What the calculator assumes
A simple gable roof: two flat slopes that meet at a ridge. The panels are ribbed, screw-down panels, and each one runs in one piece from the eave up to the ridge. The eave is the low edge of each slope. The gables are the sloped edges at the two ends of the house.
How it works
- Eave length = building length + 2 overhangs
- Panels on each side = eave length ÷ panel coverage width, rounded up
- Slope length = (half the building width + overhang) × pitch multiplier
- Panel length = slope length + the lip past the eave
- Screws = roof area in squares × screws per square
- Trim pieces = length to cover ÷ (piece length − overlap), rounded up
Example: a 40 by 30 foot house with 12 inch overhangs and a 6 in 12 roof. Each eave is 40 + 2 = 42 feet, which is 504 inches. 504 ÷ 36 = 14 panels on each side, so 28 in all. The slope length is (15 + 1) × 1.118 = 17.89 feet, which is 17 ft 10 11/16 in. Add the 1 inch lip and each panel is 17 ft 11 11/16 in. The roof area is 2 × 42 × 17.97 = 1,510 square feet, or 15.1 squares, so screws are 15.1 × 80 = 1,208. A 10 ft 6 in trim piece covers 10 feet after a 6 inch overlap. The ridge is 42 feet, and 42 ÷ 10 = 4.2, so 5 ridge caps. The two eaves add up to 84 feet, so 9 pieces of eave trim. The four gable edges add up to 4 × 17.89 = 71.6 feet, so 8 pieces of gable trim.
Round the panel length up to the next full inch when you order.
Coverage width is not the sheet width
Each panel laps over the edge rib of the one beside it, so it covers less than its full width. A Fabral Grandrib 3 sheet is 37 3/4 inches wide and covers 36 inches. Metal Sales lists 36 inches of coverage for Classic Rib and Pro-Panel II, and 24 inches for Delta Rib and 5V-Crimp. Fabral's Horizon hidden fastener panels cover 16 inches. Use the coverage number on the spec sheet for your panel.
Screws
Metal Sales says to plan on about 80 screws per square for 3 foot wide panels and 40 per square for 2 foot wide panels. Fabral also recommends 80 per square for Grandrib 3. That count goes with rows of screws 24 inches apart. Closer rows take more. Fabral lists 110 per square with rows 18 inches apart and 160 with rows 12 inches apart. Metal Sales notes that high wind areas may need the closer spacing.
These counts are for the screws that hold the panels down. On a roof, Metal Sales also shows a short stitch screw every 12 inches along each side lap, where one panel overlaps the next. Trim takes its own screws too. The fastening pattern in the guide for your panel is the one to follow.
Trim and closure strips
Fabral makes its trim in 10 ft 6 in pieces and says to lap the ends by at least 6 inches, so each piece covers 10 feet. Other makers and local shops use other lengths. Change the trim length above to match what you can buy.
Foam closure strips fill the gaps under the ribs. Inside closures sit under the panels at the eave. Outside closures sit on top of the panels, under the ridge cap. Each strip is as long as one panel is wide, so the count is one for each panel at each spot.
Lowest pitch for metal panels
The International Residential Code sets the lowest pitch by how the seams are made:
- 3 in 12 for lapped panels without lap sealant.
- 1/2 in 12 for lapped panels with lap sealant.
- 1/4 in 12 for standing seam roofs.
The panel maker can ask for more. Metal Sales lists 3 in 12 as the lowest pitch for Classic Rib. If you do not know your pitch, use the roof pitch calculator.
What it leaves out
- Hips, valleys and dormers. Panels there are cut at an angle, and each one is a different length.
- Underlayment, sealant tape, pipe boots and trim screws.
- Long panels. Metal Sales cuts panels from 5 to 45 feet long, and says to plan for the metal growing and shrinking with heat on panels over 20 feet.
Common questions
How many metal roofing panels do I need?
Divide the eave length in inches by the panel coverage width and round up. That is one side of a gable roof, so double it. A 42 foot eave with 36 inch panels takes 14 panels on each side, or 28 in all.
How many screws do I need for a metal roof?
About 80 per square for 3 foot wide panels with screw rows 24 inches apart, according to Metal Sales and Fabral. A 1,500 square foot roof is 15 squares, so about 1,200 screws. Closer rows take more: 110 per square at 18 inches and 160 at 12 inches.
How wide is a metal roofing panel?
Most ribbed screw-down panels cover 36 inches. The sheet itself is wider because the edges overlap. Fabral Grandrib 3 is 37 3/4 inches wide and covers 36. Some panels cover 24 inches, and Fabral Horizon hidden fastener panels cover 16.
How long should metal roof panels be?
As long as the slope from the eave to the ridge, plus a short lip past the eave. Take half the building width plus the overhang, and multiply by the pitch multiplier. Fabral says to let the panel hang 1 to 2 inches past the eave.
What is the minimum pitch for a metal roof?
Under the International Residential Code it is 3 in 12 for lapped panels without lap sealant, 1/2 in 12 for lapped panels with lap sealant, and 1/4 in 12 for standing seam. The panel maker can require more.
How many pieces of ridge cap do I need?
Divide the ridge length by what one piece covers after the overlap, and round up. A 10 ft 6 in piece with a 6 inch overlap covers 10 feet, so a 42 foot ridge takes 5 pieces.
Tools for this job
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- Aviation snips. Cut metal panels and trim by hand. A set has left, right and straight cutting pairs.
- Hand seamer. Bends and flattens the edges of panels and trim.
- Magnetic nut driver bits. Drive the hex head screws that hold the panels down.
- Chalk line. Snaps a straight line so the rows of screws line up.
- Cut resistant gloves. Protect your hands from the sharp edges of cut panels.
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Sources
- International Residential Code 2021, Chapter 9: roof drainage (R903) and roof coverings (R905)
- Metal Sales, Post Frame Installation Guide: Classic Rib, Pro-Panel II and other panels
- Fabral, Standard Details and Total Systems for Metal Roofing and Siding
Facts on this page were last checked against these sources in October 2026.