How do I work out how many metal roof panels I need?

Divide the length along the eave — rake overhangs included — by the panel's coverage width, the net width after the side lap, and round up for each side. A 30' x 40' gable with a foot of overhang all round is 42 ft along the eave; at 36" coverage that is 14 panels a side, 28 in all. Metal is ordered by the piece and cut to length at the mill, so the length matters as much as the count.

How long should metal roof panels be?

The roof's run from the eave edge to the ridge, times the pitch multiplier, plus whatever hangs past the eave into the gutter. On a 30' building with a foot of overhang at 4/12, the run is 192" and the slope 1.0541 times that; with 1 1/2" past the edge each panel is 203 7/8", ordered at 17 ft. Round up, never down — a short panel leaves a gap under the ridge cap that no trim closes.

How many screws does a metal roof take?

As many as the panel maker's fastening pattern puts at each purlin, times the purlin rows, times the panels. At four a panel per row on purlins 24" apart, the 30' x 40' barn above takes 1,120 — about 79 per 100 sq ft. Stitch screws along the laps and the trim screws come on top. Eaves, ridges and high-wind zones often want more than the field, so read the pattern before you order.

What is the minimum pitch for a metal roof?

Under the 2021 IRC (R905.10.2), 3:12 for lapped exposed-fastener panels without sealant in the laps, 1/2:12 with applied lap sealant, and 1/4:12 for standing seam. Many panel makers ask for more than the code does for their exposed-fastener profiles, and their install guide is what the warranty is written against.

Metal roof panels and trim

Enter the building, the pitch and the panel, and get how many panels to order and the length to have them cut to, with the ridge cap, eave and rake trim, purlin rows and screws.

Roof

Building width

Eave wall to eave wall

Building length

Along the eave

Pitch

:12

Rise per foot of run

Advanced settings36" panels · 12" eave, 12" rake · 1 1/2" past the edge · purlins 24" · 4 screws a row · trim 10 ft

Panel coverage

Net width after the lap — 36 for R-panel

Panel past the eave

= 1.5"

Eave overhang

Level, past the wall

Rake overhang

Past each end wall

Purlin spacing

On centre, up the slope

Screws per panel per purlin

From the panel maker's pattern

Trim piece covers

After its lap — check what your supplier sells

Panels cut to 17 ft

28panels

Each

204"

Roof area

1,417 sq ft

Screws

1,120

Order this

Panels14 a side, 203 7/8" of slope and overhang rounded up to the inch — the last panel covers 36" of its 36": 28 × 17 ft
Ridge cap42.0 ft of ridge, rakes included: 5 pieces
Eave trim84.0 ft along both eaves: 10 pieces
Rake trim4 rake edges of 203 7/8" each: 8 pieces
Panel screws4 a panel at each of 10 purlin rows: 1,120
Purlin rows24" apart up 202 3/8" of slope: 10 a side

Against the model code

At least 3:12 for exposed-fastener panels — passesThis roof is 4:12, steep enough for lapped exposed-fastener panels without lap sealant (IRC R905.10.2).

The slope limits are the 2021 IRC's (R905.10.2), defaults rather than law where you are building. The panel maker's minimum pitch and fastening pattern are often stricter — read the install guide, and ring your building department.

What this does not count

Stitch screws along the side laps, trim screws, foam closures at the eave and ridge, butyl tape and sealant, and any panel splice on a roof longer than a panel can be shipped. A hip, a valley or a dormer changes every number here — count each roof plane separately, or have the supplier take off the plans.

Where to buy this

28 panels, 17 ftHome Depot Lowe's
5 ridge capHome Depot Lowe's
10 eave trimHome Depot Lowe's
8 rake trimHome Depot Lowe's
1,120 roofing screwsHome Depot Lowe's

Panels, trim and screws have to match in profile and colour, and panels are cut to length to order — buy the roof from one supplier.

Plain search links, nothing sponsored. Prices and stock are the shop's, and a local lumberyard will usually beat both on solid stock.

Worked example: a 30 × 40 ft roof at 4:12 in 36" panels

A 30 × 40 ft gable with 12" rakes is 42 ft along the eave, so 36" panels take 14 a side — 28 panels.

At 4:12 the slope multiplier is 1.0541: 192" of run with a 12" eave overhang, plus 1 1/2" past the edge, makes each panel 203 7/8", ordered at 17 ft.

5 pieces of ridge cap, 10 of eave trim and 8 of rake trim, and 1,120 panel screws at 4 a panel on each of 10 purlin rows.

Panels: 28
Panel length: 17 ft
Roof area: 1,417 sq ft
Screws: 1,120

These are the figures the calculator above opens on. Change any field and it works the same problem for your numbers.

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About metal roofing

How do I work out how many metal roof panels I need?: Divide the length along the eave — rake overhangs included — by the panel's coverage width, the net width after the side lap, and round up for each side. A 30' x 40' gable with a foot of overhang all round is 42 ft along the eave; at 36" coverage that is 14 panels a side, 28 in all. Metal is ordered by the piece and cut to length at the mill, so the length matters as much as the count.
How long should metal roof panels be?: The roof's run from the eave edge to the ridge, times the pitch multiplier, plus whatever hangs past the eave into the gutter. On a 30' building with a foot of overhang at 4/12, the run is 192" and the slope 1.0541 times that; with 1 1/2" past the edge each panel is 203 7/8", ordered at 17 ft. Round up, never down — a short panel leaves a gap under the ridge cap that no trim closes.
How many screws does a metal roof take?: As many as the panel maker's fastening pattern puts at each purlin, times the purlin rows, times the panels. At four a panel per row on purlins 24" apart, the 30' x 40' barn above takes 1,120 — about 79 per 100 sq ft. Stitch screws along the laps and the trim screws come on top. Eaves, ridges and high-wind zones often want more than the field, so read the pattern before you order.
What is the minimum pitch for a metal roof?: Under the 2021 IRC (R905.10.2), 3:12 for lapped exposed-fastener panels without sealant in the laps, 1/2:12 with applied lap sealant, and 1/4:12 for standing seam. Many panel makers ask for more than the code does for their exposed-fastener profiles, and their install guide is what the warranty is written against.

You might also need

Read up, or take it to the shop

Roof pitch chart — printable chart