Formula
When you enter the slope area, tiles are that area times tiles per square metre times one plus waste, rounded up.
When you only have the plan, the slope factor is 1 divided by the cosine of the pitch. Slope area is plan area times that factor. A pitch of 0° has a factor of 1.
The tile count uses the slope area before it is rounded for display. Pitch outside 0–60° is rejected.
Worked example
Slope area 30 m² at 10 tiles per m².
With 10% waste that is 330.0 tiles before rounding, exact. The count uses the unrounded slope area.
Assumptions
Interlocking concrete starts at 10 tiles per square metre, between the two Marley Modern laps. Plain tiles start at 60, the Marley plain-tile figure.
The main example is 30 m² of slope. The plan example, behind the extra fields, is 28 m² at 22.5°.
This is a count of tiles, not a roof design.
Checked
Checked 10 November 2026. Marley Modern interlocking tiles are published at 9.9 per m² with a 75 mm headlap and 10.7 with a 100 mm headlap, so this page starts at 10. Marley concrete plain tiles are listed at 60 per m².