Choosing the right panel length is one of the most important decisions in a metal roofing project. The right length minimizes waste, eliminates horizontal laps, and simplifies installation. The wrong length creates extra work, potential leak points, and wasted material. Here’s how to choose correctly for any project.

The Basic Rule: Measure Your Roof Run

The roof run is the distance from the eave edge to the ridge, measured along the slope (not the horizontal projection). Choose the standard panel length equal to or slightly longer than your run. The panel should overhang the eave by 1–1.5 inches at the bottom; any excess at the ridge can be trimmed to fit.

A panel that’s 2–3 inches longer than your run is ideal — you get the eave overhang and don’t need to worry about trimming at the ridge. A panel that’s 6+ inches longer will need significant trimming, creating waste and more cut-edge treatment work.

Standard Lengths and Typical Applications

• 6ft — small garden sheds (5ft roof runs), accent walls, dog kennels, short carport sections
• 8ft — standard shed roofs (7ft runs), small garages, kennels with overhangs
• 10ft — medium garages, workshops, carports; covers runs up to 9.5ft
• 12ft — most common residential garage length; covers runs up to 11.5ft
• 14ft — steeper-pitched residential roofs, barn lean-tos, tall garages
• 16ft — two-story structures, deep agricultural bays
• 18ft — large agricultural buildings, deep commercial bays
• 20ft — commercial warehouses, very large agricultural structures
• 22ft — maximum standard length; covers most single-run applications without horizontal laps

When to Use Horizontal Laps

If your roof run exceeds 22ft, you’ll need to lap panels horizontally. Overlap sections by at least 6 inches, with the upper panel over the lower. Seal the lap with butyl tape before closing. Each horizontal lap is a potential point of water infiltration if not properly sealed — minimize laps by using the longest available panel that covers as much of the run as possible.

Calculating Panel Quantity

Each panel covers 33 inches of width after overlap. Divide your roof width in inches by 33 to get the number of panels per row. Round up to the next whole number. Add 5–10% for waste and trimming at the rake edges. For a gable roof, multiply by 2 for both sides.

Frequently Asked Questions

What if my run falls between standard lengths?

Always choose the next longer standard length. You’ll trim the bottom edge at the eave (or the top edge at the ridge) to fit. The small amount of trim waste is less costly than trying to use a panel that’s marginally too short and won’t provide adequate eave overhang.

Can I order custom lengths?

Our standard inventory includes lengths from 6ft to 22ft in 2ft increments. For projects requiring non-standard lengths, call 916-940-5700 to discuss options.