Drainage Fall

Gradient 1:N → fall over length

Pipe run length

Reading 6 m

Pipe type

Gradient

Choose your own to check the gradient you are actually laying.

Top invert level (datum)

Reading 0 mm

Results

Fall

150mm

Gradient

1:40

Length6 m

Top invert (datum)0 mm

Bottom invert150 mm

Slope2.5 % (25 mm per m)

Gradient used1:40 — typical for this pipe, not a minimum

Approved Document H (indicative)

Within the selected threshold: 1:40 is at least as steep as 1:40, the 100 mm minimum for any peak flow (Approved Document H 2015, Table 6). Indicative result.

What to buy — 5 items

110mm underground soil pipe 3m

2 pc

110mm × 90° bend (or 87.5°)

Estimate — count the changes of direction

2 pc

110mm straight coupling

One between each pair of lengths

1 pc

Pea shingle bedding 10mm

~50kg per m of trench

1 t

Marker tape — drainage

1 roll

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Indicative design aids — the main figures and where each comes from are listed under "Figures based on" below.

An indicative fall and gradient check. It does not size the drain for its flow and does not demonstrate compliance with the Building Regulations.

Figures based on

Minimum gradients checked: 100 mm foul 1:40 (any flow), 1:80 (1 l/s or more with a WC); 150 mm foul 1:150 (five or more WCs): Approved Document H 2015 edition (England), Table 6 "Minimum gradients for foul drains". Consistent across secondary sources (Unda, LABC, Flood & Water Pumps) but still to be confirmed against the gov.uk PDF, which could not be read from here. Wales has its own Part H guidance (2023)
Fall, slope and bottom invert: Geometry — fall = length ÷ N; slope = 1 ÷ N; bottom invert = top invert + fall
Typical gradient for each pipe (1:40, 1:80, 1:20, 1:18, 1:60, 1:600, 1:200, 1:50): No source — workshop figures, adjust to suit: "My own gradient" replaces them. Only the two foul-drain rows above are checked against a minimum; for condensate, follow the boiler maker's instructions
Two bends, clips at 0.5 m (waste, condensate), gutter brackets at 1 m, bedding 50 kg per m: No source — workshop estimate: fixed estimating assumptions. The pipe or gutter maker's instructions win
Pipe in 3 m lengths, gutter in 4 m: Typical — common stock lengths. Check what your merchant sells

Indicative design aids. A standard is named with its edition; each is still to be checked against the published text. "Typical" means no standard sets it: check the bag, tin, data sheet or supplier named. "No source" means a workshop estimate: adjust to suit, or, where the box says it is fixed, a fixed estimating assumption.