How to dig a drainage trench
Updated: 21 October 2026
You are about to cut a short garden run for a land drain or a pipe, and you need a narrow slot with a fall. This page is that method: mark the line, cut small bites, set the fall, and stop if you hit a service. It is not a product ranking. Tool criteria are on the [drainage spade buying guide](/hub/digging-tools/drainage-spade/sub/drainage-spade-buying-guide). The listings are in the [drainage spade comparison](/hub/digging-tools/drainage-spade). Peter F. compared the listings and public UK sources in October 2026. There was no lab test. A qualifying purchase can earn a commission at no extra cost to you.
You are about to cut a short garden run for a land drain or a pipe, and you need a narrow slot with a fall. This page is that method: mark the line, cut small bites, set the fall, and stop if you hit a service.
You are about to cut a short garden run for a land drain or a pipe, and you need a narrow slot with a fall. This page is that method: mark the line, cut small bites, set the fall, and stop if you hit a service. It is not a product ranking. Tool criteria are on the drainage spade buying guide. The listings are in the drainage spade comparison. Peter F. compared the listings and public UK sources in October 2026. There was no lab test. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Pattern | Shaft | Weight | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Spear & Jackson Spear & Jackson 2154HK draining shovel | Tubular draining shovel | 711 mm metal | 2.79 kg | 9.2 | about £16.20 | View offer |
 Roughneck Roughneck drainage and post-hole shovel | Post-hole shovel | 1,070 mm fibreglass | 1.98 kg | 9.0 | about £21.54 | View offer | |
| Best value |  Bulldog Bulldog 5CLAM cable-laying shovel | Cable-laying shovel | YD grip | 2 kg | 8.3 | about £11.54 | View offer |
Mark the run and the services
Start with an outfall you already have: a soakaway, a ditch you are allowed to use, or an existing drain a competent person has confirmed. Mark the route with string and pegs before you lift turf, and mark where the fall has to finish. DIY Doctor uses a string line because eye level lies once the spoil is piled beside you.
Then find what is already in the ground. HSE’s HSG47 sequence is to plan, locate services, and only then dig. A short lawn still crosses electric cables, water pipes and drains. A steel drainage spade is not an insulated tool. HSE says use insulated tools when hand digging near electric cables, and dig alongside a service rather than straight down onto it. None of the plain steel or fibreglass listings in our comparison is sold as insulated. If a locator or the plans show a cable on the line, stop and reroute or get a competent person. A neighbour’s pipe and a public sewer are not a weekend experiment. PUWER 1998 applies when the spade is work equipment on a job, which is a different duty from a private lawn. The order is the same either way: know the line, then cut.
Cut the slot
Cut only as wide as the pipe plus its bedding. A TradeCalculator land-drain guide suggests about 300 mm wide and about 450–600 mm deep for that kind of install. Three hundred millimetres is wider than a drainage-spade slot. Use it as a reminder that a land drain needs room for gravel, not as an order to open a ditch. The narrow blade keeps the sides close and the spoil smaller.
Work in small bites. Drive the blade a short way, free the soil, and lift it out. Reviews of thin blades describe bends after the shovel is used as a crowbar, including one Bulldog account after about 10 m and Neilsen comments about a blade that bent again. Those are anecdotes, not a rate, and they are the same mistake: a long pry on a narrow head. If a root stops the blade, cut the root or take a smaller bite. Keep spoil on one side, turf separate if you want it back, and do not stand in an unsupported trench as the normal method. HSE says no ground can be relied on to stand unsupported, and a cubic metre of soil can weigh more than 1.5 tonnes. A shallow slot you reach from the side is a different task from a trench you climb into. Masseys, writing about a Newcastle, notes that you should clean the head so clay does not hold damp. Wipe carbon steel before you store it wet.
Set the fall
Set the fall before you backfill, and use the figure that matches the pipe. For a garden land drain, TradeCalculator’s guide uses about 1:100. That is that guide’s land-drain figure. It is not the foul-drain figure.
