5 Costly Sheet Metal Roofing Mistakes GCs Can’t Afford to Make

Every general contractor knows that a metal roof job can make or break both reputation and profit. Unfortunately, a single small mistake—like the wrong fastener or a miscalculated panel—can lead to days of rework, unexpected costs, and unhappy clients. In this post, we’ll walk you through the five most common sheet metal roofing mistakes we see on job sites—and exactly how to avoid them.

1. Inaccurate Measurements & Templates

Why It Happens: Crews hand-measure long roof pans with chalk lines or tape measures, then transfer those dimensions to cardboard or plywood templates. One slip of the tape, and panels arrive too short or too long.

Consequences:

• Gaps between panels that let in water and wind

• Wasted material and extra labor to re‐cut or reorder

• Delayed inspections and signoffs

How to Prevent It:

1. Digital Takeoffs: Use software like Bluebeam or PlanSwift to pull measurements directly from your plans.

2. Laser Distance Tools: Bring a laser meter onsite to verify lengths—especially on irregular roof shapes.

3. Durable Templates: Build reusable plywood templates (instead of flimsy cardboard) and label them clearly for each roof plane.

4. Double-Check Protocol: Institute a “measure twice, cut once” rule: have two crew members confirm every critical dimension before cutting.

2. Wrong Fasteners & Flashing Details

Why It Happens: It’s easy to grab generic “roofing screws” off the shelf without checking coating type or length.

Consequences:

• Fastener corrosion in coastal or industrial environments

• Wind uplift or clip failure in storm zones

• Leaks at seams when flashing is cut or bent incorrectly

How to Prevent It:

• Fastener Selection Guide: Match screw material (stainless vs. galvanized), length, and washer type to your gauge and environment.

• Flashing Cheat Sheet: Keep a laminated detail sheet in your toolbox showing correct flashing profiles for valleys, eaves, ridges, and penetrations.

• Pre-Job Mock-Ups: Build and inspect a small flashing mock-up in the shop before going onsite to ensure bends and laps seal properly.

3. Skipping Thermal-Expansion Gaps

Why It Happens: Under pressure to finish quickly, crews sometimes butt-together panels tightly—forgetting that metal expands and contracts with temperature.

Consequences:

• Buckling or oil‐canning in hot weather

• Cracks and seam separation in cold snaps

• Creaking noises and compromised waterproofing

How to Prevent It:

1. Gap Calculation: Use the formula ΔL = α × L × ΔT (thermal expansion coefficient × panel length × temperature change) to determine required gaps.

2. Slip-Clip Systems: Install clips designed to allow controlled panel movement.

3. Field Jigs: Make a simple wood spacer (e.g., ⅛″ thickness) to maintain consistent gap spacing as crews lay panels.

4. Inadequate Sealants & Underlayments

Why It Happens: Cost-cutting drives the purchase of generic sealants and basic felt underlayments.

Consequences:

• Water infiltration at laps and fasteners

• Trapped moisture causing mold or corrosion under the metal

• Premature coating failures

How to Prevent It:

• Choose ASTM-rated, UV-stable sealants matched to your metal finish.

• Use breathable synthetic underlayments (minimum 5 perms) to allow moisture to escape.

• Apply sealant beads at all seams and verify coverage with a quick “tap test” in the shop.

5. Neglecting Post-Install Maintenance

Why It Happens: Contractors finish a project and move on—forgetting that metal roofs require periodic inspections.

Consequences:

• Loose or missing fasteners over time

• Sealant shrinkage leading to drip leaks

• Unspotted paint chips that become rust spots

How to Prevent It:

• Offer annual maintenance contracts with tiered services:

  1. Visual Inspection Only

  2. Fastener Torque Check & Seal Refresh

  3. Full Gutter & Flashing Re-seal

• Provide your client with a simple checklist and calendar reminders.

Conclusion & Next Steps

These five mistakes account for the majority of costly re-works and call-backs on sheet metal jobs. Download our Free QC Checklist for Metal Roof Installs to ensure your next project sails through inspections.