Gambrel Roof Calculator

Metal Roofing Panel Calculator

Work out metal panel counts for a gambrel roof by segment, using panel width and rafter length rather than area alone — plus a rough, clearly-labeled materials cost range.

ft
°
°
%
ft
ft
in
Nominal coverage width; 36″ is common for exposed-fastener panels.
%
Effective coverage lost to side-lap.
$ /ft²
$ /ft²
Rough planning range, materials only — not a quote.

Panels by segment

Lower slope Upper slope

16 panels 16 panels
Effective coverage / panel
2.82 ft
after side-lap
Panels across, per face
8
same for both segments
Panels needed, total
32
16 lower + 16 upper
Lower panel length
14.00 ft
R1 + sloped overhang
Upper panel length
6.62 ft
R2
Material cost range
$2,681 – $5,361
rough planning figure only

Segment area comes from the area calculator's same geometry. Weighing panels against strip shingles? See the shingle calculator. Either way you still need a sheathing calculator result for the deck underneath.

Why panel count isn't just area ÷ coverage

Shingles and sheathing are ordered by area because they come in small, cuttable units that tile a roof plane in any direction. Metal panels do not work that way — each panel is a fixed-width, full-length piece that runs from eave to ridge (or eave to knuckle, on a gambrel), so the number you need is set by how many whole panels fit across the building, not by dividing total square footage by panel area.

On a gambrel that split matters twice over: the lower and upper slopes need the same panel count across the building length (since both run the same length), but two different panel lengths, because the lower segment's rafter run (R1, plus the sloped overhang) is not the same as the upper segment's (R2). Order one panel length for the whole roof and you will either waste material trimming the upper-segment panels down, or come up short on the lower segment.

Gambrel roof profile split into a lower and upper panel run Cross-section of a gambrel roof showing the lower-slope panel run in orange and the upper-slope panel run in teal, each labeled with its panel length. Lower slope Upper slope 32 panels total Lower run: 14.00 ft Upper run: 6.62 ft

In the worked example below, both segments need 8 panels across a 20 ft building length, but the lower-segment panels are cut to 14.00 ft and the upper-segment panels to 6.62 ft — over twice as long. Ordering "16 panels, one length" instead of two lengths would be a costly mistake here.

Panel formulas

Effective coverage width first, then panels across, then two segment lengths.

Effective coverage width

effective_width = panel_width × (1 − overlap% / 100)

Nominal panel width minus what the side-lap overlap eats into actual coverage.

Panels across, per slope face

panels_per_side = ceil(building_length / effective_width)

Same value for the lower and upper segment — both run the full building length.

Panel length, per segment

lower_length = R1 + overhang / cos θ     upper_length = R2

Panel run length equals rafter length for that segment, not the flat span.

Total panels = panels_per_side × 2 (both roof sides) × 2 (lower and upper segments) — but two different order lengths.

Worked example: 36″ panels, 6% overlap allowance, 20 ft building length
  1. Effective width = 3.00 ft × (1 − 0.06) = 2.82 ft.
  2. Panels per side = ceil(20 / 2.82) = 8.
  3. Lower segment: 8 panels × 2 sides = 16 panels, each cut to 14.00 ft.
  4. Upper segment: 8 panels × 2 sides = 16 panels, each cut to 6.62 ft.
  5. Total: 32 panels, in two different cut lengths.

Standing seam vs. exposed fastener

Both are legitimate choices for a gambrel roof — the right one depends on budget, timeline, and how long you want to go before re-roofing.

Standing seam vs. exposed-fastener metal panels
  Standing seam Exposed fastener
Fastener visibility Concealed, under the seam Exposed, through the panel face
Typical panel width 12″ – 24″ 24″ – 36″
Relative material cost Higher Lower
Typical service life 40 – 70 years 25 – 40 years
Thermal movement handling Floating clips absorb it Fastener holes can work loose over time
Best suited for Long-term primary roofing Barns, outbuildings, budget builds

For a barn or outbuilding on a budget, exposed fastener is the common choice and the 36″-wide default this calculator starts from. For a house-scale gambrel meant to outlast a couple of re-roofing cycles, standing seam's concealed clips are usually worth the extra material cost.

Cost range and panel specs

Rough planning figures for the worked example (824.8 ft² total area) — adjust the cost inputs in the calculator above to your own supplier's quote.

Material cost, low

$2,681

At $3.25/ft², materials only.

Material cost, high

$5,361

At $6.50/ft², materials only.

Common panel width

36″

Typical exposed-fastener nominal coverage width.

override above
Overlap allowance

4% – 8%

Typical range depending on panel profile and rib spacing.

These figures are a rough, clearly-labeled planning aid, not a stamped quote — installation cost, panel gauge, coating, and regional pricing all move the real number. Get a local quote before you commit to a budget.

Ordering steps

  1. Solve the roof geometry

    Enter span, pitches, knuckle position, building length, and eave overhang — this gives R1 and R2, the panel run length for each segment.

  2. Pick a panel width and overlap allowance

    36″ nominal coverage with a 6% overlap allowance is a common exposed-fastener default; standing seam often runs narrower.

  3. Read panels-across-length

    This count is driven by building length divided by effective panel width, and it is the same for the lower and upper segment since both run the same length.

  4. Order two panel lengths, not one

    Lower-segment panels are cut to R1 plus the sloped overhang; upper-segment panels are cut to R2. Tell your supplier both lengths, not just a panel count.

  5. Treat the cost range as a planning number

    Multiply total area by the low and high $/ft² figures for a rough materials-only budget range, then get an actual quote before committing.

Common questions

Why does panel count depend on panel width, not just roof area?
Because metal panels are discrete units that each cover a fixed width, running the full length of a slope. You cannot buy 6.4 panels — you round up to 7 — so the driving number is how many whole panels fit across the building length, not the raw square footage. Two roofs with identical area but different panel widths need different panel counts.
Why are the lower and upper panel counts the same in the calculator?
Because "panels across" is set by building length ÷ effective panel width, and the building length is identical for both slopes — a gambrel's lower and upper segments both run the full length of the barn. What differs is panel length: lower-segment panels are cut to R1 plus the sloped overhang, upper-segment panels are cut to R2, so the two segments need the same panel count but different panel lengths.
What is the difference between standing seam and exposed-fastener panels?
Standing seam panels are held down by concealed clips under a raised, interlocking seam — no fasteners penetrate the panel face, so there is nothing for water to work through over time. Exposed-fastener panels (sometimes called corrugated or ribbed panels) are screwed straight through the panel into the framing; they cost less up front but the fastener holes are the roof's weak point long-term.
Is the cost range on this page an actual quote?
No — it is explicitly a rough planning figure, materials only, not installation. Metal roofing material costs vary by panel gauge, coating, region, and supplier, and installed cost typically runs well above materials-only cost. Use the range to sanity-check a quote, not to budget a final number.
How does the overlap allowance affect panel count?
Each panel's actual installed coverage is slightly less than its nominal width because adjacent panels overlap at the side-lap. A 36″ nominal panel with a 6% overlap allowance covers about 2.82 effective feet, not 3.00 — so a longer building can quietly need one more panel per row than a naive width-only calculation suggests.