How it works
The calculator works out the panel length along the slope, then counts how many panels fit side by side along each eave.
Slope length = measured or Run × √(1 + (Rise ÷ Run)²)
Panel length = Slope length + Overhang
Panels per side = ⌈ Eave length ÷ Net coverage width ⌉
Panels = Panels per side × Sides
Total panel length = Panels × Panel length
Screws = Panels × (⌈ Slope length ÷ Row spacing ⌉ + 1) × Screws per panel per row
⌈ ⌉ means rounding up to the next whole panel. All lengths are converted to one internal unit, so you can enter the eave in feet and the coverage width in inches, or in metres and millimetres.
How metal roof panels are ordered
Exposed fastener panels – ribbed, corrugated and 5V profiles – are usually cut to the length you order, so a metal roof order is a list of panel counts and lengths rather than a number of bundles. On a simple gable or shed roof every panel on a side is the same length: from the eave edge to the ridge plus whatever overhangs the eave. The panels are laid side by side along the eave, each one overlapping the previous one at the edge rib.
Net coverage width
The figure that matters is the coverage width: how much roof one panel covers once it overlaps its neighbour. Union Corrugating lists 36 in coverage for MasterRib and 24 in coverage for its 2-1/2 in Corrugated and 5V panels. Dividing by the full sheet width would leave you short, because the overlap isn’t counted. If your profile isn’t in the list, take the coverage figure from its product page or data sheet.
Slope length, run and pitch
The surest panel length comes from measuring the roof surface from the eave edge to the ridge. If you can’t get on the roof, use the horizontal run – on a symmetrical gable, half the building width plus the eave overhang – and the pitch. The slope is the run times the slope factor √(1 + (rise ÷ run)²): a 15 ft run at 4:12 is 15 × 1.054 = 15.81 ft of slope. At the ridge the panels are covered by the ridge cap; follow the maker’s ridge details for where the panels stop.
Overhang at the eave
Panels normally extend a little past the eave edge or into the gutter so water drips clear. The distance comes from the panel maker’s eave trim details, so the field starts at 0 – enter your figure and it is added to every panel.
Screws
The number of screws depends on the panel profile, the purlin or deck spacing and the wind zone, so no figure is assumed. If you have the fastening schedule, enter the spacing between screw rows along the panel and how many screws go across one panel at each row. The calculator counts one row at each spacing plus one at the end, for every panel. Stitch screws at side laps and trim screws are extra.
Worked example in words
A gable roof with 40 ft eaves and a 16 ft slope, in MasterRib panels with 36 in coverage: 40 ÷ 3 = 13.3, so 14 panels per side and 28 panels in all, each cut to 16 ft – 448 ft of panel. The roof itself is 2 × 40 × 16 = 1,280 sq ft. The 14 panels cover 42 ft, so the last panel on each side is trimmed or the layout is centred to suit the rake trim.
Worked example
Inputs
- Roof shape
- Gable – two sides
- Eave length (one side)
- 40 ft
- Panel length from
- Slope length (measured)
- Slope length
- 16 ft
- Panel overhang at the eave
- 0 in
- Panel profile
- Union Corrugating MasterRib – 36 in coverage
Result
- Panel length
- 16 ft (192 in)
- Panels per side
- 14
- Panels
- 28
- Total panel length
- 448 ft
- Roof area (to the eave edge)
- 1,280 sq ft
Common mistakes
- Dividing by the full sheet width. Exposed fastener panels overlap at the side rib, so a panel adds only its net coverage width – 36 in for a MasterRib panel, for example.
- Using the horizontal run as the panel length. Panels run up the slope, which is longer than the run – a 15 ft run at 4:12 is about 15.8 ft of slope.
- Forgetting the eave overhang. Panels usually extend past the eave edge or into the gutter; add that to every panel.
- Measuring the eave length between the walls. Panels also cover the rake overhangs at both ends of the roof.
- Using this for hip roofs. On a hip roof, panels are cut to different lengths along each hip, so count each roof face separately or ask the supplier for a cut list.
Assumptions and limits
- Each roof side is a rectangle, with panels side by side along the eave, all the same length from eave to ridge.
- Panels per side are rounded up to whole panels; the last panel may be cut or the layout adjusted at the rake.
- Panel length = slope length + overhang. In run-and-pitch mode, slope length = run × √(1 + (rise ÷ run)²).
- Each panel is one piece from eave to ridge. End laps for panels longer than your supplier can make are not included.
- Screws are counted only if you enter a fastening pattern; trim, closure and stitch screws are not included.
Frequently asked questions
How many metal roof panels do I need?
Divide the eave length of each roof side by the panel’s net coverage width and round up. A 40 ft eave covered with 36 in (3 ft) MasterRib panels needs 40 ÷ 3 = 13.3, so 14 panels per side, or 28 for a gable roof.
What is net coverage width?
The width a panel adds across the roof once it overlaps its neighbour. Union Corrugating lists 36 in coverage for MasterRib and 24 in for its 2-1/2 in Corrugated and 5V panels. The sheet itself is wider; the overlap isn’t counted.
How long should metal roof panels be?
As long as the slope from eave to ridge plus the overhang at the eave. Panels are usually cut to length, so measure along the roof or calculate the slope from the run and pitch, then add the overhang from the panel’s trim details.
How many screws per square foot for a metal roof?
It depends on the panel, the purlin or deck, and the wind zone, so the calculator doesn’t assume a figure. Enter the row spacing and the screws per panel per row from your panel’s fastening schedule; the calculator multiplies them out for every panel.
What is the minimum pitch for a metal roof?
It depends on the panel. Union Corrugating lists 3:12 as the minimum recommended roof pitch for MasterRib, 2-1/2 in Corrugated and 5V. Other profiles have their own minimum. Check the maker’s figure for the profile you choose.
Sources
- MasterRib panel – specifications — Union Corrugating Company. Exposed fastener roof and wall panel: "Coverage: 36" Coverage"; "3:12 minimum recommended roof pitch".
- 2-1/2" Corrugated panel – specifications — Union Corrugating Company. Exposed fastener panel: "Coverage: 24" Coverage"; "3:12 minimum recommended roof pitch".
- 5V panel – specifications — Union Corrugating Company. Exposed fastener roof panel: "Coverage: 24" Coverage"; "3:12 minimum recommended roof pitch".
- Estimating How Much Roofing Is Required — IKO. Actual area = horizontal area × area/rake factor; 6/12 → 1.118. The factor equals √(1 + (rise/run)²) (Pythagoras).
Method reviewed November 4, 2026