How do you calculate roof pitch?
Roof pitch is the rise (vertical) over the run (horizontal), usually written as "rise-in-12". Divide the rise by the run and multiply by 12: a roof that rises 5 units over a 12-unit run is a 5-in-12 pitch. The angle is arctan(rise ÷ run) — a 5-in-12 pitch is about 22.6°.
What is the slope factor and why does it matter?
The slope factor is √(rise² + run²) ÷ run — the ratio between the true sloped length of the roof and its flat (plan) length. Multiply a horizontal measurement by it to get the actual roof-surface length. A 5-in-12 roof has a slope factor of about 1.083, so its surface is 8.3% longer than the footprint, which affects how much roofing material you buy.
What is a good roof pitch?
Most residential roofs fall between 4-in-12 and 9-in-12 (about 18° to 37°). Low-slope roofs below 3-in-12 need special membranes to stay watertight, while steep roofs shed water and snow well but are harder and costlier to work on.
How do I measure the pitch of a roof I cannot climb on?
From inside the loft is easiest and safest. Hold a spirit level horizontally against the underside of a rafter, mark 12 inches (or 300 mm) along it, then measure vertically from that mark up to the rafter. That vertical distance is the rise for a 12-inch run, giving the pitch directly. Outside, photograph the gable end square-on from a distance and measure the angle in any image editor, or hold a smartphone inclinometer flat against a rafter or a bargeboard. Avoid measuring against fascia or guttering — they are often set at a slightly different angle from the roof plane itself.
What is the difference between pitch, slope and angle?
They describe the same steepness three different ways, and mixing them up is a common source of ordering errors. Slope is rise over run, written as a ratio such as 6:12 — six units of rise for every twelve of horizontal run. Traditional pitch is rise over the full span rather than the run, so the same roof is a quarter pitch, because the rise is a quarter of the total width. Angle is simply the slope expressed in degrees: 6:12 works out at 26.57 degrees. When a supplier or a building inspector quotes a figure, confirm which of the three they mean before you cut anything.
Does the pitch limit which roof covering I can use?
Yes, and it is usually the deciding constraint. Every covering has a minimum pitch below which wind-driven rain gets forced back under the laps. Standard interlocking concrete tiles generally need about 17.5 degrees, plain clay tiles considerably more at 35 to 40 degrees, natural slate around 20 to 25 degrees depending on slate size and headlap, while standing-seam metal will go down to about 3 degrees and single-ply membrane lower still. Fitting a covering below its stated minimum normally voids the manufacturer's warranty as well as risking leaks, so check the pitch before choosing the material rather than after.