Drainage Fall & Gradient Calculator
Calculate the required fall (drop) for correct drainage gradient. Find the right pipe angle for foul water drainage, surface water drainage, and gutters.
Drainage Gradient Guide
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-cl
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 flexi
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. Bo
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
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