Patio Fall Calculator

Total fall, high and low points, puddling warnings.

All calculators

Inputs

Run(m)

Fall ratio (1:x)

Site context (optional)

Helps tailor algae, weather and longevity advice.

Aspect

Exposure

Confidencelow· 42Complexitylow· 10

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Run: 4m
Fall ratio: 1:80
Total fall: 50mm
High point (above low): 50mm
Low point: 0mm

Heads-up: Fall is always away from the house (or to a channel drain that takes it away from the house). 1:80 = 12.5 mm drop per metre of run. Why it matters: Reverse falls or shallow falls cause damp, frost lift and slip risk.

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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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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.

Worked example

A 4.0 m patio falling away from the house at the standard 1:80.

Inputs

Run: 4.0 m
Fall ratio: 1:80

Step by step calculation for this example
Step	Working	Result
Total fall	4000 mm ÷ 80	50 mm
High point	set at the house, below the damp-proof course	50 mm
Low point	the far edge, where the water leaves	0 mm

What to order for this example
What to order	Quantity
Total fall	50 mm
High point (above low)	50 mm
Low point	0 mm

In short: Over a 4.0 m run at 1:80 the patio drops 50 mm, so string your levels 50 mm lower at the far edge than at the house.