Figuring metal roofing from the roof pitch
A metal roof order comes down to three questions, and the pitch answers all three. How much roof surface is up there? How many panels cross the eave? How long does each one have to be? Take the 24-foot-wide, 40-foot-long gable this page works through at 6/12: it is 1073 square feet of roof — 10.7 squares — crossed by 14 panels a slope at 36” of coverage, each 13’5” long before you add an inch of overhang.
None of that is a code requirement and none of it is cited to one — it is the rise-and-12 triangle and division. What the 2021 IRC does have to say about a metal roof is the slope it may be laid on, and that is further down this page with the section beside it.
How much roof is up there
A roof plane is its own footprint times the pitch multiplier — the hypotenuse of the rise-and-12 triangle, and the same figure the pitch pages publish. Run is the horizontal distance from eave to ridge, which on a symmetric gable is half the building width; eave length is the other dimension. Multiply the two, multiply by the multiplier, double it for the second slope.
The column below is the 24-by-40-foot gable at every whole-inch pitch, so the cost of pitch is visible as a number rather than an impression.
| Pitch | Roof per ft² of footprint | Panel length over a 12 ft run | Both slopes, 24 × 40 ft | Squares |
|---|---|---|---|---|
| 1/12 | 1.0035 | 12’0½” | 963 ft² | 9.6 |
| 2/12 | 1.0138 | 12’2” | 973 ft² | 9.7 |
| 3/12 | 1.0308 | 12’4⅜” | 990 ft² | 9.9 |
| 4/12 | 1.0541 | 12’7¾” | 1012 ft² | 10.1 |
| 5/12 | 1.0833 | 13’0” | 1040 ft² | 10.4 |
| 6/12 | 1.1180 | 13’5” | 1073 ft² | 10.7 |
| 7/12 | 1.1577 | 13’10¾” | 1111 ft² | 11.1 |
| 8/12 | 1.2019 | 14’5⅛” | 1154 ft² | 11.5 |
| 9/12 | 1.2500 | 15’0” | 1200 ft² | 12.0 |
| 10/12 | 1.3017 | 15’7½” | 1250 ft² | 12.5 |
| 11/12 | 1.3566 | 16’3⅜” | 1302 ft² | 13.0 |
| 12/12 | 1.4142 | 16’11⅝” | 1358 ft² | 13.6 |
A symmetric gable, 24 feet wide and 40 feet along the eave, with no overhang. Panel length is the sloping distance over the run and nothing more — add whatever you want past the eave, and add nothing at the ridge until you have read how your profile closes there. A square is 100 square feet.
How many panels cross the eave
Eave length over the panel's coverage width, rounded up. Coverage width is not the width of the sheet — it is what the sheet covers once it is lapped into its neighbour, it is published on the profile's own specification sheet, and it varies by profile. That is why it is a column here and not an assumption: read the one your panel publishes.
| Eave length | 12” coverage | 16” coverage | 24” coverage | 36” coverage |
|---|---|---|---|---|
| 12 ft | 12 | 9 | 6 | 4 |
| 16 ft | 16 | 12 | 8 | 6 |
| 20 ft | 20 | 15 | 10 | 7 |
| 24 ft | 24 | 18 | 12 | 8 |
| 30 ft | 30 | 23 | 15 | 10 |
| 40 ft | 40 | 30 | 20 | 14 |
| 50 ft | 50 | 38 | 25 | 17 |
| 60 ft | 60 | 45 | 30 | 20 |
Panels across ONE slope. A gable takes that twice. Where the eave does not divide evenly the count rounds up, and the last panel is cut narrower than the ones beside it — on the 40-foot eave worked through this page at 36” of coverage, 24 inches of that last panel's coverage is surplus.
Panel length, run by run
The same multiplier, read the other way: how long a panel has to be to reach from eave to ridge over a given run. This is the rafter length, because a panel and a rafter cross the same triangle — which is also why R802.4.1 measures a rafter span on the horizontal projection and not up the slope.
| Pitch | 8 ft run | 10 ft run | 12 ft run | 14 ft run | 16 ft run | 18 ft run | 20 ft run |
|---|---|---|---|---|---|---|---|
| 1/12 | 8’0⅜” | 10’0⅜” | 12’0½” | 14’0⅝” | 16’0⅝” | 18’0¾” | 20’0⅞” |
| 2/12 | 8’1⅜” | 10’1⅝” | 12’2” | 14’2⅜” | 16’2⅝” | 18’3” | 20’3¼” |
| 3/12 | 8’3” | 10’3¾” | 12’4⅜” | 14’5⅛” | 16’5⅞” | 18’6⅝” | 20’7⅜” |
| 4/12 | 8’5¼” | 10’6½” | 12’7¾” | 14’9⅛” | 16’10⅜” | 18’11⅝” | 21’1” |
| 5/12 | 8’8” | 10’10” | 13’0” | 15’2” | 17’4” | 19’6” | 21’8” |
| 6/12 | 8’11⅜” | 11’2⅛” | 13’5” | 15’7⅞” | 17’10⅝” | 20’1½” | 22’4⅜” |
| 7/12 | 9’3⅛” | 11’6⅞” | 13’10¾” | 16’2½” | 18’6¼” | 20’10⅛” | 23’1⅞” |
| 8/12 | 9’7⅜” | 12’0¼” | 14’5⅛” | 16’9⅞” | 19’2¾” | 21’7⅝” | 24’0½” |
| 9/12 | 10’0” | 12’6” | 15’0” | 17’6” | 20’0” | 22’6” | 25’0” |
| 10/12 | 10’5” | 13’0¼” | 15’7½” | 18’2⅝” | 20’9⅞” | 23’5⅛” | 26’0⅜” |
| 11/12 | 10’10¼” | 13’6¾” | 16’3⅜” | 18’11⅞” | 21’8½” | 24’5” | 27’1⅝” |
| 12/12 | 11’3¾” | 14’1¾” | 16’11⅝” | 19’9⅝” | 22’7½” | 25’5½” | 28’3⅜” |
Eave to ridge, to the nearest eighth of an inch, with no overhang and no allowance for how the panel finishes at the ridge. The rafter under it is a separate question with a code answer — see the rafter span tables.
