Set a drainage fall with a rotary laser
Updated: 22 October 2026
A level laser has not yet given your 100 mm foul drain a fall. This page shows how to set a drainage fall with a rotary laser: what 1:80 means in millimetres, how to put that drop on a staff while the laser stays level, and when slope mode tilts the plane instead. Prices are on the [rotary laser comparison](/hub/measuring-equipment/rotary-laser). Peter F. took the figures from Approved Document H for England. A qualifying purchase can earn us a commission at no extra cost to you.
A level laser has not yet given your 100 mm foul drain a fall.
A level laser has not yet given your 100 mm foul drain a fall. This page shows how to set a drainage fall with a rotary laser: what 1:80 means in millimetres, how to put that drop on a staff while the laser stays level, and when slope mode tilts the plane instead. Prices are on the rotary laser comparison. Peter F. took the figures from Approved Document H for England. A qualifying purchase can earn us a commission at no extra cost to you.
Our top picks
| Badge | Product | Slope claim | Stated accuracy | Staff in the box | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  KayEsampra KayEsampra red rotary laser | Manual X/Y, no angle | n/a | Not listed | 7.0 | about £97.00 | View offer |
 Huepar Huepar RL200HVG green rotary laser | 8° dual, claimed | ±3 mm at 30 m | Not listed | 8.1 | about £329.99 | View offer | |
| Best value |  Grettz Grettz G60 red rotary laser kit | Not stated | ±3 mm at 30 m | 2 m metric staff | 6.5 | about £365.99 | View offer |
On this page
The fall in millimetres
A gradient of 1:80 is 12.5 mm per metre, because 1,000 divided by 80 is 12.5, and 100 mm over 8 m. For England, Approved Document H (2015) Table 6 is the minimum-gradient row used here for a 100 mm foul drain with at least one WC. Set the fall to the invert, the inside bottom of the pipe. Marking the crown on a 100 mm pipe puts you about a diameter off the bore. Write “invert” on the peg.
A horizontal laser can still set it
You do not need a slope button. Read the staff at the upstream invert and call that reading R. A higher reading means a lower invert. Over 8 m at 1:80 the downstream reading should be R plus 100 mm. If R is 1.400 m, the downstream invert should read 1.500 m. At 4 m, check R plus 50 mm so the run has no belly. A stated ±3 mm at 30 m is still a band on that reference. Staff basics are on how to use a rotary laser.
Slope mode when the laser has it
Laser Levels Online’s method is: find level, drop the receiver by the fall at the far end, then tilt until the on-grade tone returns. For 8 m at 1:80 that drop is 100 mm. Only Huepar claims dual slope, and the amazon bullet says 8° in both X and Y. A 1:80 gradient is about 0.7°, because the angle whose tangent is 1/80 is under a degree. Do not dial 8° and call it 1:80. The other four listings do not print a slope in degrees. Only Grettz and Huepar print ±3 mm at 30 m. That check is on the rotary laser buying guide.
What this folder can and cannot do
Stop for a public sewer, a boundary, or guidance from a nation you have not read. Stop if the drawing is tighter than the laser’s label, if the staff cannot reach the trench, or if the tool was dropped. Do not open the housing. Nothing here sets a legal calibration interval or approves the connection. A stated ±3 mm at 30 m is the maker’s claim on the two listings that print it.
Common questions
What does 1:80 mean in millimetres per metre?
It means 12.5 mm of fall for every metre of run, because a metre is 1,000 mm and 1,000 divided by 80 is 12.5. Over 8 m that is 100 mm. Over 4 m it is 50 mm. A 1:40 fall is twice as steep, 25 mm per metre, which is 250 mm over 10 m. Those are proportions. They are not a survey of your garden until you measure the run between the two inverts.
Which gradient does Approved Document H show for a normal house drain?
For England, Table 6 of Approved Document H (2015) shows a minimum of 1:80 for a 100 mm foul drain with at least one WC when the peak flow is above 1 litre per second. Table 5 gives one dwelling as 2.5 litres per second. Other diameters have other rows. Scotland, Wales and Northern Ireland are not covered by pasting that England table onto the job. Read the guidance for the nation where the drain is being built.
Can I set that fall with a laser that only does horizontal?
Yes. Hold a level plane, read the staff at the upstream invert, and add 12.5 mm for every metre of run at the downstream end. A higher staff reading means a lower invert. Over 8 m at 1:80 the downstream reading is 100 mm higher than the upstream reading. Check the middle at half the fall. You write the numbers on the pegs, so the fall does not depend on a slope button or on the laser staying switched on.
How do I use slope mode, and which listing claims it?
Find level, drop the receiver by the fall at the far end, and tilt until the on-grade tone returns, which is the Laser Levels Online method. Peg the result, because their guide says the grade can clear when you switch off. Only the Huepar listing in this folder claims dual slope, and the amazon bullet says 8°. A 1:80 drain is about 0.7°, so do not dial 8° and call it 1:80. The other four listings do not print a slope in degrees.
What is the difference between invert and crown?
The invert is the inside bottom of the pipe, where the flow runs. The crown is the inside top. You set a foul-drain fall to the invert unless the drawing tells you to use another point. Marking the crown on a 100 mm pipe puts you about a pipe diameter away from the invert, and the fall in the bore will not match the fall you calculated. Write the word invert on the peg before you take R.
When must a drainage contractor or building control take over?
When the run joins a public sewer, crosses a boundary, or sits in a nation whose drainage guidance you have not read. England’s 1:80 is not a default for Scotland, Wales or Northern Ireland. Hand the job over if the specification is tighter than the laser’s stated tolerance, if the staff cannot reach the trench, or if the instrument was dropped. This page is a method for the millimetres. It is not an approval to connect.
Related pages
Sources
Public UK sources from the research digest, last checked on 21 October 2026. Shelf prices and star lines are the amazon.co.uk figures stored with these five listings. The price band is this set, not a market average. The OPSS report is a different laser level.
- UKHSA: Laser radiation safety advice (retrieved 21 October 2026)
- The Control of Artificial Optical Radiation at Work Regulations 2010 (retrieved 21 October 2026)
- HSE: Lasers (retrieved 21 October 2026)
- Approved Document H: Drainage and waste disposal (2015) (retrieved 21 October 2026)
- OPSS product safety report 2404-0147 (a different laser level) (retrieved 21 October 2026)
- Protrade: A guide to rotary laser levels (retrieved 21 October 2026)
- Laser Levels Online: Buying advice (retrieved 21 October 2026)
- Huepar UK: RL200HVG product page (retrieved 21 October 2026)