How to Set Falls for Drainage With a Rotary Laser

Get the fall wrong on a drainage run and you'll know about it – either the water sits in the pipe, or it runs so fast the solids get left behind. A rotary laser takes the guesswork out of setting falls, letting you peg out a consistent gradient over long runs far faster than a spirit level and straightedge ever could. Here's how to do it properly.

What a fall actually is

A fall (or gradient) is simply the drop over a given distance. A fall of 1:60 means the pipe drops 1 unit for every 60 units of run – so over a 6 m trench, the invert at the downstream end sits 100 mm lower than at the upstream end.

For a standard 100 mm foul drain in the UK, you'll typically be working somewhere between 1:40 and 1:80, depending on the design and what the drawings specify. Always work to the engineer's drawings where you have them – the figures below are just to show the maths:

• 1:40 = 25 mm drop per metre (2.5%)
• 1:60 ≈ 16.7 mm drop per metre (1.67%)
• 1:80 = 12.5 mm drop per metre (1.25%)

To convert a ratio to a percentage grade, divide 100 by the second number: 100 ÷ 40 = 2.5%.

Two ways to set a fall with a rotary laser

There are two approaches, and which you use depends on the laser you've got.

Method 1: Dial in the grade (dual-grade rotary)

A dual-grade rotary like the Grettz G70 or the green-beam G80 lets you tilt the laser plane itself to a set percentage. Instead of a level plane, the laser projects a sloping plane that matches your required fall.

1. Set the laser on its tripod at one end of the run, aligned so the grade axis points straight down the trench.
2. Convert your fall to a percentage (1:60 → 1.67%) and dial it into the grade setting.
3. Take a reading at the upstream end with the receiver on your levelling staff.
4. Keep the receiver at that same height on the staff and work down the run – wherever the staff bottoms out on grade, the beam reads on-grade. Every point at that reading is on your fall.

This is the fastest method for long runs because you never touch the maths again after setup. If you want the full background on how slope functions work, see our guide to what a dual-grade rotary laser is.

Method 2: Level plane plus calculated offsets

Any self-levelling rotary – including the Grettz G60 kit – will do the job with a level plane and a bit of arithmetic:

1. Set the laser up mid-run and take a reading at your upstream peg. Say the receiver reads on-grade at 1.200 m on the staff.
2. Work out the required drop at each peg. At 1:60 with pegs every 3 m, that's 50 mm per peg.
3. At the next peg, slide the receiver up the staff by 50 mm (to 1.250 m) and drive the peg down until the beam reads on-grade. A higher staff reading means lower ground – exactly what you want going downstream.
4. Repeat down the run, adding 50 mm at each peg.

Write your target staff readings on each peg before you start – it keeps things simple when you're in and out of the trench.

Setting up on site

Whichever method you use, the routine is the same: establish a site datum first (our guide on setting a datum point covers this), work out your invert levels at each end of the run from the drawings, then peg the run and check the fall before any pipe goes in the ground.

Common mistakes to avoid

• Grade axis out of line. On a dual-grade laser, the sloping axis must point straight down the trench. A few degrees off-line and your fall reads shallow.
• Checking only one end. Always verify the reading at both ends of the run before backfilling.
• Soft ground under the tripod. A tripod leg sinking mid-job shifts every reading after it. Re-check your benchmark reading periodically.
• An out-of-calibration laser. A small calibration error compounds over a long run. If your laser has had a knock or hasn't been checked in a while, book it in for Service 72 calibration – back to you within 72 hours.

The right kit for the job

If drainage and groundworks are your bread and butter, a dual-grade rotary pays for itself quickly – the G80 green-beam dual-slope kit gives you grade on both axes plus a high-visibility beam for outdoor work. Browse the full rotary laser range to find the right setup for your site.