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110 mm drain fall: 1:40, 1:80, or 1:100

Updated: October 2026

Someone has told you a house drain must fall at 1 in 40, and someone else has said 1 in 80. Both numbers are in Approved Document H, and they are different rows. A 110 mm drain fall is the plastic way of talking about a 100 mm nominal drain. Foul water with a WC, a low flow, and rainwater do not share one gradient. This page picks the row. It does not rank pipes or describe a chamfer. The [underground drainage pipe comparison](/hub/rods-and-pipes/underground-drainage-pipe) is the product table. The Products Compared editorial team is following the 2015 document, checked on 16 October 2026, not a private calculator. Scotland and Northern Ireland use their own regulations.

Someone has told you a house drain must fall at 1 in 40, and someone else has said 1 in 80. Both numbers are in Approved Document H, and they are different rows.

Our top picks

GIONAR GIONAR catch-basin and downspout kit

Best overall

GIONAR GIONAR catch-basin and downspout kit
6.2/10about £20.99
View offer
eco-filtration eco-filtration 110 mm solvent-weld pipe, 1 m

Alternative

eco-filtration eco-filtration 110 mm solvent-weld pipe, 1 m
4.6/10about £19.99
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lcyen lcyen flexible downpipe, 21 to 60 inches

Best value

lcyen lcyen flexible downpipe, 21 to 60 inches
4.0/10about £12.99
View offer
On this page
  1. 1.Foul water, 1:40 or 1:80
  2. 2.Rainwater is 1:100
  3. 3.What a flatter or steeper trench does

Foul water, 1:40 or 1:80

Table 6 sets the flattest gradients for foul drains. Under 1 litre a second, a 100 mm foul drain is shown at 1:40. Above 1 litre a second, with at least one WC, a 100 mm drain may be as flat as 1:80. Table 5 puts one dwelling at about 2.5 litres a second. That flow is over 1 litre a second, so a single house with a WC is the 1:80 row. A fall of 1:80 is 12.5 mm per metre. A fall of 1:40 is 25 mm per metre. Over 8 m of garden that is 100 mm of drop, or 200 mm. If the trench cannot find 100 mm between the house invert and the chamber, you are flatter than 1:80 and outside the table. Steepening to 1:40 is still inside it. The document calls these the flattest gradients, not a ceiling. A 150 mm pipe at 1:150 in Table 6 needs at least five WCs. A single house does not use that line to lay a 110 mm pipe flatter than 1:80.

Rainwater is 1:100

Paragraph 3.15 says a 100 mm rainwater drain should be no flatter than 1:100. That is 10 mm per metre. It is a third row. Roof water and a WC share a pipe only on a combined sewer the undertaker accepts. A buried downpipe kit does not become a foul drain, and it does not inherit a foul fall. None of the three kept listings states a fall. The GIONAR kit, the lcyen extender, and the eco-filtration solvent-weld pipe are silent on millimetres per metre. If you are moving rainwater a few feet across a lawn, paragraph 3.15 is the row to read. A 1:40 trench would be steeper than that rainwater minimum, and steeper is still allowed. Do not copy 1:80 from a WC conversation onto a shed gutter and call it the regulation.
GIONAR GIONAR catch-basin and downspout kit

Testsieger

GIONAR GIONAR catch-basin and downspout kit
6.2/10about £20.99
View offer

What a flatter or steeper trench does

Flatter than the row means the table no longer supports the size you are using. Solids settle in a foul drain that is too flat. Rainwater flatter than 1:100 on a 100 mm pipe is outside paragraph 3.15. The remedy is a steeper fall or, where Table 6 allows it, a larger diameter with the WC count that row demands. Steeper than the minimum still complies. A 1:40 trench on a house that could have used 1:80 is a deeper dig at the lower end, not a breach. 1:40 is not wrong. 1:80 is not a favour you have to request if Table 5 and Table 6 already put your house there. If the site needs a staged drop, that design is beyond marking 12.5 mm on a garden line. Paragraph 2.5 asks for at least three weeks’ notice before a public connection. None of the Amazon kits prints a gradient to copy.

Common questions

Is 1:40 compulsory on a house?
It is the flattest fall when the 100 mm foul flow is under 1 litre a second, or when no WC is connected. One dwelling in Table 5 is about 2.5 litres a second. With a WC, Table 6 allows 1:80. Using 1:40 on that house still meets the table. It is not the only figure printed.
When is 1:80 allowed?
When the 100 mm foul drain carries more than 1 litre a second and at least one WC is connected. One house at about 2.5 litres a second meets that test. 1:80 is 12.5 mm per metre. It is the flattest row for that case, so any steeper fall also meets the table.
What fall is rainwater?
Paragraph 3.15 says a 100 mm rainwater drain should be no flatter than 1:100, which is 10 mm per metre. That row is for rainwater. A short downpipe extension that only carries roof water stays on this row. It does not pick up the foul ratios.
How many millimetres is that per metre?
Divide 1,000 mm by the second number. 1:40 is 25 mm per metre. 1:80 is 12.5 mm per metre. 1:100 is 10 mm per metre. Over 4 m those drops are 100 mm, 50 mm, and 40 mm. Measure along the pipe, not along a lawn surface that hides a flat invert.
Is steeper than the minimum a problem?
The tables state flattest gradients. A steeper trench still meets them. You dig deeper downstream. On a very steep site you may need a designed drop rather than one slope. Flatter than the row is the breach. Steeper by itself is not.
Does a 160 mm pipe change the row?
A larger diameter has its own lines. The 150 mm foul row at 1:150 assumes at least five WCs. A single house cannot quote that line to lay 110 mm pipe flatter than 1:80. If you change size, you change row, and you keep the WC count the table asks for.
Who checks the fall on a new connection?
Building control on work that needs it, and the sewerage undertaker when you join a public sewer. Paragraph 2.5 asks for at least three weeks’ notice. None of the Amazon listings in this comparison states a fall, so the check is on your trench, not on the product page.

Related pages

Underground drainage pipe buying guide

How to lay underground drainage pipe

Clay or plastic underground pipe

Underground drainage pipe kits

Best underground drainage pipe among this Amazon set

Sources

Sources from the research digest, last checked on 16 October 2026.

  1. Approved Document H, 2015 PDF (retrieved 16 October 2026)
  2. GOV.UK Approved Document H (retrieved 16 October 2026)
  3. FloPlast 110 mm system (retrieved 16 October 2026)
  4. Kalsi Plastics: underground 110 mm specification (retrieved 16 October 2026)
  5. Wickes FloPlast 3 m pipe (retrieved 16 October 2026)
  6. Screwfix 110 mm underground category (retrieved 16 October 2026)
  7. Toolstation 110 mm underground drainage (retrieved 16 October 2026)
  8. Ofwat private sewer transfer (retrieved 16 October 2026)
  9. CPDA structural design (retrieved 16 October 2026)