Metal Roofing Calculator — Panels, Screws and Trim

How to work it out

1. Measure the building, not the roof

   You do not need to get on the roof. Measure the building's footprint on the ground — length along the eave and width gable to gable — and let the pitch multiplier convert that into true sloped area.

   Add the overhang if the roof extends past the walls, since the panels have to reach the edge of the fascia and usually a little past it into the gutter.

2. Find the pitch from the ground

   Hold a level out horizontally against the gable end, mark 12 inches along it, and measure straight down from that mark to the roof line. That drop in inches is the rise, and the pitch is that number over 12.

   A phone with a pitch app held flat against the underside of a rafter in the attic works too, and is safer than a ladder. Pitch matters twice here: it sets the area, and many panel profiles have a minimum pitch below which they are not rated.

3. Count panels across, not over

   This is where metal differs completely from shingles. Panels run from ridge to eave in one continuous piece, so the number you need is the eave length divided by the panel's coverage width — nothing to do with the slope.

   Coverage width is not the sheet width. A panel sold as 39 inches wide typically covers 36 once it laps its neighbor. Use the coverage figure or you will order short.

4. Order the length, do not cut it

   Metal panels are cut to order at the supplier, usually to the quarter or half foot. Running ridge to eave in a single piece is the whole point — no horizontal seams, nothing to leak.

   Measure twice and add a little. Cutting metal on site is slow, the cut edge loses its factory coating, and a panel cut an inch short cannot be fixed.

Roof pitch multipliers

Multiply the flat footprint by this to get true sloped area.

Pitch	Multiplier	Angle	1,000 sq ft footprint becomes
2/12	1.014	9.5°	1,014 sq ft
3/12	1.031	14.0°	1,031 sq ft
4/12	1.054	18.4°	1,054 sq ft
5/12	1.083	22.6°	1,083 sq ft
6/12	1.118	26.6°	1,118 sq ft
8/12	1.202	33.7°	1,202 sq ft
10/12	1.302	39.8°	1,302 sq ft
12/12	1.414	45.0°	1,414 sq ft

The multiplier is the square root of 1 plus (rise ÷ 12) squared. It is pure geometry — no allowance in it.

Panels needed across a 40 ft eave

Per roof plane. Double it for a gable roof.

Coverage width	Panels per plane	Gable roof total
36 in.	14	28
26 in.	19	38
24 in.	20	40
16 in.	30	60
12 in.	40	80

Always round up. The last panel gets cut down the length, and that cut-off is rarely usable elsewhere.

How many metal panels for a 40x28 building?

A gable roof on a 40 by 28 ft building at 6/12 pitch. Each rafter runs 14 ft horizontally, times the 1.118 multiplier, so the true run is 15.7 ft. Add a 3 inch overhang and the panels want to be 16 ft long.

Across the 40 ft eave with 36 inch coverage panels: 480 inches divided by 36 is 13.3, rounded up to 14 per side, so 28 panels in total.

True sloped area is 40 × 15.7 × 2 = 1,252 sq ft, or 12.5 squares. That calls for five boxes of 250 screws, five pieces of ridge cap, 27 closure strips and two rolls of underlayment.

Coverage width versus panel width

Every metal panel overlaps its neighbor, and the overlap is not free. A common exposed-fastener panel is 39 inches of actual metal that covers 36 inches once installed — three inches disappear into the lap.

Suppliers quote both numbers, and it is easy to take the wrong one. Order using the coverage width and you will be right. Order using the sheet width and you will be roughly 8% short, which on a big roof is several panels.

Standing seam is usually quoted in coverage directly — 16 or 24 inches are the common widths — because the seam is formed on site and the panel arrives flat.

How many screws does a metal roof need?

The usual planning figure for exposed-fastener panels is 80 screws per square — that is 80 per 100 square feet of roof. A 12.5 square roof therefore needs about 1,000 screws. Add the usual 10% spare and that is five boxes of 250.

That splits roughly into 60 for the field, fixing panels down to the purlins or decking, and 20 for the laps, trim, ridge and eave. The exact number depends on your panel's fastening pattern and your purlin spacing, both of which the manufacturer specifies.

Buy screws with a bonded EPDM washer and a coating that matches the panel — color-matched heads on a colored roof. And buy 10% more than you calculate: some will strip, some will go in crooked and have to come out, and running out on a Sunday stops the whole job.

Underlayment, and whether you need a solid deck

Metal panels can go over solid decking or straight onto purlins, and which one you have changes what goes underneath.

Over a solid deck, you need underlayment. Synthetic is now standard — it is lighter, tougher underfoot and does not wrinkle like felt. A roll typically covers 10 squares, so a 12.5 square roof takes two. In snow country, a self-adhering ice and water membrane goes along the eaves as well, usually far enough up to reach two feet inside the exterior wall line.