What slope a metal roof may go on — 2021 IRC R905
This is the part of the page that is code rather than arithmetic. Chapter 9 gives every roof covering a minimum slope, and it does not give metal one answer: a standing-seam panel and a lapped panel without lap sealant are more than ten times apart on how flat they may go.
| Metal roof covering | Minimum slope | Section | Clears at |
|---|---|---|---|
| Metal roof panels — standing seam | ¼:12 | R905.10.2 | 1/12 |
| Metal roof panels — lapped, nonsoldered seam with lap sealant | ½:12 | R905.10.2 | 1/12 |
| Metal roof shingles | 3:12 | R905.4.2 | 3/12 |
| Metal roof panels — lapped, nonsoldered seam without lap sealant | 3:12 | R905.10.2 | 3/12 |
Minimum slopes as stated in the 2021 IRC R905 sections named on each row. The other coverings the chapter lists are on the roof pitch page. chapter source
R802.4.4 rafter supports. Below a 3:12 pitch, the members carrying rafters — ridges, hips and valleys — are designed as beams rather than sized off a table, with bearing provided for the rafters per R802.6. A shallow metal roof is exactly where that turns on, so it is worth knowing before the panels are ordered. chapter source
The runs around the edge
Trim is bought by the foot, and on a plain gable the three runs come off the same two dimensions. For the 24-by-40-foot roof at 6/12: 40 feet of ridge, 80 feet of eave across both sides, and 54 feet of rake up the four slope edges at the two gable ends.
That is a plain gable and nothing else. A hip, a valley, a dormer, a chimney or a skylight changes every one of those numbers and adds runs that are not on this list — and each of them also makes offcuts, which is where a waste allowance comes from. Which trim pieces the profile needs, and how they lap, is in the manufacturer's own instructions.
Common questions
How do you figure metal roofing?
Three numbers do it, and two of them are measured on the ground. Multiply the horizontal run by the pitch multiplier to get the length each panel has to cover; divide the eave length by the panel's coverage width and round up to get how many panels cross one slope; multiply run by eave length by the multiplier for the area of one slope. On the 24 by 40 foot gable worked through this page at 6/12, that is 14 panels of 36” coverage per slope, each 13’5” long before any overhang, over 1073 square feet of roof.
How much bigger is a roof than the floor under it?
By exactly the pitch multiplier, and by nothing else. A 6/12 roof is 1.1180 times its own footprint — so the 24 by 40 foot plan under the roof on this page, 960 square feet, carries 1073 square feet of roof surface. Steeper roofs open that gap fast: at 12/12 the same footprint carries 1358 square feet. The multiplier for every whole-inch pitch is in the table on this page and on the pitch pages themselves.
What is panel coverage width, and why not just use the panel width?
Coverage width is the width a panel covers once it is lapped into the one beside it, and it is always less than the sheet is wide — the lap is the difference. It is a number off the profile's own specification sheet, and it varies by profile, so this site will not state one as though it were universal. The columns on this page are common coverage widths; read the one your profile publishes, and where the eave does not divide evenly into it, the last panel is cut down.
What roof pitch can a metal roof go on?
It depends on which kind of metal roof, and the 2021 IRC states a minimum for each. standing seam panels go as low as ¼:12 (R905.10.2); lapped, nonsoldered seam without lap sealant want 3:12 (R905.10.2). All 4 metal entries in R905 are on this page with their sections. Below a 3:12 pitch there is a second thing to know, and it is structural rather than a covering rule: R802.4.4 has ridges, hips and valleys designed as beams rather than sized off a rafter table.
Does this include waste, trim and fasteners?
No. What is on this page is the roof itself: the surface, the panels that cross it, and the length each one has to be. Waste depends on how the roof is cut up — hips, valleys, dormers, skylights and chimneys all make offcuts, and a plain gable makes almost none. Trim and fastener quantities belong to the profile and its manufacturer's instructions. Nor does this page size anything: what the panels are fastened to is a separate question with a code answer, and that one is on the rafter span tables.
Every pitch, page by page
Each pitch page carries the triangle drawn to scale, rafter lengths run by run, and the coverings that slope may and may not carry: 1/12 pitch · 2/12 pitch · 3/12 pitch · 4/12 pitch · 5/12 pitch · 6/12 pitch · 7/12 pitch · 8/12 pitch · 9/12 pitch · 10/12 pitch · 11/12 pitch · 12/12 pitch
The wing's hub, with the pitch-and-rafter-length tool: roof pitch tables. What the roof weighs down onto is a different question with a code answer: rafter spans and ceiling joist spans.