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Roof Pitch Calculator

Enter the building span and roof pitch to get rafter length, rise, roof angle and slope factor — with a scaled cross-section diagram.

Building span

Roof pitch (X/12)

Slope factor: 1.118 · 50% grade · 26.57° angle

Eave overhang

Run: 14.00 ftRise: 7.00 ftRafter: 17.33 ft26.6°6/12 pitch

Right-triangle cross-section. Rafter shown includes overhang.

You'll need
17.33 ft
17 ft 3 15/16 in
+ 7 ft of 7 ft 0 in

28 ft span at 6/12 pitch · 18 in eave overhang.

Materials list

Framing nails1 box
16d 3-1/2 in galvanized — for nailing rafters to the top plate and ridge board.
Rafter ties / hurricane clips43 pieces
One per rafter on both bearing walls — code-required in most jurisdictions to resist uplift.

Tools you'll want

Buy-once items — skip any you already own.

Speed square / rafter square
Marks rafter cuts directly from the pitch — the fastest way to lay out birdsmouths and plumb cuts.
Framing naileroptional
Pneumatic framer dramatically speeds up rafter installation — rent for a weekend if you don’t own one.
4-ft leveloptional
A long level confirms the ridge runs true and the rafters are plumb.

Where to buy (optional)

Happy with your list? Tap any item below to open it at a retailer. These are affiliate links that support the site at no extra cost — but shop wherever you like.

Estimates only — verify quantities before buying.

How to calculate roof pitch

Roof pitch is the ratio of vertical rise to horizontal run, expressed as X-in-12. A 6/12 pitch means 6 inches of vertical rise for every 12 inches of horizontal travel. Once you know the pitch and the building span, every other measurement falls out of basic right-triangle geometry:

run    = span ÷ 2
rise   = (pitch ÷ 12) × run
rafter = √(run² + rise²)            ← body length, no overhang
slope factor = √(pitch² + 144) ÷ 12
total rafter = rafter + (overhang × slope factor)
angle  = arctan(pitch ÷ 12)

The slope factor is the same value you multiply a roof footprint by to get the actual sloped roof area — it shows up in every roofing material calculation, not just rafters.

Common pitches and what they look like

  • 2/12 to 3/12 — Low slope. Shingles need extra underlayment; below 3/12 most codes require membrane roofing.
  • 4/12 to 5/12 — Standard low residential. Easy to walk, sheds water adequately.
  • 6/12 — The North American residential default. Visible but not steep.
  • 7/12 to 9/12 — Steeper styles like Cape Cod and Colonial. Safety harness recommended.
  • 10/12 and up — Steep — roof jacks or staging required to walk safely.
  • 12/12 — Exactly 45°. Equal rise and run.

Common mistakes

  • Forgetting overhang. The rafter has to extend past the wall — typically 12 to 24 inches. Cut to body length only and the eave will be flush with the siding.
  • Using span instead of run. The pitch formula uses the run (half the span). Plugging in the full span doubles the rise.
  • Measuring pitch from the underside. A 12 in level held against the underside of a rafter gives the right number — but only if the rafter is straight. Old sagging rafters lie.
  • Mixing units. Always work in inches for the X/12 ratio, then convert the run and rise to whichever unit you want for the final cut.

Frequently asked questions

What does a 6/12 roof pitch mean?+

A 6/12 roof pitch means the roof rises 6 inches vertically for every 12 inches of horizontal run. It is the most common residential pitch in North America — steep enough to shed water and snow well, but shallow enough to walk on safely during installation. The number before the slash is the rise; 12 is always the run.

How do I calculate rafter length from pitch and span?+

Split the span in half to get the run, multiply (pitch ÷ 12) by the run to get the rise, then take the square root of run² + rise². That gives you the body length of the rafter from ridge to top plate. For the total cut length, add the overhang multiplied by the slope factor: √(pitch² + 144) ÷ 12.

What is the slope factor?+

The slope factor is how much longer a sloped surface is than its horizontal projection. For a 6/12 pitch the slope factor is about 1.118 — meaning every 1 ft of horizontal distance equals 1.118 ft along the rafter. Use it to convert horizontal eave overhang into the extra rafter length you need to cut.

What is the steepest pitch I can walk on?+

Most roofers consider anything up to 6/12 walkable in dry conditions without specialized gear. Between 7/12 and 9/12 you should be wearing rubber-soled boots, using a safety harness, and ideally roof jacks for footing. Anything 10/12 or steeper requires staging, scaffolding, or a roof ladder — it is no longer reasonable to free-climb.

How do I measure the pitch of an existing roof?+

Set a 12-inch level horizontally against the underside of a rafter so one end touches the rafter and the other end sticks out into the attic. Measure straight down from the 12-inch mark to the rafter. That distance in inches is the rise — so a 5-inch measurement means a 5/12 pitch. You can also use a smartphone protractor app held flat against the roof surface.

What pitch counts as a "low slope" roof?+

Anything below 4/12 is generally considered low slope. Asphalt shingles can technically be installed down to 2/12 with extra underlayment, but below 3/12 most building codes require a fully adhered membrane (modified bitumen, EPDM, or TPO) instead of shingles to prevent wind-driven rain from backing under the joints.

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Reviewed by the RenoSheets team for calculation accuracy. Method: right-triangle geometry with the standard slope factor √(pitch² + 144) ÷ 12 for the overhang projection. Last updated 2026-06-02. Estimates only — verify rafter dimensions against your framing plan before cutting.