Drainage Fall & Gradient Calculator

Drainage Gradient Guide

What's the key thing to understand about Correct Gradients for Drainage?

Building Regulations Approved Document H (drainage) gives minimum gradients. Foul water (waste from WCs, sinks, baths): 100mm diameter minimum 1:80 gradient (12.5mm fall per metre). 150mm diameter minimum 1:150. Surface water drainage: 1:100 to 1:200. Gutter: minimum 1:600 fall (BS EN 12056-3). Too flat: waste and solids settle, blockages occur. Too steep: water runs fast but solids lag behind, also causing blockages. The 'Maguire Rule': flow velocity should be at least 0.75m/s to remain self-cleansing, preventing solid waste from settling and building up inside the pipe over time.

What should I know about Gradient Calculation?

Fall (mm) = length (mm) ÷ gradient (the X in 1:X). For 8m pipe at 1:60: fall = 8000mm ÷ 60 = 133mm = 13.3cm. Practical application: if laying a 110mm soil pipe from a house extension to connect to the main soil stack 8m away, at 1:80 gradient the pipe invert must drop 8000/80 = 100mm (10cm). Measure from the invert (inside bottom of pipe) not the outside. Clay or concrete pipes: gradients are more critical — minor movement after installation can alter the fall. Push-fit plastic pipes: more flexible to work with and forgiving of minor ground movement after installation, compared to the rigid joints of clay or concrete pipework.

What's the key thing to understand about Gutter Falls?

Half-round gutters (most common UK type): minimum 1:600 fall (approximately 1.7mm per metre). Practical approach: for a 10m gutter run: drop = 10000/600 = 16.7mm ≈ 17mm total fall. Install the downpipe bracket at the correct height, then string a line to the high end at +17mm, fitting all intermediate brackets to the string. High end furthest from the downpipe. For gutters draining to both ends (double fall): the highest point is at the centre — each half falls toward its respective downpipe. Both halves should have an identical fall to ensure water drains evenly from each end rather than pooling at the centre.

What do I need to know about Underground Drainage?

Building Regulations Part H for underground drainage: minimum 110mm diameter for any soil drain (carrying WC waste). Minimum gradient: 1:80 for 100-150mm pipes. Minimum depth of cover: 600mm in soft ground, 900mm under roads and drives. Bedding: 100mm granular material (pea gravel or granite chips) below pipe invert, haunch and surround with granular to above pipe crown. Inspection chambers (manholes): maximum 45m between inspection chambers on straight runs. Rodding eyes at junctions. Flushing eyes provide access for rodding a blockage from ground level without needing to excavate down to the pipe itself.