50–100mm Sub Base for Artificial Grass Drainage in UK Gardens
Yes, artificial grass needs drainage, and getting it right comes down to one thing: a permeable, compacted aggregate sub-base beneath the turf. The turf backing itself lets water pass through easily, but without the right layers underneath, that water has nowhere to go and simply pools. The standard fix is a geotextile membrane, compacted MOT Type 1, a levelling layer of grano dust, and then your turf. Hard bases or heavy clay sites are the exceptions that need extra thinking.
TL;DR:
- Proper drainage relies on a permeable, compacted aggregate sub-base, typically 50 to 100mm of MOT Type 1 for most lawns, to prevent water pooling.
- On hard surfaces like concrete or paving, a dedicated drainage layer such as foam underlay or drainage matting is necessary to carry water away effectively.
- Heavy clay sites or areas with persistent pooling may require professional solutions like gravel-filled trenches with perforated pipes and soakaways, especially if water drainage fails after base correction.
- Diagnosing drainage problems involves checking sub-base depth, testing runoff with a hose, and inspecting outlets for blockages, not assuming a failed turf or surface.
- Correct installation techniques, including material choice, proper lifts, and proper falls towards outlets, are critical for long-term drainage performance and to avoid future repairs.
What to put under artificial grass: materials, layer order and depths
Most drainage problems trace back to one mistake: treating the sub-base as an afterthought rather than the part of the job that actually does the work. The turf is just the finish. The base is the drainage system.
MOT Type 1 is the standard choice for a reason. It compacts into a firm, stable surface while remaining permeable enough to let water pass through to the ground below. For most residential lawns, aim for 50 to 100mm of compacted MOT Type 1, building it up in stages rather than dumping it in one go.
Some sites need something more open. If you’re working on heavy clay, or the lawn will see regular use from dogs, MOT Type 3 or 20mm granite and limestone chippings drain faster because they contain fewer fine particles clogging the gaps. The trade off is depth: these materials often need around 125mm to achieve the same stability MOT Type 1 gives you at half that thickness.
On top of the aggregate goes your levelling layer. Installers generally prefer grano dust over sharp sand here, since it compacts to a firmer, flatter finish and is less likely to migrate into the stone below. Sharp sand and limestone fines work as alternatives, but grano dust remains the practitioner’s default for a reason worth respecting.
Underneath it all, lay a permeable, non-woven geotextile membrane. This stops the aggregate mixing with weaker subsoil and suppresses weeds, but never substitute it with solid polythene sheeting. That traps water rather than letting it through, defeating the entire point of the build-up.
- Geotextile membrane (permeable, non-woven)
- MOT Type 1, compacted to 50 to 100mm
- MOT Type 3 or 20mm chippings where drainage needs are higher, compacted to roughly 125mm
- Grano dust levelling layer, 10 to 15mm
- Artificial turf
When you’re ordering materials, be specific with your supplier. Ask for “MOT Type 1 compacted in 40 to 50mm lifts” rather than just “sub-base”, so there’s no ambiguity about the depth or the compaction method you need.
Pro Tip: If you’re unsure which pile height suits your drainage setup, shorter pile turf such as a 35mm option tends to shed surface water faster than longer pile styles, which can hold moisture closer to the surface after heavy rain.
How do you drain artificial grass on concrete or patios?
Concrete, paving and old patio slabs don’t let water through at all, so you can’t rely on the usual stone sub-base to carry water away. Instead, you need a dedicated drainage layer sitting above that impermeable surface, doing the job the ground would normally do.
- Lay a thin drainage layer, either 5 to 10mm of free-draining foam underlay, specialised drainage matting, or a micro-honeycomb grid designed for hard surfaces.
- Build in an edge outlet or channel so water has somewhere to escape rather than building up against a wall or step.
- Direct that outfall somewhere useful: a gully, a downpipe, or a soakaway sized for the area.
- Seal all edges and seams carefully. Any gap lets water track underneath the structure, which causes far worse problems than a slow-draining lawn ever would.
Skipping the drainage layer on a hard surface is the single most common reason artificial lawns on old patios turn into shallow ponds after rain.
When do you need French drains or a soakaway?
Some sites simply won’t cope with a standard sub-base, no matter how well you build it. Watch for these warning signs before you commit to a straightforward MOT Type 1 build:
- A clay subgrade that won’t absorb water even once the aggregate above it is correctly compacted
- Pooling that persists after you’ve corrected the base, ruling out a simple compaction or levelling fault
- A soakaway test showing water sitting for hours rather than draining away within a reasonable time
Where these signs show up, professional designs typically use a gravel-filled trench with an 80mm perforated collector pipe, feeding laterals into a perimeter drain or soakaway. Spacing and pipe runs vary by site, but this pattern turns up repeatedly in larger installations and sports-pitch style projects. Field research on artificial turf drainage shows porous sub-bases attenuate peak rainfall well, though heavy storms can exceed what the base alone can hold, which is exactly when a collector system earns its keep.
One caveat worth flagging early: discharging water to a public sewer or a nearby watercourse may need formal consent. If your site backs onto either, it’s worth a conversation with a drainage professional before you dig.
