Gambrel Roof Calculator

Roof Area Calculator

Get the sloped roof area of a gambrel roof split by lower slope and upper slope, plus the combined total — so you can order shingles, metal panels, or sheathing by the segment instead of guessing off one number.

ft
Wall plate to wall plate.
°
°
%
Percent of the half-span, eave to knuckle.
ft
Ridge-line length, eave to eave along the barn.
ft
Horizontal projection past the wall plate.

Area split, lower vs. upper

Lower slope Upper slope

560 ft² 265 ft²
Lower-slope area
560.0 ft²
both sides, includes overhang
Upper-slope area
264.8 ft²
both sides
Total roof area
824.8 ft²
= lower + upper, matches roofArea
Footprint (plan) area
480.0 ft²
width × length, flat
Sloped : flat ratio
1.72×
roof area over footprint

Feed the total into the shingle calculator, the metal panel calculator, or the sheathing calculator — or use the lower/upper split directly since each segment is often ordered as a separate line item.

Why the segment split matters

On a gambrel roof the lower slope and the upper slope are cut at different pitches, and material suppliers largely treat them as two small roofs stitched together at the knuckle line rather than one continuous plane. If you only know the combined total area, you can still buy enough material overall, but you cannot split the order into two purchase lines, check a delivery against each slope, or apply a different waste allowance where it actually belongs.

The lower slope runs steeper and meets the wall plate, so it usually needs a different cut allowance than the shallower upper slope near the ridge — more hip-and-valley-style waste if there are dormers cut into it, more starter-course and drip-edge detail at the eave. The calculator above solves the same rafter geometry as the homepage calculator — lower rafter length R1, upper rafter length R2, and the sloped eave overhang — then multiplies each by two (both sides of the ridge) and by the building length to get an area per segment.

Gambrel roof profile split into a lower slope and an upper slope Cross-section of a gambrel roof showing the lower slope from eave to knuckle in orange and the upper slope from knuckle to ridge in teal, each with its own area label. Lower slope Upper slope Total: 824.8 ft² Lower slope: 560.0 ft² Upper slope: 264.8 ft²

In the worked example above (24 ft span, 60° lower pitch, 25° upper pitch, 20 ft building length, 1 ft eave overhang), the lower slope alone is 560.00 ft² and the upper slope is 264.81 ft² — the lower slope carries roughly two-thirds of the total roof area even though it is not the longer rafter, because it is also the steeper one and it absorbs the eave overhang.

That split is not a fixed ratio. Move the knuckle toward the eave, or steepen the lower pitch further, and the lower segment's share climbs higher still. Recompute it for your own numbers before ordering — do not reuse the 68/32 figure above for a different span or pitch pair.

The area formulas

Both segment formulas fall directly out of the same finish() step that produces the homepage calculator's combined roof area — this page simply keeps the two halves of that sum separate instead of adding them together first.

R1
Lower rafter length, eave to knuckle.
R2
Upper rafter length, knuckle to ridge.
θ
Lower pitch angle, in degrees.
L
Building length, ridge-line run.
oh
Horizontal eave overhang.

Lower-slope area, both sides

A_lower = 2 × (R1 + oh / cos θ) × L

The term oh / cos θ is the overhang's sloped length — a horizontal overhang gets longer once it is measured along the roof plane instead of along the ground.

Upper-slope area, both sides

A_upper = 2 × R2 × L

No overhang term — the overhang is a feature of the eave, which only the lower slope touches.

Total

A_total = A_lower + A_upper

Algebraically identical to 2 × (R1 + R2 + oh / cos θ) × L — the exact roofArea formula the homepage calculator reports.

A_lower + A_upper always equals the combined roof area — this page just shows the two halves of that number instead of only the sum.

Worked example: W = 24 ft, θ = 60°, φ = 25°, knuckle 50%, L = 20 ft, overhang = 1 ft
  1. R1 = 12.00 ft (half-span 12 ft at the knuckle, θ = 60° gives a 12.00 ft lower rafter).
  2. Sloped overhang = 1 ft ÷ cos(60°) = 2.00 ft.
  3. A_lower = 2 × (12.00 + 2.00) × 20 = 560.00 ft².
  4. R2 = 6.62 ft (remaining 12 ft half-span, φ = 25°).
  5. A_upper = 2 × 6.62 × 20 = 264.81 ft².
  6. A_total = 560.00 + 264.81 = 824.81 ft², matching the homepage calculator's roof area for the same inputs exactly.

