Figuring metal roofing comes down to three numbers: how many panels you need, how long each one is, and how much trim finishes the edges. Unlike shingles, which you order by the square, metal panels are cut to length and counted by coverage width, so the math is a little different. The good news is it’s simple once you see it laid out.

Here’s the step-by-step method we’d use on a simple gable roof, with a worked example you can follow along with.

What You Need Before You Start Figuring Metal Roofing

• Building footprint: width and length of the walls.
• Overhangs: how far the roof extends past the walls at the eaves and at the gable ends.
• Roof pitch: rise over run, like 4/12 or 6/12.
• Panel coverage width: the net width each panel covers once it’s lapped or seamed to the next one. This is on your quote, and it’s different from the panel’s full width.

If you don’t know your pitch yet, our roof pitch chart shows how to measure it and gives you the slope multiplier for every common pitch.

Step 1: Find the Panel Length

Roof panels run from the eave up to the ridge, so each one has to cover the horizontal run plus the overhang, measured along the slope.

Panel length = (half the building width + eave overhang) × slope multiplier

On a gable roof, half the building width is the horizontal run of each side. The slope multiplier converts that flat distance into the real length up the slope (it’s 1.118 for a 6/12, for example).

The math gets you close, but measure the finished framing before you order. Rafters and trusses are rarely perfect, and custom length metal roofing panels are cut to the number you give us, so a tape measure on the real roof beats a calculator.

Step 2: Count the Panels

Panels lay side by side along the eave, so the count depends on how long the eave is and how much each panel covers.

Panels per side = eave length ÷ coverage width, rounded up

Eave length is the building length plus both gable overhangs. Round up to the next whole panel, because a partial run at the end still needs a full panel ripped down to fit. A gable roof has two sides, so double it.

Step 3: Figure the Trim

Trim is measured in linear feet along the edges, not by area. On a simple gable roof:

Trim comes in set stick lengths and overlaps at the joints, so ask for the stick length on your quote and add enough for the laps. Our rundown of metal trim pieces explains what each one does and where it goes.

A Worked Example: 30 by 40 Shop at 6/12

Say you’re roofing a 30 by 40 foot shop with a 6/12 pitch and 1 foot overhangs all around. For the example, we’ll assume a 36 inch coverage width. Use the coverage on your own quote, since it varies by profile.

• Run per side: 15 feet (half of 30) plus 1 foot of overhang = 16 feet.
• Panel length: 16 × 1.118 = 17.89 feet, or about 17 feet 11 inches.
• Eave length: 40 feet plus two 1 foot overhangs = 42 feet.
• Panels per side: 42 feet ÷ 3 feet = 14 panels.
• Total panels: 14 × 2 = 28 panels at about 17 feet 11 inches each.
• Roof area: 32 × 42 × 1.118 = about 1,503 square feet. Use this for underlayment.
• Trim: 42 feet of ridge cap, 84 feet of eave trim, and about 72 feet of rake trim, plus laps.

That’s the whole order for a simple gable, minus fasteners and closures.

What About Screws, Underlayment, and Closures?

Screws: Fastener count depends on the panel profile, your purlin or batten spacing, and the manufacturer’s screw pattern, so don’t guess at it from roof area. Our metal roofing fasteners and screws guide covers how to pick the right ones.

Underlayment: Order it off the sloped roof area from the multiplier, plus overlap. See our guide to metal roofing underlayment for the options.

Closures: Exposed fastener panels need closures at the eave and ridge to seal the gaps under the ribs. Figure them by eave and ridge length and match them to your panel profile.

Common Mistakes When Figuring Metal Roofing

• Using the full panel width instead of the coverage width. Panels lap, so the full width overstates what each one covers and leaves you short.
• Forgetting the overhangs. They add to both the panel length and the eave length.
• Skipping the slope multiplier. Ordering panels at the horizontal run gets you panels that stop short of the eave.
• Treating a hip roof like a gable. Hips, valleys, and dormers mean angled cuts and panels of many lengths. Send us your plans and we’ll figure those for you.

Footprint also changes how cleanly panels lay out. Our guide to standard pole barn sizes covers how width and length affect the panel order.

FAQ

How do you calculate how many metal roofing panels you need?

Divide the eave length (overhangs included) by the panel’s coverage width, round up, and multiply by the number of roof planes. A simple gable has two.

How do I figure the length of metal roofing panels?

Add the eave overhang to the horizontal run, then multiply by the slope multiplier for your pitch. Measure the real roof before ordering, since framing is rarely exact.

Should I order extra panels?

On a simple gable with custom cut panels, you don’t need much extra, since every panel is already cut to fit. On hips and valleys, the angled cuts create waste, so plan on more.

The Bottom Line

Figuring metal roofing is three steps: panel length from the run and the slope multiplier, panel count from the eave length and the coverage width, and trim from the edge lengths. Do that math, check it against a tape measure on the real framing, and you’ve got an order you can trust.

If you’d rather have us check your numbers, give us a call or request a quote with your dimensions and pitch. We’ll lay out the panels and trim for you and send free color samples while we’re at it.