How do you diagnose artificial grass drainage problems?
Water pooling on an artificial lawn almost always has a specific, findable cause. Work through these checks before assuming you need to rip the whole thing up:
- Probe the base at a few points to confirm the sub-base depth actually matches what was specified, not just what looks right from the surface.
- Run a hose over a problem area and time how long it takes to drain, comparing it against a section of lawn that behaves normally.
- Check every edge outlet and channel for leaves, sand, or moss blocking the exit point.
The usual culprits are insufficient compaction, using ordinary soil where a proper sub-base should sit, poor falls that leave no route for water to travel, blocked outlets, or fine levelling material migrating down into the aggregate and clogging the gaps that should let water through.
Pro Tip: A localised low spot rarely needs a full rebuild. Lift the turf, top up with grano dust, re-compact, and relay. It’s a half-day fix rather than a weekend one.
Practical installation checklist for correct drainage
Getting this right the first time saves you from digging it all back up later. Work through materials, technique, and falls in that order, and verify before you call the job finished.
Materials to have ready:
- Permeable, non-woven geotextile membrane
- MOT Type 1, plus MOT Type 3 or 20mm chippings if the site needs extra drainage capacity
- Grano dust, 10 to 15mm for the levelling layer
- A plate compactor rated for the aggregate depth you’re working with
Compaction technique matters as much as material choice. Work in lifts of roughly 40 to 50mm rather than compacting one thick layer in a single pass. This achieves a firm, stable surface without sealing the aggregate so tight that water can’t pass through it. Over-compaction is a real risk. A base compacted too hard behaves almost like concrete, defeating the purpose of choosing a permeable material in the first place.
Set your falls before you finish levelling. Most residential lawns work well with a gentle fall of around 1:80 to 1:100 toward a chosen outlet point. Finish edges cleanly and check that seam joins sit flush rather than creating a lip where water can collect.
Once the turf is down, verify the job properly. Run a hose test across the whole lawn, watch how it performs during the first heavy rainfall, and check back periodically for silting or moss build up around edges and outlets, which are the earliest signs a base is starting to clog.
For readers building on ground recently disturbed by tree removal or root work, it’s worth checking the soil condition first, since compacted or disturbed subgrade drains very differently to settled soil.
Why installation detail matters more than the turf itself
Homeowners spend a surprising amount of time comparing turf pile heights and yarn colours, and almost no time thinking about what sits underneath. That’s backwards. The turf is largely cosmetic once you’re past a basic quality threshold. The sub-base is the part of the job doing the actual engineering, and it’s also the part nobody sees once the lawn is finished, which is exactly why corners get cut there.
The conventional advice to “just use MOT Type 1” isn’t wrong, but it’s incomplete without knowing when to deviate from it. A clay-heavy garden in the north of England behaves nothing like a sandy plot in the south east, yet plenty of generic guides give both the same specification. Reading the site before reading the spec sheet is the actual skill here, not memorising a single formula.
Product ranges, installation guides, and free samples can give homeowners and landscapers a practical way to match turf choice to a properly specified base, rather than treating the two as separate decisions. Anyone specifying a job at scale would do well to request technical advice before ordering, not after the aggregate is already delivered.
— Amanda Cheetham
Get your artificial grass drainage right from the start
Suppliers of artificial grass, membranes, and installation accessories offer materials needed to build a lawn that actually drains; some provide free samples so you can check pile height and drainage behaviour before committing to a full order.
Whether you’re weighing up a shorter pile like the 35mm option for faster surface drainage or a longer, denser feel with the Austrian 40mm, the right choice depends on how your specific site copes with water, not just how it looks in photos. Deck Kingdom’s artificial grass advice hub walks through installation detail, and the team is on hand for technical guidance if your site has clay, poor falls, or an existing hard surface to work around. Order a free sample, check the pile against your intended base, and request a quote once you’re confident the specification matches your ground conditions.
Primary sources and useful technical references
For the technical specifications behind this guide, see the GOV.UK highway works specification and Loughborough University’s drainage research.
Sources
FAQ
Do you need drainage under artificial grass?
Yes, almost always. The turf backing lets water through, but without a permeable, compacted sub-base beneath it, that water has nowhere to go and pools on the surface.
What is best for drainage under artificial grass?
Compacted MOT Type 1 is the standard choice for most residential lawns, typically laid 50 to 100mm deep over a permeable geotextile membrane, with a grano dust levelling layer on top.
What can I put under artificial grass for drainage on concrete or patios?
Use a dedicated drainage layer such as 5 to 10mm free-draining foam underlay or specialised drainage matting, since concrete and paving don’t let water through and need help from an added layer rather than aggregate alone.
Will water drain through artificial turf?
Water passes through the turf backing itself without issue. Whether it actually drains away depends entirely on what’s underneath, which is why the sub-base matters more than the turf you choose.
How do I fix a patch of artificial grass that keeps puddling?
Lift the turf, check the base depth with a probe, top up any low spots with grano dust, re-compact, and check nearby outlets for blockages before assuming you need a full rebuild.
