Drain fall 1 in 40: why a centred bubble is wrong

In short

A drain fall 1 in 40 spirit level check is made by a calculated drop, not by simply centring the bubble. A normal centred vial means zero fall. 1:40 means a 25 mm drop per horizontal metre, and 1:80 means 12.5 mm per metre.

You have laid out a pipe, clipped a small level to a taut line and adjusted the trench until the bubble sits between its marks. If that line is level, the pipe has no designed fall. Create a level datum and measure calculated offsets from it, or place a known packer under a straight edge so a centred bubble stands for the intended height difference.

What Table 6 actually says

Approved Document H is often reduced to the claim that a foul drain must be 1 in 40. Table 6 is conditional. The minimum gradient depends on pipe size, peak flow and, on some rows, the appliances connected. Read it as a system, not as one universal number.

For peak flows below 1 litre per second, the researched Table 6 rows give a minimum gradient of 1:40 for both 75 mm and 100 mm pipes. Where peak flow is above 1 litre per second, the cited rows give 1:80 for a 75 mm pipe, 1:80 for a 100 mm pipe where at least one WC is connected, and 1:150 for a 150 mm pipe where at least five WCs are connected. A 100 mm label printed on the pipe does not, by itself, tell you which gradient to use.

Divide the horizontal run by the second number. 1,000 mm divided by 40 is 25 mm, so 1:40 falls 25 mm in 1 m and 100 mm in 4 m. At 1:80 the drop is 12.5 mm per metre, or 50 mm over 4 m. Approved Documents guide ways to meet Building Regulations. They do not design the drainage. England and Wales publish their own guidance, and rules elsewhere in the UK differ. Establish which rules apply before you dig or buy pipe.

What a centred bubble really means

A standard spirit-level vial reads horizontal when the bubble sits between the marks. On a sound straight surface that is zero gradient, within the limits of the tool and the setup. A string spirit level hung on a line reads that line, not the drain below it. Prices and review counts on that comparison are amazon.co.uk listing figures, not a lab test. A qualifying purchase can earn a commission at no extra cost to you.

The bubble can still finish centred on a sloping pipe, once a known difference has been added elsewhere. A correctly sized packer under one end of a straight edge tilts the edge relative to the pipe. The packer creates the target slope. The vial only checks that the edge is then level.

None of the ten product listings considered for this topic states a calibrated fall vial or a millimetres-per-metre reading. A specialist slope marking or display only counts when its scale and instructions match the job. A plain two-line vial, while centred, cannot tell 1:40 from 1:80.

The string can sag, stretch, catch the trench edge or shift when you clip the vial on. How to stop a builders line sagging is the check for that curve. A repeatable bubble can still sit on a wrong datum, so check the calculated end heights separately.

How to show a fall with a straight edge

For a short, accessible run, a straight edge and a measured packer make the geometry visible. A 1.2 m straight edge needs a 30 mm difference for 1:40, because 1,200 divided by 40 equals 30. For 1:80, the difference over the same length is 15 mm. Those differences are large enough to measure carefully only if the edge is actually straight and bears on the intended points.

Place the straight edge along the direction of flow, with a packer of the calculated thickness at the low end. If the pipe has the target fall, the straight edge should then be horizontal and an ordinary vial should centre.

Reversing the setup, or putting the packer at the high end, tests the opposite relationship and gives a misleading result. Mark the intended direction before you start.

Check successive sections, both ends and the overall fall. A true 1.2 m bay can hide a backfall at the next joint. Read the packer and the horizontal run with a tape measure, not the length along the sloping pipe. Dirt, a rocking pipe, a rounded fitting or a bowed edge can swallow several millimetres. At 1:80, 3 mm is nearly a quarter of the 12.5 mm fall in one metre. Clean the contacts and repeat the reading after you lift the tool.

Using a string or laser datum

Treat the string as a datum, not as the drain gradient. Set a level reference above the trench, then measure down to the pipe at each checkpoint. If the crown should be 500 mm below the datum upstream, a 4 m run at 1:40 puts it 600 mm below downstream.

Work through the setup in a fixed order.

1. Mark the direction of flow and measure the horizontal run between defined pipe points.
2. Confirm the specified ratio, then calculate the total fall before moving soil or bedding.
3. Establish and verify a stable level datum that will remain in place during the work.
4. Calculate the required offset from the datum at each end and at intermediate checkpoints.
5. Bed and support the pipe, recheck after jointing, and record the readings before the trench is covered.

The offset grows towards the low end, because the pipe sits farther below a horizontal datum. Write the required measurement beside each stake. Measure the same feature every time. Switching between crown, invert and a fitting invents a difference that is not fall.

A laser is a convenient datum across a site. You still calculate the offsets and still check the staff reading. The string spirit level vs laser level page sets out where each reference is useful. Verify the tool, and keep it from being knocked between readings.

Patio fall is a different decision

A patio needs water to shed in a controlled direction, and Table 6 does not set that fall. The finished slope depends on the paving, the joints, the threshold, the drainage route and the specification. Copying 1:40 from a foul-drain row can be the wrong surface.

Calculate both finished levels before you excavate, and include bedding and slab thickness. A centred bubble still means level unless a packer, a slope vial or measured offsets introduce the specified fall. A sub-base set correctly can lose that fall in later layers.

Send the water somewhere lawful. A slope towards a blocked channel, a neighbour's property or the building has failed. Thresholds, damp protection and accessible routes can force a different layout. A patio reading does not validate a foul drain, and a drain reading does not set the patio.

When the job leaves DIY

Call in competent help when you cannot identify the Table 6 row, cannot hold a trustworthy datum, or cannot show the pipe is supported without backfalls. The same applies to a public sewer connection, uncertain existing drainage, deep excavation, or work Building Control must inspect. When a line level is the wrong tool sets the wider limit of a hanging vial.

Confirm the approval route before you cover the pipe. Building Control may need to see drainage under the Building Regulations, and a public sewer can need the sewerage undertaker's consent. Flows, pipe size, access, ventilation and the connection are design work. This article explains the measurement error. It does not design a sewer or a safe excavation.

Trench collapse, buried services, contaminated ground and unsafe access remain risks when the gradient is right. If the trench needs support or the depth is in doubt, keep people out. On a small permitted job, leave the trench open if the readings disagree, the pipe rocks, water ponds, or the inspection requirement is unclear.