Garden SurveyorSurface Water Runoff Calculator
Runoff risk and SuDS-aware drainage recommendations.
4 advanced inputs hidden. Switch to Advanced to fine-tune.
Order what you need for this job
ACO channel drainage
Drainage Superstore
£18–£40 / linear m
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Heavy-duty weed-control membrane
Amazon
Need ~17 m²
£15–£40 / roll · ~£15–£40
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- Catchment area
- 15m²
- Perimeter m
- 0m
- Peak runoff (l/hr)
- 712
- Peak runoff (m³/hr)
- 0.71
- Runoff coefficient (C)
- 0.95
Heads-up: Where the water goes matters more than how much. Discharging onto a neighbour, the pavement, or the highway is an offence — design the runoff to a legal point on your own land. Why it matters: Part H of the Building Regs covers surface water; councils can require you to dig it up if you skip it.
Drainage & falls
Surface drainage is advised here — set a gentle fall away from the house and any structures.
Lay paving with falls of 1:80 (recommended) or 1:60 (minimum) away from buildings.
Why it matters: Smooth paving needs a positive fall to overcome surface tension and shed water before it ponds in low spots.
If ignored: Insufficient falls cause standing water, algae growth, freeze-thaw damage and water tracking back toward the building.
- • Recommended falls: 1:80
- • Falls 1:80 minimum
- • Surface water must drain to a permeable area or drain
Quantities pre-filled from your calculation (15.0 m²). Partner links — we may earn a small commission if you buy, at no cost to you.
Essentials for this job
Quantities include a ~10% waste allowance. Guide prices are indicative UK ranges (materials only, inc. VAT) — always confirm with the supplier before ordering.
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Ad · Some links are sponsored. We may earn a small commission at no extra cost to you. Suppliers are chosen for fit, not commission.
This app provides general UK guidance and material estimates only. It is not legal, planning, engineering or building-control advice. Always confirm requirements with your local planning authority, building control, utility providers, manufacturers or qualified professionals.