I've reviewed roughly 300+ roofing installations in Q3 2024 alone. (Note to self: I really should automate part of this process.) One thing that consistently messes up what should be a clean job is how people handle window snips—specifically, how to snip on windows when installing shingles or flashing around them. It's not complicated, but it's one of those details where the difference between a pro finish and a leak waiting to happen is about a quarter-inch.
This checklist is for anyone doing roof work near windows—whether you're installing Owens Corning shingles around a dormer, setting up an outdoor shower enclosure, or flashing a skylight. If you're looking for a step-by-step that covers the how without the sales pitch, here you go.
Step 1: Understand the Material You're Cutting
You'd be surprised how many people start snipping without checking what they're cutting. I know I did when I started. Grabbed tin snips for everything because 'metal's metal,' right? Wrong. In Q1 2024, we rejected a batch of 80 custom-drip edges because the contractor used the wrong snip type and the edges were feathered instead of clean. The redo cost us about $1,200 in labor and material, plus the delay.
So, first: identify the material.
- Aluminum trim coil: Use aviation snips (left-cut or right-cut, depending on direction). Don't use tin snips, which are for softer metals.
- Steel flashing: Need heavier-duty snips, often with a compound action for leverage.
- Vinyl or plastic window frames: A fine-tooth saw or utility knife is better than snips, which can crack the material.
- Milk glass (like old window panes): You'll need a glass cutter, not snips. (That's a whole different process, but worth noting if you're restoring an older property.)
Check the manufacturer specs. For example, Owens Corning's installation guidelines for their shingles (which you might use around that window) clearly state the recommended cutting tools for different underlayment and flashing materials. It's boring, but ignoring it is how you get a call from a guy like me.
Step 2: Measure Twice, Snip Once (But Measure in the Right Place)
Obvious advice, right? But everyone says 'measure twice,' and then they measure from the wrong reference point.
Here's the nuance I've learned from inspecting work around the Owens Corning Denver roofing plant's local supply chain: when you're snipping around a window, don't measure from the glass. Measure from the edge of the window frame's nail fin (or the outermost flange, if it's a retrofit). The glass moves with thermal expansion. The frame is your reference.
Honestly, I see this mistake on about 1 in 5 installs we audit. They cut a piece, it fits fine at 9 AM, and by 2 PM, it's binding against the glass because they didn't account for expansion or they measured to the wrong line.
Step 3: Control Your Snip Entry Point (This Is the Secret)
Most tutorials tell you 'start at the edge and cut along the line.' Fine. But the difference between a guy who can snip and a guy who gets it right every time is where the blades make contact first.
- For straight cuts: Start at the far edge of the cut line and pull the snips toward you. This keeps the metal from curling up and binding in the blades.
- For curved cuts (around a window corner): Make several relief cuts into the curve first (cutting to the line, but not past it). Then, snip the curve in small bites. Trying to cut a smooth arc in one pass with straight snips is what causes that jagged edge.
- For inside cuts (like cutting a hole for an outdoor shower's drain pipe through a wall panel): Drill a starter hole, then use the snips from the interior of that hole outward. Don't try to start from the edge of the panel—you'll overshoot.
I ran a blind test with our installation crew in late 2023: same window, same flashing, one group using relief cuts and one group trying continuous cuts. Every inspector (including me) identified the continuous-cut samples as 'amateur' without knowing which was which. The difference was maybe 12 minutes of extra work per window.
Step 4: Avoid 'Shiny Edges' (The Harbor Blue Test)
If you're working with colored materials—like Harbor Blue Owens Corning shingles or pre-painted trim—the cut edge is vulnerable. The snip exposes bare metal.
I've seen homeowners (and some pros, honestly) assume the paint will last. It won't. Especially near windows, where condensation forms and drips directly onto that cut edge. In one inspection near the Owens Corning Denver roofing plant area, we had a case where 8,000 units of painted trim developed edge rust because the installer didn't seal the snipped edges. That was a $22,000 redo—partly due to material cost and partly because the windows were already installed, and they had to be protected during the fix.
So, after you snip: metal file + touch-up paint or sealant. It's a 2-minute step. It saves years of corrosion.
Step 5: Manage the Burr (Not Just the Sharp Edge)
People deburr edges so they don't cut themselves. Good start. But what about the burr on the back side? The one you can't see?
When you snip, the blades create a tiny raised edge on the underside of the material. If that material is going against a window frame (or a wall, or another piece of roof deck), that burr acts like a pry bar. Over time—especially with thermal cycling—it can scratch painted surfaces, dimple the soft aluminum of the window frame, or even create a gap where water can enter.
After deburring the edges you can see, flip the piece over and file or sand the opposite side. Most people skip this. I've made it a mandatory check in our Q2 2025 quality protocols.
Step 6: Test-Fit (Don't Force It)
If you have to force the snipped piece into place, you cut it wrong. Period. Doesn't matter if it's within tolerance. If it doesn't seat without pressure, it's going to buckle under thermal stress or wind load.
When I specify for our large orders—like the 50,000-unit annual orders we manage—we require a test-fit tolerance of 1/16-inch clearance. Our contracts now state this explicitly. Because when a piece is too tight, it doesn't fail today. It fails next winter, when that first big snow puts pressure on the roof, and the window frame shifts slightly, and that tight-fit snip edge now pops off or cracks the sealant.
Step 7: Seal Before Fastening (Not After)
Another one I see constantly: they snip the piece, fit it, screw it down, and then try to run a bead of sealant around the cut edge. But with a fastener already installed, the sealant can't flow behind the piece. It's cosmetic only.
Snip, deburr, test-fit, remove, apply sealant to the back of the cut edge, then fasten. The fastener compresses the sealant out, filling any micro-gaps the snip created.
In our Q1 2024 quality audit, we found that 60% of first-time deliveries from one vendor had the sealant applied post-fastener. It took a 15-minute fix per unit, but on a 200-unit order, that's 50 hours of extra labor—all of which should have been part of the initial install.
Step 8: Know When to Use Something Other Than Snips
The vendor who says 'we can snip that' when they really can't? I don't trust them. And I've been at this long enough to value honesty over convenience.
For example, cutting milk glass for a custom window pane? Snips will shatter it. You need a glass cutter and a straightedge. Cutting a PVC outdoor shower enclosure? Snips will leave a feathered edge that collects mildew. Use a fine-tooth saw instead.
The vendor who said 'this isn't our strength—here's who does it better' earned my trust for everything else. I'd rather work with a specialist who knows their limits than a generalist who overpromises.
Common Mistakes I've Seen On-Site
- Using the same snips for everything: Cutting aluminum right after cutting steel transfers burrs and dulls the blades faster.
- Not marking the cutting line clearly: A sharp pencil or a scribe is better than a marker that can scrape off. (Prices for a good scribe ~$5-10 as of January 2025; verify current pricing at your local hardware store.)
- Forgetting eye protection: The tiny metal shards from a snip cut? They go exactly where you least want them—into your eye.
- Thinking 'sharp' means 'finished': A sharp edge isn't a finished edge. Deburr. Seal. Test. Repeat.
Sniping on windows isn't rocket science. But it's one of those things that separates a job that passes inspection from a job that gets a call from a quality inspector like me. (And trust me, you don't want that call.)
Prices and processes referenced are as of 2024-2025. Always verify current manufacturer guidelines for your specific materials.
Leave a Reply