Metal Roof Calculator

The metal roof calculator converts your building half-span and pitch into a slope length, adds the eave overhang along the slope, and reports the panel length each sheet must be. It then divides your roof length by the panel's net coverage width (18" for standing seam, 24" for 5V crimp, 26" for corrugated, 36" for R-Panel) to count panels per slope, doubles the count for a gable, and applies your waste factor to produce an order quantity. For a 24' × 36' gable at 6/12 with an 8" overhang, it returns a 13' 5-5/8" panel length, 24 panels per slope, and a 53-panel order at 10% waste.

Panel length = (half-span ÷ cos(pitch angle)) + (overhang ÷ cos(pitch angle)). The half-span is half the building width for a gable roof, and the overhang is measured along the rafter, not horizontally. For a 24'-0" wide building (12' half-span) at 6/12 (26.57°): slope length = 12 ÷ 0.8944 = 13.417', and an 8" horizontal overhang adds 8 ÷ 0.8944 = 8.94" along the panel, so the panel length is 14' 2-3/8". Order panels cut 1/2" short of the exact dimension if the manufacturer charges for over-length cutoffs — but never order short enough to lose the eave drip edge flange.

The minimum slope for a metal roof depends on the profile: mechanically double-locked standing seam is rated to 1/4:12 (1.2°), R-Panel/PBR with continuous lap sealant goes to 1:12 (4.8°) — 3:12 unsealed — and water-shedding profiles — snap-lock standing seam, 5V crimp, corrugated, and metal shingles — require 3:12 (14.0°). The dividing line is seam behavior: double-locked seams are hydrostatic (they hold water), while lap-type seams are hydro-shedding and rely on gravity to drain every lap. Below 3:12, an exposed-fastener panel without sealant lines will leak at the first wind-driven rain, not gradually.

Count panels per slope as: roof length ÷ net coverage width, rounded up, then multiply by two for a gable roof. A 36'-0" long roof with 18" standing seam coverage needs 36 ÷ 1.5 = 24 panels per slope, 48 total. Add your waste factor next: 10% is standard for simple gables (the offcut from each panel is reused on the next course), 15% for hips and valleys where panel ends are cut at angles, and 5% only for straight sheds with standard lengths. The calculator applies the waste factor to the panel count, not the area, because panels cannot be pieced together from offcuts the way shingles can.

Standing seam panels are ordered in single full-length sheets, so laps do not apply — the length calculator simply reports the slope length plus overhang. Exposed-fastener panels over the trucking limit (40' on most carriers, 45' max) must be split into lapped courses: courses = panel length ÷ 40 rounded up, and each course is cut to (panel length + 8" × (courses − 1)) ÷ courses so the 8" end laps stack in the correct water-shedding order. The calculator performs this split automatically and reports per-course cut lengths; always lap the upper course over the lower so water flows over the joint, never into it.

Yes — corrugated and 5V crimp panels need 3:12 while standing seam can go as low as 1/4:12. Corrugated profiles seal with washered screws and overlapping ribs, which depends on water moving downhill fast enough that capillary action cannot pull it back up through the laps. Standing seam mechanical panels fold two seam edges together 180°, creating a joint that does not depend on slope to stay watertight. That is why a porch roof tied into a house at 2:12 gets standing seam while the main shingle roof stays at 6:12 — the two profiles coexist at different pitches on the same building.

Use 10% for a simple gable, 15% for a hip roof, and 5% for a single-slope shed roof with standard-cut panels. Unlike asphalt shingles, metal panels are ordered in exact lengths; the offcut from a 14' panel used on a 12' slope is 2' of prime material you can reserve for ridge caps, valley flashings, and porch tails, which is why the waste factor is lower than shingles. Do not reduce below 5% even on a plain gable — shipping damage, a miscut hole for a pipe boot, and one dropped panel are common enough that ordering exact quantities leaves no margin for a single mistake.

A common planning figure is one screw per square foot of roof, which the calculator applies to the total area. In practice the fastener layout in the installation manual controls: most 36" R-panel manuals call for screws in every other rib at the eave and ridge and in every rib at each purlin crossing, which lands between 0.8 and 1.2 screws per square foot. Screws must land in the flat or the rib crown exactly as the manual specifies — driven in the wrong spot, a screw tents the panel and the washer never seals. Add one extra box (250 screws) per 20 squares; stripped and dropped screws are a certainty on installation day.

Per 100 square feet (one square), a metal roof consumes roughly four 18"-coverage panels totaling about 56 linear feet of coil, while asphalt consumes three bundles of shingles plus underlayment and starter strip. Metal wins on weight — 1 to 1.5 psf versus 2.5 to 4 psf for asphalt — which is why metal overlays on an existing shingle layer are frequently approved without new decking. Asphalt wins on piece-size flexibility: its 3' tabs fill hips, valleys, and dormers with far fewer custom cuts. Over a 50-year horizon, one metal install typically replaces two asphalt roofs, which is where the material ledger truly flips.

Often yes, subject to local code and the panel manufacturer's written approval. The standard assembly adds 1×4 purlins (or furring strips) at 24" o.c. through the shingles into the rafters, then fastens panels to the purlins — this keeps the metal off the old shingle grain and creates an air gap that stops condensation and interior dripping. Weight is not the constraint (metal plus purlins adds only ~1.5 psf); the constraints are the IRC's two-layer maximum for roof coverings and whether the existing shingles are flat enough for the panel profile's minimum slope. Confirm both in writing before ordering panels cut to length.

For a 40'-0" long gable, panel length is set by the slope direction, not the building length — a 24' × 40' gable at 6/12 still orders 14'-long panels, just 27 of them per slope (40 ÷ 1.5 = 26.7). The building length controls panel count and the ridge cap length (order 41' of ridge cap to cover 40' with 1' of overlap). Only on a single-slope roof does the 40' dimension become the panel length — and at 40' you are at the trucking limit, so confirm the carrier can deliver uncrimped full-length sheets, or plan a lapped two-course installation with an 8" end lap sealed with butyl tape.

Standing seam is roll-formed on site or at the distributor to your exact slope length, so quotes list linear feet (roof length × number of slopes × slope length) rather than fixed 18' stock panels. A 24' × 36' gable at 6/12 quotes about 648 linear feet plus trim — the same footage the calculator reports. If a quote shows a fixed panel count like 48 panels of a stock length, it is snap-lock from inventory, which means field-cut ends, exposed cut edges, and a 3:12 slope minimum. Ask which system you are buying before comparing prices, because the two are not the same product.