Setting out
Fall from a 1:n gradient
- where
- fall (mm)
- horizontal run (m)
- gradient denominator (1:n)
- end level (m)
- start level (m)
Explain this equation
Works out how much a surface drops over a given distance at a stated 1:n gradient, and the level at the far end. Use it for patios, drainage runs, flat roofs and anything laid to a fall.
x is the horizontal run in metres. n is the gradient denominator: 1:80 means 1 unit of drop for every 80 units along. f is the resulting fall in millimetres. z1 is the starting level and z2 the finished level at the far end, both in metres.
A 1:n gradient drops by 1/n of the run, so the fall is x/n in metres. The × 1000 converts it to millimetres for marking out. The end level is the start level minus that same fall.
5 m at 1:80 falls 62.5 mm. At 1:40 the same run falls 125 mm: halving n doubles the fall.
Watch out: the calculator does not choose a gradient. Use the fall specified for that detail.
Calculate a vertical fall from a specified horizontal distance and gradient.
Enter your values to see the result.
Good to know
Use the fall specified for the particular detail. No universal paving, DPC or drainage fall is assumed.
Sources
- Geometry / arithmetic
This is a calculation aid, not a structural design or compliance certificate. Every default is an example, not a specification for your project.