuts to the right, and yellow cuts straight. Pick the colour so the waste curls away from the blade, which stops it binding. Many roofers carry all three because it makes cuts cleaner and faster.
Can you cut metal roofing with tin snips?
You can cut thin flat metal, flashing and trim with snips, and short cuts on panels. For long cuts or ribbed panels, power shears or a nibbler are faster and leave a cleaner edge. Snips also distort ribs and can be tiring for long cuts. Do not use an abrasive wheel on prefinished metal.
How do you cut a curve with aviation snips?
Use the snip that matches the direction of the curve: red for a curve to the left and green for a curve to the right. Make a few short relief cuts in the waste area first, cut in small smooth strokes and follow the line gradually. Do not close the jaws fully, and drill a hole for sharp inside corners.
How do you sharpen tin snips?
Open the snips, rub a fine sharpening stone along the factory bevel of each blade with light strokes, and remove burrs from the inside face. Keep the original angle and take off as little metal as possible. Adjust the pivot bolt so the blades meet with light contact, and put a drop of oil on the pivot.
What is the difference between tin snips and aviation snips?
Traditional tin snips are simple scissor-style cutters with long handles, strong but with little leverage. Aviation snips have compound leverage, which multiplies your hand force, and they are colour coded for the cutting direction. Most roofers prefer aviation snips for flashing and trim because they cut cleanly with less effort.
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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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