Master Panel Coverage and Overlap Guide

Correct panel coverage and overlap planning is the most common source of material shortages, leaks, and installation failures in metal roofing. This guide explains how panel coverage works in real-world conditions — especially in wet Pacific Northwest climates.

Educational reference only. Always follow manufacturer instructions and local building code. Valley Outdoor Supply does not provide engineering or installation services.

What Panel Coverage Really Means

Usable coverage is the width of a panel after required overlap is applied. Panel width is not the same as usable coverage.

Example: A 26.5-inch corrugated panel typically provides only ~21.5 inches of usable coverage on a roof because corrugated roofing requires a double-rib overlap for waterproofing.

Planning based on raw panel width is the most common DIY and contractor mistake.

Corrugated Metal Panels — The Siphon Break Rule

Standard corrugated metal panels are manufactured at approximately 26.5 inches wide. Roofing applications require a two-rib overlap.

Why it matters: A double-rib overlap creates a siphon break. In heavy rain or wind-driven storms, water can wick uphill through surface tension if the seam does not include an air break.

The double-rib overlap intentionally interrupts that water path.

Effective Coverage for Corrugated Panels

• Nominal panel width: 26.5 inches
• Overlap required: ~5 inches (two ribs)
• Usable coverage: ~21.5 inches

This reduced coverage is intentional and required for leak prevention. Ignoring it increases seam leak risk and causes material shortages.

Painted Rib and 36-Inch Panels

Painted rib panels (often called R-panel, Pro-Rib, or Ag panel) use engineered interlocking ribs and a flatter profile.

When installed correctly, these panels provide a full 36 inches of effective coverage.

Why it is safe: Moisture control is built into the seam geometry. Anti-siphon features manage condensation and wind-driven water internally.

Manufacturer fastening and pitch requirements must still be followed to maintain watertight performance and warranty compliance.

Roof Pitch and End Laps

Panels should ideally run from ridge to eave in a single piece. When that is not possible, horizontal end laps are required.

Beginner rule: The lower the pitch, the harder it is for water to exit. Low slopes require longer end laps and sealing.

End laps are one of the most common “looks fine today, leaks later” failure points if not sealed.

Fastener Placement Logic (Theory)

Fastener placement varies by panel profile and application. Incorrect placement causes leaks, deformation, and warranty issues.

• Corrugated roofing: Fasten through the high rib to keep screws out of pooled water.
• Rib panels: Fasten in the flat next to the rib per manufacturer guidance.

Beginner check: If the washer is crushed flat or mushroomed out, the fastener is over-driven.

Polycarbonate Panels — Expansion Rules

Polycarbonate panels share the corrugated profile but expand and contract significantly with temperature.

Beginner warning: Tight screws and normal holes will restrain movement and cause cracking over time.

Closures, Ridge Caps, and Sealing

Corrugated profiles leave natural gaps that invite insects, rodents, wind-driven rain, and snow.

Closure strips and ridge caps are functional weather components, not decorative trim.

Proper installation includes profile-matched closures, correct overlaps, and sealing at all ridge and trim laps.