How to Calculate Roof Pitch: A Practical UK Guide

How to calculate roof pitch from two measurements

Roof pitch describes how steeply a roof rises compared with its horizontal distance. The repeatable method is to measure the rise and run of the same roof section, then divide the rise by the run. In the UK, millimetres are a convenient unit because tape measures, roof plans, tiles and many manufacturer instructions use metric dimensions.

The rise is the vertical distance from the eaves level to a higher point, such as the ridge, over the part of the roof being assessed. The run is the horizontal distance from the same vertical reference to that higher point. For a symmetrical pitched roof measured from the eaves to the ridge, the run is usually half the horizontal span, not the full span.

For example, if a roof rises 2,400 mm over a horizontal run of 4,000 mm, divide 2,400 by 4,000. The result is 0.6, which can be written as a ratio of 3:5, a percentage of 60%, or an angle of about 31 degrees. These are different ways of describing the same slope, so label the format clearly when recording it.

• Measure rise and run in the same unit.
• Use half the span when measuring from eaves to ridge on a symmetrical roof.
• Record whether the result is a ratio, percentage or angle.
• Round only after the calculation, not before it.

Choose a safe measuring point

The most reliable measurement is taken from a roof plan, a section drawing, or a safe accessible location such as a loft. Look for a clear vertical line from the eaves or wall plate to the ridge, and a horizontal line between those same reference points. Avoid measuring along the sloping roof surface because that gives the rafter or slope length rather than the run.

In a loft, use a spirit level, straightedge and tape measure if the structure can be accessed safely. Hold the level horizontally to identify a true horizontal line, then measure the vertical gap to the underside of the roof covering or rafter at the chosen point. If insulation, plasterboard, storage, services or restricted access obstructs the view, use drawings or an external survey instead of disturbing the construction.

Check that the roof is not made up of several different pitches. Dormers, side returns, hips, gables, porches and extensions may each have their own slope. A single average figure can be misleading when ordering roof windows, replacement tiles, flashing or ridge components.

• Use structural or drawing reference points rather than the outer edge of a tile.
• Keep the level horizontal and the tape vertical.
• Measure more than one location if the roof may have settled or changed.
• Record the location, date and access limitations with the result.

Convert the result into the format you need

A roof pitch ratio is commonly written as rise:run. A measurement of 2,400 mm rise and 4,000 mm run becomes 2,400:4,000, which simplifies by dividing both numbers by 800 to 3:5. Some roofing information instead uses a format such as 3 in 5, while technical documents may specify an angle in degrees.

To calculate the percentage pitch, multiply rise divided by run by 100. In the example, 2,400 ÷ 4,000 × 100 equals 60%. This percentage is useful for a quick comparison, but it is not the same as a 60-degree angle; percentage describes vertical change against horizontal distance, while degrees describe the angle between the slope and the horizontal.

To calculate degrees, use the inverse tangent function on a calculator: arctan(rise ÷ run). Set the calculator to degrees rather than radians. For the example, arctan(2,400 ÷ 4,000) is approximately 31.0 degrees, which can be recorded as 31 degrees or 31.0° depending on the precision required.

• Ratio: rise:run, simplified where practical.
• Percentage: rise ÷ run × 100.
• Degrees: arctan(rise ÷ run), with the calculator set to degrees.
• Use the same format as the manufacturer, designer or contractor requests.

Check the calculation against the roof covering

The calculated pitch is a measurement, not approval that a particular covering is suitable. Roof tiles, slates, membranes, roof windows, solar equipment and flashing systems can have product-specific pitch limits and installation conditions. Check the current instructions for the exact product, including requirements for battening, laps, underlay, fixings, ventilation and exposure.

Common UK roof coverings include interlocking concrete tiles, plain tiles, natural slate and fibre-cement slate. Their suitability can differ even when they look similar. A roof with a low pitch may need a product or construction detail specifically designed for that condition, so do not substitute a familiar tile solely because its dimensions appear to fit.