Over open purlins there is no underlayment, but condensation becomes the issue instead. Warm moist air from inside hits cold metal and drips. Panels with a factory-applied condensation control membrane on the underside, or a layer of rigid insulation, solve it. A barn roof with no vapour control will rain on the inside every clear cold night.

The trim list nobody prices first

Panels are the cheap part. Trim is what turns a pile of metal into a roof, and it is easy to forget until it is missing.

The list for a simple gable: ridge cap along the top, in 10 ft pieces lapped 6 inches so you get about 9.5 ft of coverage each. Closure strips — profiled foam that fills the corrugations at the ridge and the eave, keeping out wind-driven rain, birds and insects. Rake trim down both gable ends. Eave or drip trim along the bottom. Then sealant tape for the laps, and pipe boots for anything that penetrates.

Add valleys, hips or a chimney and the trim list grows quickly. Every one of those is a separate part number with its own lead time, so order them with the panels rather than after.

Expansion, and why the fasteners matter

Metal moves. A 16 ft steel panel changes length by roughly a quarter of an inch between a freezing night and a hot afternoon, and aluminum moves about twice that.

Exposed-fastener systems handle this by letting the panel flex slightly around the screw, which is why the screw must be driven straight and just tight enough to compress the washer — not crushed, and not left loose. An over-driven screw dimples the panel and the washer fails early; an under-driven one leaks straight away.

Standing seam handles it properly, with concealed clips that let the panel slide. That is the real reason standing seam costs more and lasts longer: no holes through the weather surface at all. On panels longer than about 30 ft, that difference stops being theoretical.

How this is worked out

No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.

1. rafter run = building width ÷ 2 × pitch multiplier For a gable. A shed roof uses the full width.
2. panel length = rafter run + overhang ÷ 12 Panels run ridge to eave in one piece, plus the projection.
3. panels per plane = eave length × 12 ÷ coverage width, rounded up Counted across the eave, not up the slope.
4. roof area = eave length × rafter run × number of planes True sloped area, not the footprint.
5. squares = roof area ÷ 100 A roofing square is 100 square feet.
6. screw boxes = squares × 80 × 1.1 ÷ 250 About 80 screws per square, plus 10% spare, in boxes of 250.
7. ridge cap = eave length ÷ 9.5 10 ft pieces lapped 6 inches.
8. underlayment = area × 1.1 ÷ 1000 A synthetic roll covers about 10 squares; 10% for overlaps.

Common questions

How many metal roofing panels do I need?

Divide the eave length in inches by the panel's coverage width and round up — that is the count for one side of the roof. Double it for a gable. A 40 ft eave with 36 in. coverage panels takes 14 per side, so 28 in total.

What length should I order the panels?

The rafter run from ridge to eave, plus the overhang. Take half the building width, multiply by the pitch multiplier, and add the projection. A 28 ft wide building at 6/12 pitch with a 3 in. overhang wants 16 ft panels.

How many screws per square for a metal roof?

About 80 — roughly 60 in the field and 20 in the laps and trim. Buy 10% more than the calculation, because some strip and some go in crooked and have to come back out.

What is the minimum pitch for a metal roof?

It depends on the profile. Exposed-fastener panels generally want 3/12 or steeper. Some standing seam systems with sealed seams go down to 1/4:12. Below the rated minimum the manufacturer's warranty does not apply, so check the specific panel.

Do I need underlayment under metal roofing?

Over a solid deck, yes — synthetic underlayment is standard, plus ice and water membrane at the eaves in snow country. Over open purlins there is no underlayment, but you need a plan for condensation instead, such as a backed panel or rigid insulation.

Can I install metal roofing over shingles?

Often yes, and it is one of metal's advantages — but check locally, because codes limit the number of roof layers. You generally need furring strips over the shingles to create an air gap and a flat plane, and the existing deck has to be sound.

What is coverage width?

The width a panel actually covers once it overlaps its neighbor, which is less than the sheet width. A 39 in. sheet commonly covers 36 in. Always order using coverage, or you will be about 8% short.

How much does a metal roof expand?

A 16 ft steel panel moves about a quarter inch between a freezing night and a hot day; aluminum moves roughly double. Exposed-fastener panels absorb it around the screw, which is why screws must be driven straight and to the right depth. Standing seam uses sliding clips instead.

Where these figures come from

Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.

• Metal Construction Association guidance — panel coverage widths, minimum slopes and fastener practice
• International Residential Code, section R905.10 — metal roof panel installation and underlayment requirements
• Panel manufacturer installation manuals — screws per square, closure strip and trim requirements

Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 12, 2026.