The short answer
Measure the angle of a rafter from inside the loft with a digital angle finder or a phone inclinometer, or hold a level against the rafter and measure the rise over a set run. Divide the rise by the run and convert the result to degrees. UK tile and slate makers state their minimum pitch in degrees, so degrees are the figure to quote.
Measuring from inside the loft
The safest place to measure a pitch is from below the covering, standing on proper boards in the loft. Pick out a rafter, meaning one of the angled timbers that climb from the wall plate at the eaves to the ridge board at the top. Choose one where a long stretch of the underside is clear of wires, pipes and insulation.
A digital angle finder laid along that rafter reads the pitch in degrees immediately. A phone with an inclinometer app does the same job if it lies flat against the timber, though its accuracy depends on how well the phone is calibrated. Taking it twice, once with each long edge down, shows whether it reads true. If you have neither device, a tape and a level will do the job.
Hold a spirit level horizontal against the rafter so that one end touches it. Measure a run of exactly 1 metre along the level, then measure straight up from the level to the rafter at that point. That vertical distance is the rise over a 1 metre run. A rise of 0.5 metres gives a pitch of about 26.6 degrees, and a rise of 1 metre gives 45 degrees. Take readings in two or three places, as old rafters are rarely perfectly straight, and use the average. Write each number down with the slope it belongs to, so a front and rear reading do not get mixed up.
Degrees, ratios and what the trade uses
Roof angles are quoted in more than one way, which causes confusion when you compare advice. British manufacturers and the British Standards that cover tiling and slating give pitch in degrees. Older books and some builders still talk in ratios, such as 1 in 2, meaning a rise of one unit for every two units of run.
American sources often use inches of rise per 12 inches of run, and you will meet that figure on overseas websites. It does not map directly onto UK tile data, so it is best converted to degrees before you use it. To convert a rise-over-run ratio, divide rise by run and take the inverse tangent on a scientific calculator. A ratio of 1 in 2 gives 26.6 degrees, 1 in 1 gives 45 degrees, and 1 in 4 gives 14 degrees.
If a supplier quotes a minimum pitch, it is measured from the horizontal along the rafter, not from the ridge. Use the same method when you compare your roof with their figure.
Finding the pitch from plans or a photograph
Architects' elevation drawings often state the pitch, and the planning documents for an extension or loft conversion will show it as a label on the roof outline. If the drawing is to scale, you can measure the angle with a protractor, or measure the rise and run on the drawing and work it out.
A photograph taken square-on from the gable end can do the same job. Stand well back, keep the camera level, and draw a horizontal line at the eaves and a line along the slope on the printed image. The angle between them is the pitch. Perspective distorts angles in photographs taken from an angle, so treat the result as a guide and confirm it from the loft.
Mixed roofs need more than one reading. A main slope, a rear extension and a porch may each sit at a different angle, and the part with the lowest pitch sets the choice of covering for that section.
Why pitch decides which covering you can use
No covering works at every angle. Below its stated minimum, wind-driven rain can be forced up beneath the laps and into the roof space, so the maker publishes a lowest safe pitch for each product. Marley's published table lists its Canterbury clay plain tile at 40 degrees, its concrete plain tile at 35 degrees, and its interlocking concrete tiles at figures from 12.5 degrees up to 25 degrees. BS 5534 sets a minimum of 20 degrees for natural slate, with the headlap increasing as the pitch comes down, and Cupa Pizarras gives 115 mm at that angle for a 500 by 300 mm slate in moderate exposure.
So a measurement of 22 degrees does not rule out a tiled roof, but it rules out a lot of tile profiles. It also affects the slate headlap, how much of the covering is visible and whether solar equipment sits well on the slope. Our pages on the most economical roof pitch and on roof valley types show how angle affects other choices on the same roof.
When to get someone to take the reading
If you are planning a re-roof, a solar installation or a change of covering, a reading from a roofer is worth having, because they will measure each slope and check the rafters at the same time. Our page on solar panels on a tiled roof explains what else to check. The age and condition of the roof are part of the same picture, and our page on how long a pitched roof lasts covers that.
Please do not walk the slope to measure it; a fall from a roof is far worse than a wrong number. Our survey records each pitch, photographs the slopes and the loft, and is followed by a written quote. Work then carries a 10-year guarantee on workmanship, given in writing. A quick note of your readings, with a few loft photographs, helps us prepare before the visit. More on tiled roofs is under pitched roofing.