Calculating roof pitch in degrees and as a percentage

Percentage gradient is 100 times the vertical rise divided by horizontal run. For example, a rise of 0.5 m over a horizontal distance of 10 m gives 5%. Use the same units for both distances. The rafter length measured up the slope is not the denominator.

This is convenient on shallow roofs because it gives the height difference between supports directly. A 7% gradient across an 8 m horizontal run requires 0.56 m of rise. That is a geometric result. Check the proposed panel’s permitted application separately.

Multiply the tangent of the pitch angle by 100 to obtain the gradient in percent. To convert back, divide the percentage by 100 and take the inverse tangent. Select degrees on the calculator when entering or reading an angle in degrees. Radian mode gives a different numerical value.

A pitch of 5° is about 8.75%, while a 10% gradient is about 5.71°. At 15° the gradient is about 26.79%. At 45° the rise equals the horizontal run and the gradient is 100%. This is why a requirement of 5° describes a steeper roof than 5%.

Measure the geometry relevant to the task. Drawings may give a main pitch even though local deflection or uneven supports alter the drainage path. During renovation compare the drawing with the actual structure. A measurement on one short section may miss a hollow elsewhere.

For roof area and sheet orders, you also need the true length along the slope. On a simple plane this can be calculated, but final fabrication dimensions must include the eaves and ridge details. A unit error can affect both the choice of covering and the ordered sheet lengths.

Product documentation sets the conditions under which a profile and its laps can shed water. Rib height, sheet lengths, overlaps and seals all matter. Even within the T18 or T35 families, individual products can have different limits. Apply the instructions for the exact profile ordered.

Tile-effect sheets also have model-specific pitch ranges. Check the required underlay as well. Near the minimum pitch, workmanship at junctions is particularly important. Do not assume the manufacturer allows construction tolerances beyond the stated limit.

For roof sandwich panels, a continuous eaves-to-ridge panel may have a different minimum pitch from shorter panels with end laps. Introducing rooflights can change the limit again. Product specifications differ, so there is no single minimum pitch for every PIR roof panel installation.

A single long panel avoids some transverse joints but requires suitable transport and lifting. Check available production lengths, thermal movement and supports. If a design changes to split panels, verify the pitch and lap detail again before amending the order.

If the pitch is insufficient, first establish what system can handle the geometry and loads. Extra sealant at overlaps is no substitute for that assessment. Changes to the structure or roof build-up may be required. On a flat roof, structural steel decking can support a separate waterproof membrane, serving a different function from exposed profiled roof sheeting.

State the pitch and its unit in the enquiry, together with slope length, joints and openings. Ask the supplier to confirm the requirements of the exact product. This gives the designer, estimator and installation crew the same basis for checking both geometry and suitability.