How Many Metal Roofing Panels Do I Need?

Metal roofing panels are not ordered by area. Each panel runs from the eave to the ridge in one piece, so you order a number of panels at a cut length. The count comes from the eave length and the panel’s coverage width; the length comes from the building width and the pitch. Area still matters for underlayment and for checking a quote, but it does not tell you how many panels to buy.

Coverage width is not the sheet width

A panel overlaps its neighbor, so it covers less than it measures across. Suppliers state the net figure as coverage width. Two examples from manufacturer literature:

• Exposed-fastener rib panel. MBCI lists its PBR panel with a 36 in coverage width, ribs 12 in apart and 1¼ in high, fastened through the panel.
• Standing seam. Western States Metal Roofing’s Western Lock snap-lock panel has a 1¾ in rib, with 16, 14 and 12 in listed as alternate widths and custom widths from 12 to 20 in.

Always count with the coverage width of the exact panel you are buying. A narrower panel means more panels, more seams and more clips or screws for the same roof.

Step 1: panels along the eave

Measure the eave from rake to rake, including overhangs, convert to inches and divide by the coverage width. Round up.

Panels per slope = round up(eave length in inches ÷ coverage width)

The example roof sits on a 40 × 30 ft building with 1 ft overhangs, so each eave is 42 ft, or 504 in. With 36 in panels that is 14.00, so 14 panels per slope and 28 for the two slopes of the gable roof. When the division does not come out even, the last panel on each slope is cut lengthwise, or the layout is shifted so that both rakes end on a similar width. Either way you buy the whole panel.

Step 2: panel length along the slope

The panel has to reach from the eave edge to the ridge. That distance is the level run of one slope, including the eave overhang, times the pitch multiplier.

Panel length = (building width ÷ 2 + overhang) × √(1 + (pitch ÷ 12)²)

For the example the run is 16 ft and the multiplier for 6:12 is 1.1180, giving 17.89 ft, about 17 ft 11 in. Where panels are cut to order, the figure you give the supplier matters. Whether to add length for an eave overhang into the gutter or deduct for a ridge gap depends on the trim details of the system; follow the manufacturer’s installation guide and have the supplier confirm the cut length before production.

Going from 36 in to 16 in coverage raises the count from 28 to 64 panels for the same roof. The lineal footage of panel rises by the same ratio.

A shed roof example

A shed roof has one slope, so the panel runs the full building width. For a 20 × 12 ft porch roof with 6 in overhangs at 3:12, the eave is 21 ft and the run is 13 ft. With 36 in coverage that is 7 panels at 13 ft 5 in.

Check the slope before choosing a panel

Metal panels have minimum slopes that depend on how the seams are made. The model residential code text, as adopted in California, sets 3:12 for lapped, nonsoldered seams without lap sealant, ½:12 for the same seams with applied lap sealant, and ¼:12 for standing-seam systems. Manufacturers publish their own limits per product, which can be stricter or, with sealant and specific details, lower: MBCI lists ½:12 for PBR, and the Western Lock data sheet recommends 3:12 and asks buyers to inquire for lower pitches. The porch example at 3:12 sits right at the code line for unsealed laps, so the product’s own minimum decides.

Hips, valleys and other shapes

On a hip roof the panels on each face get shorter toward the hips, and every one is cut at an angle. Counting them by formula wastes material or leaves you short. Ask the supplier for a cut list made from a roof drawing, with a length for every panel. The same applies to valleys and dormers. What you can prepare is an accurate drawing: the footprint and overhangs of each section, the pitch, and the positions of ridges, hips and valleys. The roof area calculator gives ridge and hip lengths for rectangular hip sections, which is a useful check on the supplier’s list.

What else goes on the order

• Trim. Eave trim, rake trim and ridge cap are sold in fixed lengths and overlapped. Count them from the eave, rake and ridge lengths.
• Fasteners or clips. Spacing comes from the manufacturer’s tables for your panel, purlin or deck spacing and wind zone. Do not estimate these from area alone.
• Closures and sealant. Foam closure strips at eave and ridge match the panel profile and are counted by eave and ridge length.
• Underlayment. This one is by area. Use the sloped area from the calculator and your roll’s effective coverage; the underlayment guide shows how.