Construction and design
How to read a roof pitch shown in degrees or per cent
Before choosing a covering, establish whether the stated pitch is in degrees or percent — these units are not interchangeable. Check that the value applies to the specific roof slope and accounts for joints and roof penetrations. Percent describes the rise relative to horizontal distance, not the length measured along the slope. For T18 and T35 sheets and roof panels, instructions may use different units, so before requesting a quote, state the angle or pitch with its unit, the slope length and the planned details.
What does pitch expressed as a percentage mean?
Percentage pitch is the ratio of the height difference to the horizontal distance multiplied by one hundred. If the roof rises by 1 metre over a horizontal distance of 4 metres, the pitch is 25%. The denominator does not use the length measured along the slope. Confusing these two distances changes the result and can lead to the wrong choice of roofing.
On a gable roof, the calculation relates to a specific roof slope. With a symmetrical layout, the horizontal distance from the eaves to the ridge is half the width between the relevant points. In a real building, however, the geometry and the eaves overhang must be taken into account. The best basis is a dimensioned section drawing, not an estimate of the angle from a photograph.
How do you convert percentages to degrees?
The relationship is: pitch in percent = 100 × tangent of the angle. Conversely, the angle is obtained by the arctangent of the pitch divided by one hundred. For a pitch of 25%, the result is approximately 14°. A roof inclined at 15° has a pitch of approximately 26.8%. These figures show why 15% is not equivalent to 15°.
At low pitches, the error is particularly significant. A requirement of 5% means approximately 2.9°, not 5°. Conversely, 5° is approximately 8.7%. Before using a calculator, the unit of the angle must be checked. The result should be retained with a precision appropriate to the project. Rounding down near the permissible limit should not be the basis for a decision on the suitability of the roofing.
Why does the angle affect the choice of material?
At a lower pitch, water drains more slowly, and the joints are more susceptible to water being drawn up and to wind action. The behaviour of snow also changes. The roofing therefore has specific conditions of use. It cannot be assumed that any steel sheet is watertight at the same pitch. The geometry of the laps and the method of installation are of key importance.
T18, T35, tile-effect metal roofing and roof sandwich panels may have different requirements. Even within one category, different values occur. The data must be read from the instructions for the specific product. The name trapezoidal steel sheet is not a technical parameter sufficient for assessing a roof. If the material is changed, the pitch must be checked again.
What changes when joining sheets and installing skylights?
A transverse joint creates an additional detail along the drainage path. It may require a greater pitch, a suitable lap and sealing. The instructions may treat a full sheet from eaves to ridge differently from roofing joined along its length. The lowest value in the data sheet should not be selected without checking which configuration it is assigned to.
Skylights and other penetrations behave in a similar way. In roofs with sandwich panels, the conditions for slopes without skylights may differ from those with rooflights. The actual geometry after deflection of the structure must also be taken into account. The nominal fall from the design does not guarantee drainage if a local depression forms at poorly executed support.
How can the pitch of an existing roof be determined safely?
It is best to use documentation and measurements taken from a safe position. The measurement should determine the height difference and the horizontal distance in the same section. It is not worth climbing onto a wet slope just to hold a phone against the sheet. The uneven profile of the metal sheet and incorrect positioning of the device can further distort the result.
In an older roof, the fall may differ locally due to deflections or alterations. If the result determines the acceptability of a new covering, it should be verified by the contractor or designer. For renovation, the length of the slope and the position of the outlets are also important. Measuring the angle alone without assessing the condition of the structure does not describe all drainage conditions.
How should the data be recorded in a request for roofing?
State the value and unit, for example an angle in degrees or a fall in percent. Add the length of the slope, planned connections and penetrations. It is worth attaching a simple section drawing. This way the seller does not have to guess what the note fall five means. This ambiguity can have major consequences on a roof with a low pitch.
Ask for confirmation of the conditions for the selected profile and arrangement. Do not replace them with a general value found for another product. Correct conversion of units is the first step. Only by combining the angle, geometry, supports and method of connection can a covering be selected that will drain water in accordance with the design.
In the order documents, it is also worth keeping the source of the measurement and the date it was taken. This will make verification easier if the roof geometry changes during renovation.
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