Sloped area vs. flat footprint

The sloped roof area is always bigger than the flat plan area underneath it. A roof plane tilts up and away from the wall plate, so covering it takes more material per square foot of building footprint than a flat roof over the same floor plan would need — and the steeper the pitch, the worse that ratio gets.

Sloped area vs. flat footprint at a fixed 24 ft span, 20 ft length, 1 ft overhang
Lower / upper pitch Flat footprint Sloped roof area Ratio
45° / 15° 480 ft² 644.4 ft² 1.34×
60° / 20° 480 ft² 815.4 ft² 1.70×
60° / 25° 480 ft² 824.8 ft² 1.72×
70° / 30° 480 ft² 1,095.8 ft² 2.28×
80° / 35° 480 ft² 1,905.4 ft² 3.97×

At a modest 45°/15° pair the roof only costs about 34% more material than the footprint it covers. Push the lower pitch to 80° — a genuinely barn-like profile — and that multiplier climbs past 3.9×, meaning the roof surface alone is nearly four times the flat floor area below it.

A steeper lower pitch buys attic headroom and a taller, barn-style silhouette, but it also buys considerably more roofing material per square foot of floor space below. Use this ratio to sanity-check a quote that looks cheap for the pitch you are asking for — a low bid on a steep gambrel usually means someone under-measured the slope, not that they found a deal.

Turn this into a material order

Five steps from geometry to a purchase order, in the order most suppliers expect the numbers.

  1. Solve the geometry

    Enter span, lower pitch, upper pitch (or switch to half-circle), knuckle position, building length, and eave overhang into the calculator above.

  2. Read the lower- and upper-slope areas

    Note the lower-slope area and upper-slope area separately — these are the two figures material calculators need, not just the combined total.

  3. Check the total against roofArea

    Lower-slope area plus upper-slope area should equal the total roof area shown — if you are cross-checking against the homepage calculator, the two totals must match exactly.

  4. Price shingles or metal panels

    Take the segment areas to the shingle calculator or the metal panel calculator to convert square footage into an order quantity.

  5. Price the sheathing deck

    Use the same segment areas in the sheathing calculator to get a sheet count for the deck underneath the roofing material.

Common questions

Does the roof area include the eave overhang?
Yes. The lower-slope figure includes the sloped length of the eave overhang, converted from the horizontal overhang measurement by dividing by cos(θ). A 1 ft horizontal overhang at a 60° lower pitch adds 2.00 ft of sloped length, not 1 ft, because the overhang follows the roof plane, not the ground.
Why is the lower-slope area so much bigger than the upper-slope area?
Two reasons stack together: the lower slope is steeper, so a given horizontal run produces a longer sloped rafter (R1 > R2 for the same horizontal distance), and the overhang is only added to the lower segment since that is the slope that meets the wall plate. In the worked example below, the lower slope is 560.00 ft² against 264.81 ft² for the upper slope — roughly a 68/32 split, not 50/50.
Do I need the building length, or just the span?
Both. Span (W) and the two pitch angles fix the rafter lengths R1 and R2 for one cross-section; building length (L) is the ridge-line run that turns that cross-section into an actual roof plane. Area scales directly with length, so doubling the building length doubles every area figure on this page.
How is this different from the "Roof area" figure on the homepage calculator?
It is the same number, split in two. The homepage calculator reports one combined roofArea figure (both slopes, both sides, overhang included). This page runs the identical geometry engine and breaks that same total into a lower-slope figure and an upper-slope figure that sum back to it exactly — useful because shingles, panels, and sheathing are usually bought as two line items, not one.
Should I order material by total area or by segment?
By segment, if your supplier allows split orders. The lower and upper slopes often carry different waste factors (the lower slope meets the wall plate and, on some builds, dormers; the upper slope is a plainer field near the ridge), so pricing each segment separately usually gets you a tighter, cheaper order than adding 10% waste to the combined total and hoping it lands right on both slopes.