Kevin Taylor, Training and Technical Support Manager at Marley, outlines how builders can get to the right pitch number.
Roof pitch is the steepness of the roof. The standard pitch for most UK homes is between 25° and 40°, but steeper roofs may go as high as 70° and low-pitch roofs can be as little as 15°. Importantly, if a roof’s slope is less than 10°, it’s considered a flat roof.
Roofs come in a wide range of shapes and types, including the most common gable roofs and hip roofs to more complex designs. But by finding the right triangulation in the roof’s geometry, it’s possible to calculate the roof pitch in one of three ways: geometry, calculation or using a pitch finder.
Geometry
Taking a few measurements from inside the loft space can help determine the height and half span of the roof, and from there we can work out the roof pitch. The height is taken from the underside of the ridge to the bottom edge of the ceiling joists, or to the decking if boarded. Next, measure across the span (width) of the roof and divide by two. This produces two numbers, for example: height of roof = 3m, half span = 4m.
From here, draw the two lines at right angles to each other using these ratios (e.g., 3cm and 4cm, 6cm and 8 cm etc.), join up the triangle and simply measure the angle with a protractor. In this case, the angle would be 37°.
It may also be possible to roughly establish the half span of the roof and the height from the outside of the building. To do this, first, measure the half span of the building (or measure it all then half it), then count the brick courses up from the eaves line. If the bricks are a standard size, multiply the number of courses by 75mm (65mm + 10mm mortar joint) or 0.75m to give the height of the roof.
Trigonometry/calculation
The in-built calculators on most smartphones have made finding the angle much simpler.
Here’s the method using the same numbers as before: height of roof = 3m, half span = 4m. To work out the pitch using a smart phone or scientific calculator, first, divide height by half span:
3 / 4 = 0.75
Enter this number then, find and press Tan-1 on a calculator (this may mean pressing 2nd function), or turning the phone landscape to access the scientific functions. The answer should be 36.87 (37°).
Using a pitch finder
Most smartphones also now have an angle or pitch finder app as standard, if not they can be easily downloaded and are often free of charge. To use a pitch finder on a phone while inside the loft, simply align the appropriate edge to the underside of one of the rafters to give the roof pitch.
The roof pitch can be established from ground level outside the building. Stand back and square on to the side of the building and line up the pitch finder with the roof slope. Standalone digital pitch finders are generally considered to be more accurate than phone-based apps, and they are often relatively inexpensive and easy to source online and in most roofing merchants. They can prove to be an invaluable tool if a more precise reading is needed.
For more information on Marley visit Pitched Roof Tile Specialists | Marley.
