How much land drain does a garden lawn need?

Plan a main collector with a real outlet, then laterals about 5–8m apart on clay and about 8–12m apart on loam. That is the useful rule of thumb for a UK garden lawn that actually needs a land drain.

In pipe, that usually means one of two jobs:

• A single interceptor along the wet edge — often 10–20m, no herringbone, no Y junctions
• A full herringbone over the wet area — very roughly 0.2–0.5m of pipe per square metre of lawn you intend to drain, plus a short outfall

Those figures are typical DIY practice, not a regulation. Sports pitches are laid much tighter; a back lawn is not a football pitch. Diagnose the wetness first. Do not bury anything until you know where the water will go. We sell the fittings, not the pipe — coiled perforated land drain and gravel come from a builders’ merchant.

Diagnose the wetness before you buy pipe

A lawn that looks wet after rain is not automatically a land-drain job.

Compaction and thatch sit at the surface. The grass feels spongy, water beads after a shower, and a spade’s depth down the soil can still be only damp. Hollow-tine aeration, scarifying and a sandy top dressing often do more here than a grid of pipe.

True waterlogging lasts. The ground stays squelchy for days, footprints fill, and moss lingers. Dig a test hole of about 300–400mm after wet weather: if it stands with water, or the sides weep, the soil at drain depth cannot get rid of water fast enough. That is when a land drain earns its trench.

Runoff from hard surfaces is a third pattern. The wet band is next to a patio, path or downpipe, not in the middle of the grass. Intercept there — a herringbone under the whole lawn will not fix a downpipe. See garden drainage when landscaping patios.

If clay lawn and a new patio are both true, drain the source first, then decide whether the rest of the grass still needs laterals.

Interceptor drain or herringbone?

You do not have to drain every square metre.

A single interceptor is one trench on a fall, along the lowest edge of the wet area, along the patio, or across the line where water enters the lawn. Use it when the problem is a band. Pipe quantity is the length of that edge plus a few metres to the outlet. Fittings stay simple: couplings where coil lengths meet, a solid end cap on each buried high end, and a silt trap near the outfall.

A herringbone is a main collector with laterals joining it at a Y, branches pointing upstream. Use it when the whole lawn (or a large rectangle of it) stays wet. Each lateral needs a Y junction on the main.

A halfway house is often enough: two interceptors, or a main with one or two laterals into a hollow. Add pipe where the water sits, not where a diagram looks neat.

How far apart should laterals be?

Closer on clay, wider on loam. Typical UK garden DIY spacing — typical, not a regulation:

• Heavy clay: laterals about 5–8m apart
• Loam or clay-loam that still drains a little: about 8–12m
• Sandy or free-draining soil: ask whether you need a land drain at all

Laterals commonly sit around 400–600mm to the invert, in a gravel-filled trench, on a continuous fall (a 1:100 fall is a working target — 10mm per metre). If you cannot get a fall, you do not have a drain. Do not copy a tight sports-pitch grid onto a family lawn “to be safe”.

Worked examples: metres of pipe and fittings

Treat these as order-of-magnitude ranges. Merchants commonly sell 25m or 50m coils — we do not sell the pipe.

Count fittings like this:

• Y junctions: one per lateral that joins the main
• End caps: one per upstream dead end (packs of two)
• Couplings: only where you join coil or make a repair
• Silt trap: one on most garden outlets

Small lawn, about 50m²

Interceptor: about 8–15m of pipe, no Y junctions, 0–2 couplings, 1–2 end caps, one silt trap.

Herringbone on clay: about 15–30m of pipe, 1–3 Y junctions, 1–3 couplings, 2–4 end caps, one silt trap. This is the size people most often over-buy for.

Typical suburban lawn, about 150m²

Interceptor along one long edge: about 15–25m of pipe, 0 Y junctions, 1–3 couplings, 1–2 end caps, one silt trap.

Herringbone on loam at about 10m spacing: about 25–50m of pipe, 1–3 Y junctions.

Herringbone on clay at about 5–8m spacing: about 40–70m of pipe, 3–6 Y junctions, 3–8 couplings, 4–8 end caps, still usually one silt trap.

If only half the lawn is wet, drain that half.

Larger garden, around 300m²

Interceptor on the low boundary: about 20–35m of pipe.

Herringbone on loam: about 50–80m of pipe, 3–6 Y junctions.

Herringbone on clay: about 70–120m of pipe, 6–10 Y junctions, 6–12 couplings, 7–12 end caps, one or two silt traps. Add 5–15m for the outfall if the soakaway sits away from the lawn.

Beyond this, get the layout drawn. Garden rules of thumb stop being honest at field scale.

60mm, 80mm or 100mm for a lawn

Metres of trench do more for a wet lawn than a jump in pipe diameter. Short version for lawns (full detail: land drain sizes):

• 60mm — short laterals or a tiny lawn
• 80mm — the usual UK garden choice for laterals and many mains on 50–150m² plots
• 100mm — a collector that several laterals feed, or extra water from paving and downpipes

Pipe sizes differ by brand. Measure your coil and check fit before ordering. Shape Shop will release OD fittings soon for wider compatibility — no date or SKUs yet. Match fittings to the coil you actually bought.

The outlet is part of the quantity

A land drain without somewhere to empty only moves water around. Every layout above assumes an outfall: a soakaway, a ditch or watercourse where allowed, or an existing 110mm surface-water drain — not the foul sewer.

Put the silt trap upstream of that point. Cap buried high ends with solid or grid caps. If the outlet end is exposed and water must leave the pipe, see anti-return end caps for land drain outlets — that fitting is for exposed outlets, not buried dead ends, and it is not a check valve.

If you cannot identify an outlet, stop. More laterals will not create one.

What to order — fittings, not pipe

From the examples, the fittings list is small next to the pipe: Y junctions equal to the laterals, end caps for every blanked high end, a few couplings, and one silt trap unless the run is long.

We manufacture those parts. We do not sell land drain pipe, gravel, or geotextile. Buy the coil once a sketch exists — main, laterals, outlet, fall — so you avoid a single short trench that does nothing, or a full coil buried at random.

When the quantities feel right, how to install a garden land drain covers the trench, the gravel and the joints. Joining coil to coil: how to join land drain pipe.