Roof Area Calculator
This calculator sums roof area across multiple independent sections — each with its own length, width, and pitch — using roofArea = length × width × slope factor per section, added together. Built for L-shaped houses, attached garages, and porches at a different pitch than the main roof.
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Scenario presets
| Section | Length (ft) | Width (ft) | Pitch (x/12) | Roof area | Actions |
|---|---|---|---|---|---|
| 1 | 1,073 sq ft | ||||
| 2 | 126 sq ft |
Each section is a simple rectangle at its own pitch. Break an irregular roof into as many rectangles as it takes to approximate its shape.
- Total footprint area
- 1,080 sq ft
- Total squares
- 12
How it works
Each section is treated as its own simple rectangle at its own pitch, exactly like the Roof Square Footage Calculator — this tool just runs that calculation once per section and adds the results.
totalRoofArea = Σ (sectionLength × sectionWidth × slopeFactor)
Because each section keeps its own pitch, a steep main roof and a shallow porch roof are never averaged together — averaging pitches before calculating area produces a different, less accurate number than calculating each section separately.
Worked example
A house has a main roof (36 ft × 26 ft, 8/12 pitch) and an attached porch roof (20 ft × 8 ft, 3/12 pitch).
Main roof. Slope factor for 8/12 = √(144+64) ÷ 12 = 1.202. Area = 36 × 26 × 1.202 = 1,125.5 sq ft.
Porch roof. Slope factor for 3/12 = √(144+9) ÷ 12 = 1.031. Area = 20 × 8 × 1.031 = 164.9 sq ft.
Total. 1,125.5 + 164.9 = 1,290.4 sq ft, or 12.9 squares.
Tips from the field
- Measure each section to the edge of the roofline, including overhang, if you want overhang reflected in the total — this tool doesn't add a separate overhang correction per section.
- Split an L-shaped or T-shaped roof at the natural break in the roofline, not an arbitrary line through the middle.
- Keep a small sketch of which section is which while you measure — it's easy to lose track past three or four sections.
- Round the combined total, not each individual section, when ordering material.
What this tool does not do
- Does not handle triangular or curved sections — approximate them as rectangles.
- Does not apply a separate overhang correction per section — fold overhang into your length/width measurement instead.
- For a single-rectangle roof with overhang and roof-type handling, the Roof Square Footage Calculator is a better fit.
For a single simple footprint, see the Roof Square Footage Calculator. To turn this total into shingle bundles, see the Shingle Calculator.
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Frequently asked questions
These are the questions that come up when a roof is not one simple rectangle — an attached garage, an L-shaped wing, or a porch at a different pitch.
Why break the roof into sections instead of using one footprint?
Because the footprint-times-slope-factor method assumes a single rectangle at a single pitch. Real houses are often an L, a T, or a main roof with an attached garage or porch at a different pitch — each of those needs its own calculation before adding the areas together.
How do I break an L-shaped house into rectangles?
Split it into two overlapping or adjoining rectangles that together cover the full footprint, being careful not to double-count the corner where they meet. Measure each rectangle's own length, width, and pitch.
What if two sections share a wall, like a house and an attached garage?
Treat them as two separate sections with their own length, width, and pitch. The shared wall doesn't need special handling — you're calculating each roof plane's surface area, not the building's interior floor plan.
Should overhang be included in each section separately?
This calculator keeps section input simple (length, width, pitch) without a separate overhang field — measure each section's length and width to the edge of the roofline (including overhang) rather than the wall line, if you want overhang reflected in the total.
Can each section have a different pitch?
Yes — that is the main reason to use this calculator instead of the single-pitch Roof Square Footage Calculator. A porch roof might be 3/12 while the main house is 8/12; each row uses its own pitch.
How many sections can I add?
As many as your roof needs — click "Add section" for each additional rectangle. There is no practical limit.
What does "total squares" mean in the result?
It is the combined roof area from every section, divided by 100 — the standard roofing unit used for ordering shingles, underlayment, and most other roofing material.
Does this calculator handle triangular sections, like a gable end wall or a dormer?
Not directly — it works in rectangles. Approximate a triangular section as a rectangle at roughly half its full height, or treat small features like dormers as a rough percentage add-on to the nearest full section.
Why does my total come out different from a single-footprint estimate?
A multi-section, multi-pitch calculation is more accurate for irregular roofs than a single blended footprint and pitch — small sections at a different pitch than the main roof can shift the total more than expected.
Should I round each section up before adding, or round the total at the end?
Round at the end. Rounding each section up individually compounds waste allowance across every section instead of applying it once to the true total.
Is this the right tool for a simple, single-rectangle roof?
It works, but the Roof Square Footage Calculator is built specifically for that case and adds overhang and roof-type (gable/hip/shed) handling that this multi-section tool keeps intentionally simple.
How we checked this page
- Written by
- Pitch & Span Editorial
- Reviewed by
- Per-section formula checked against the same slope-factor geometry used across the site; multi-section total verified to equal the sum of independently-calculated sections.
- Last reviewed
- 2026-07-28
- Publisher
- Pitch & Span
- Scope
- Multi-section roof area estimation
- What we checked
-
- Worked example (main 36x26 @ 8/12, porch 20x8 @ 3/12) verified against the live calculator.
- Confirmed that adding sections at different pitches produces a different (correct) total than averaging pitches first.
- Confirmed the total updates correctly after adding and removing sections.
Cite this calculator
Pitch & Span. "Roof Area Calculator." Pitch & Span, https://pitchandspan.com/roof-area-calculator/. Accessed .
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