In England, Approved Document H covers foul drainage. A WC drain needs at least 100 mm internal diameter. Table 6 shows 1:80, which is 12.5 mm per metre, as the flattest gradient for a 100 mm foul drain when flow exceeds 1 litre per second and at least one WC is connected. Where peak flow is under 1 litre per second, the steeper figure is 1:40. DIY Doctor separates those foul figures from about 1:100 for surface water. Do not apply 1:80 to every perforated land drain, and do not apply 1:100 to a WC connection. Set the outfall depth first, then work to a string. Backfill only after the pipe is bedded as that pipe requires. Gravel and a geotextile do not replace the fall. If the run would go lower than nearby foundations, Approved Document H paragraph 2.25 says not to excavate below them unless the concrete-fill conditions are met: within 1 m, concrete up to the underside of the foundation; beyond 1 m, concrete to a level below the foundation equal to the distance from the building minus 150 mm. If the trench would undercut a wall, stop.
When to stop
Stop if you hit a buried service, if the sides slump, if you cannot see the bottom, or if the line heads into a sewer, a neighbour’s drain or a highway. HSE’s excavation guidance is that unsupported ground is not trustworthy because it looks firm. HASpod’s note on the old 1.2 m rule is that there is no universal safe depth. Do not copy a claim that a trench deeper than 1,200 mm must be shored by law.
A mini digger or a groundworker is the right call when the run is long, the ground is rubble, the sides will not stand, or the fall has to meet a foul connection you are not competent to make. A drainage spade does not make a drain comply with the Building Regulations. It only cuts the slot. If the blade is bending, stop levering. A post hole, rather than a run, is spade for fence post holes.
Common questions
How wide should I dig the trench?
As wide as the pipe plus the bedding it needs, and no wider for comfort. A land-drain guide that says about 300 mm is describing room for gravel, which is wider than the slot a drainage spade naturally cuts. A border spade will move spoil you then have to return. Keep the sides straight and the width matched to the pipe.
What fall does a land drain need, and what fall does a foul drain need?
A land-drain guide we used suggests about 1:100. That is not the foul figure. In England, Approved Document H Table 6 gives 1:80 (12.5 mm per metre) as the flattest gradient for a 100 mm foul drain when flow is over 1 litre per second and a WC connects, and 1:40 when peak flow is under 1 litre per second. A WC drain needs at least 100 mm internal diameter. Match the fall to the pipe.
Do I start at the high end or the outfall?
Set the outfall depth first so the low end is actually low enough, then work along a string line. You can cut from the high end if that outfall level is already marked. Guessing the slope after the pipe is in means lifting the pipe out again. The string is there because the spoil pile hides the slope.
Can I stand in the trench?
Not as the normal method. Cut from the side while you can still reach. HSE says no ground can be relied on to stand unsupported, and a cubic metre of soil can weigh more than 1.5 tonnes. Climbing into a narrow hole to gain reach is how a collapse becomes serious. A longer shaft is the alternative, not your weight in the slot.
What if the trench is next to a house wall?
Stop and read Approved Document H paragraph 2.25 before you go below the nearby foundations. Within 1 m, the stated fill is concrete up to the underside of the foundation. Beyond 1 m, the fill level relates to the distance from the building minus 150 mm. Undermining a shallow wall is also an HSE excavation warning. A soakaway hard against a footing is a building question.
How do I stop the blade bending?
Take small bites and do not use the shaft as a lever on roots or compacted clay. Bent-blade comments in this set describe that prying, not a measured failure rate. If the edge will not enter, clean a clay-caked head rather than bouncing on the grip. A forged head still bends if the pry is long enough.
Do I need a locator before a short garden run?
Yes if a cable, a pipe or a drain could be on the line. HSG47 is plan, locate, then dig, with no exemption for a short lawn. A steel spade is the wrong tool to discover a live cable. If you cannot locate the services, do not dig that line. Plans alone are weaker than plans plus a locator.
Related pages
Sources
Sources from the research digest. Last checked on 19 October 2026.
- Spear & Jackson shovel selection guide (retrieved 19 October 2026)
- Faithfull 2018 catalogue (retrieved 19 October 2026)
- BSI BS 3388:2004 Forks, shovels and spades (retrieved 19 October 2026)
- Approved Document H (England, 2015) (retrieved 19 October 2026)
- HSE HSG47 Avoiding danger from underground services (retrieved 19 October 2026)
- HSE excavations and electricity (retrieved 19 October 2026)
- HSE PUWER overview (retrieved 19 October 2026)
- Consumer Rights Act 2015, section 9 (retrieved 19 October 2026)