For a roof window or dormer, calculate the pitch of the roof plane where the fitting will be installed rather than relying on the main roof pitch. Confirm the opening size, rafter arrangement, flashing kit and product pitch range before cutting or ordering. Building regulations, planning requirements and structural changes may also need separate checks through the relevant UK authority or competent professional.

• Match the pitch to the exact product instructions.
• Check low-pitch, wind-exposure and weathering requirements.
• Confirm roof window flashing and pitch compatibility.
• Ask building control or a qualified designer about structural alterations.

Use plan dimensions and slope length correctly

Architectural drawings may show a roof span, ridge height or sloping length instead of the rise and run directly. If you know the full span of a symmetrical roof, divide it by two to obtain the run from eaves to ridge. If you know the sloping length and one of the right-triangle dimensions, use the appropriate geometric relationship or obtain the missing dimension from the designer.

The sloping length is also called the rafter length or hypotenuse in a right-triangle model. It is not interchangeable with the run. Confusing these measurements can produce a pitch that is too steep or too shallow, especially on a roof with wide eaves or a complex junction.

When working from a plan, check the drawing scale, units, revision date and whether dimensions refer to structural framing, finished roof covering or external walls. Note whether the drawing shows the roof before or after insulation, lining, fascias or roof alterations. If a drawing and site measurement disagree materially, stop the ordering process and resolve the discrepancy.

• Check whether a dimension is span, half-span, rise or slope length.
• Confirm the drawing scale and units.
• Use the current drawing revision.
• Resolve discrepancies before ordering materials.

Avoid common roof-pitch measurement errors

One common error is measuring from the floor of the loft to the ridge instead of from the eaves or wall plate. Another is measuring the roof covering’s visible edge when the required calculation relates to the structural roof plane. Mark the exact start and end points before taking each measurement.

Another error is mixing units, such as recording a rise in metres and a run in millimetres. Convert both to millimetres first: 2.4 m becomes 2,400 mm. Also avoid rounding a 2,375 mm rise to 2,400 mm until the calculation is complete, because small differences may matter when checking a product’s minimum pitch.

Hipped roofs, unequal roof slopes and extensions require separate calculations for each plane. If the roof has sagging, irregular geometry or evidence of movement, a simple two-dimensional measurement may not represent the condition of the whole roof. In those cases, arrange an inspection by a suitably qualified roofing or structural professional.

• Do not use the loft floor as the run reference unless it is demonstrably level and correctly positioned.
• Convert all measurements to millimetres before dividing.
• Do not treat a hipped or extended roof as one uniform plane.
• Investigate movement, sagging or water damage separately.

Roof pitch verification checklist

Before using the result for a quote, design decision or material order, repeat the calculation independently. Measure the rise and run again, preferably from a second reference point or drawing. If both calculations agree within the accuracy needed for the job, record the final figure and the method used.

Keep a short measurement record with the roof plane, date, rise, run, ratio, percentage, angle and any assumptions. Attach a sketch or photograph that does not expose personal information and mark the measurement points. This makes the result easier for a roofer, surveyor, designer or building control professional to review.

The final check is practical: compare the recorded pitch with the exact product literature and confirm whether the work involves structural alteration, a change to the roof covering or a new opening. Where the result affects safety, weatherproofing or compliance, obtain professional confirmation before work starts.

• Rise and run are measured on the same roof plane.
• Both measurements use millimetres and the same reference points.
• Ratio, percentage and angle agree when converted.
• The result matches the current product instructions.
• Any structural or regulatory questions have been referred to the appropriate professional or authority.

Scope and limitations

This note helps householders organise measurements and understand the calculation. It does not replace an on-site decision by a designer, installer, contractor, or manufacturer. Fabrication, structure, electrical work, gas, waterproofing, and personal safety require suitably qualified verification.