RenoSheets
ENFR

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)6/12

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

Eave overhang

6/12 pitch

6/1226.6°
Drag to rotate · scroll to zoom · double-click to reset
You'll need
17.33 ft
  • 17 ft 3 15/16 in
  • 7 ft of 7 ft 0 in

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.

Estimates only — verify quantities before buying.

What actually determines a roof's pitch

Roof pitch isn't just a design choice — it's driven by climate, roofing material, and structural type. Steeper pitches (8-in-12 and above) shed snow and rain faster, which is why they're standard in heavy-snowfall regions, while low-slope roofs are more common in dry or mild climates. But the roofing material has its own minimum: asphalt shingles typically need at least a 2-in-12 to 3-in-12 pitch to shed water properly, while anything flatter needs a membrane roofing system instead. Pitch also determines whether a roof uses rafters or trusses — steep custom rooflines are usually rafter-built on site, while simple, consistent pitches are often engineered as prefabricated trusses, which is faster to install but harder to modify later. Rafter length isn't just rise and run either: the birdsmouth cut at the wall plate and the overhang beyond it both add to the total board length you need to buy, and getting either wrong on a steep pitch can mean an expensive reorder.

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.

Related calculators

Roofing Calculator Insulation Calculator
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.