Converting a gradient such as 1:40 over a pipe length into the actual vertical fall in millimetres.
Pipes that drain by gravity need the right slope, called the fall. Gradient is written as a ratio, and this example shows how to turn that ratio into a real measurement. The figures are chosen for the example.
The situation. A branch waste pipe is 6 metres long and is to be laid at a gradient of 1 in 40 (written 1:40).
What 1:40 means. For every 40 units measured along the pipe horizontally, the pipe drops 1 unit vertically.
Step 1 — divide length by the ratio number. Fall = length / 40 = 6 m / 40 = 0.15 m.
Step 2 — convert to millimetres. 0.15 m x 1000 = 150 mm of fall over the 6 metre run.
Reading it on site. The pipe end farthest from the stack is set 150 mm higher than the end at the stack, so water runs towards the stack.
Why the slope is a balance. Too shallow and water moves too slowly, letting solids settle and block the pipe. Too steep and the water can race away and leave solids stranded. That is why gradients are specified within a range. Confirm the correct gradient range for the pipe size against Approved Document H.
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