What goes under artificial grass, and why does mine hold water?
The layer build-up artificial grass actually needs, why a laying course of stone dust and a permeable base matter more than the grass itself, and diagnosing a lawn that ponds.
- Difficulty
- intermediate
- Time
- 8 hr
- Read
- 6 min
- Safety
- caution
Short answer
Excavate 75 to 100mm, lay and compact MOT type 1 or a permeable sub-base, then a 20 to 30mm screeded laying course of granite or limestone dust, then a weed membrane, then the grass. Standing water almost always means an impermeable layer somewhere — a polythene sheet, a compacted clay subgrade, or a sub-base with too many fines and nowhere to drain to.
Artificial grass is a permeable carpet with drainage holes in its backing. It sheds water down through itself and then relies entirely on what is underneath to take it away. So every ponding complaint is a base problem, and the base is the part that is buried and expensive to correct. Getting it right is almost all of the job.
Safety
What you'll need
- MOT type 1 or a permeable sub-base
- Granite or limestone dust for the laying course
- Weed membrane, permeable
- Plate compactor, hired
- Screed rails and a long straightedge
- Timber or composite edging and pegs
- Sharp knife and joining tape and adhesive
- Kiln-dried silica sand for the infill (optional)
Step by step
- Excavate to a consistent depth, not to a consistent surface.Strip the turf and topsoil to 75 to 100mm below the intended finished level — more if the ground is soft or if the area will take heavy use. Take out all organic material: turf left in place decomposes and the surface sinks in patches.
- Test what the subgrade is.Dig a hole 300mm deep and fill it with water. If it drains away in a few hours, the ground will take the water and the build-up above just has to reach it. If it is still full the next day, you have heavy clay and the base alone will not solve drainage — the area needs a drain, a soakaway or a fall to somewhere that does drain.
- Lay and compact a permeable sub-base.MOT type 1 in layers of no more than 75mm, each compacted with a plate before the next. On soft or clay ground, a geotextile separator under it stops the stone punching into the subgrade.
- Set the edging.Treated timber, composite or metal edging pegged or haunched around the perimeter, set to the finished level. The grass is fixed to this, and without it the edges lift and the base spills out.
- Screed a laying course of stone dust.20 to 30mm of granite or limestone dust, screeded flat with rails and a straightedge and compacted. This is what gives the grass a smooth surface: every stone left proud in the sub-base will telegraph through and be felt underfoot. It must still be permeable, so do not use sharp sand with a high fines content or a sand-cement mix.
- Build a fall if the area needs one.Artificial grass on level ground over free-draining subsoil drains vertically and needs no fall. On clay, or against a building, a fall of around 1 in 100 across the base to a drained edge is what stops water sitting on the membrane.
- Lay a permeable weed membrane over the laying course.Over the base, not under it — the weeds you are keeping out arrive as seed on the surface. Overlap the joins by 100mm and take it up to the edging.
- Roll the grass out and let it relax.Lay it out and leave it for a few hours in the sun before cutting or joining. It comes off the roll with a memory and it settles considerably. Lay all pieces with the pile leaning the same way, or the joins will show as a colour change.
- Join and fix the edges.Joining tape and adhesive under the seams, pressed and weighted while it cures. Fix the perimeter to the edging with galvanised U-pins or clout nails at close centres. Do not glue the whole area down — it needs to be able to move.
- Brush in the infill if the product specifies one.Kiln-dried silica sand brushed into the pile, if the manufacturer calls for it. It weights the grass down, holds the pile upright and protects the backing. Follow the specified quantity per square metre.
Tips
- Granite dust, not sharp sand, for the laying course. It compacts to a smooth firm surface and stays permeable; sand with a high fines content can seal up.
- Never use polythene or a damp proof membrane anywhere in the build-up. It is the single commonest cause of a permanently wet artificial lawn.
- Order the grass in the fewest pieces you can. Every seam is a potential visible line, and a well-planned layout can often avoid them entirely on a small garden.
- Lay all pieces with the pile direction the same and facing the main viewpoint. Grass laid in opposing directions looks like two different colours.
- Keep a metre of offcut. Repairs to burns, pet damage and cuts are straightforward with matching material and impossible without.
Common mistakes
- Laying grass over the existing lawn — Turf and topsoil decompose, the surface sinks unevenly within a year, and the organic layer holds water against the backing.
- Putting the membrane under the sub-base only — The weeds come from seed landing on the surface and rooting in debris in the pile. The membrane belongs immediately under the grass, over the laying course.
- Using sharp sand as the laying course on clay — It can pump and seal, and on clay it holds water at exactly the level you need it to drain through.
- Gluing the whole area to the base — The grass expands and contracts with temperature. Fixed only at the perimeter it accommodates that; glued all over, it rucks and lifts.
- Assuming a permeable build-up solves clay — Permeable means the water gets down to the subgrade. If the subgrade cannot take it, the water backs up. On clay you also need somewhere for it to go.
If it doesn't work
Water sits on the surface after rain
Cause: An impermeable layer somewhere in the build-up, or nowhere for water to go once it reaches the subgrade. — Fix: Establish which. Lift a corner: if the membrane is wet on top and the laying course is dry, the membrane is not permeable. If everything is saturated, the subgrade cannot take it and the area needs a land drain or a soakaway.
The surface is lumpy or has hollows
Cause: The laying course was not screeded flat, or organic material was left under the base and has rotted down. — Fix: Lift the grass in that area, re-screed and re-compact the laying course, and relay. The grass can usually be reused if it was pinned rather than fully glued.
The edges have lifted
Cause: Insufficient fixing to the edging, or no edging at all. — Fix: Fit or repair the edging and re-pin at closer centres. Edges lift because there is nothing holding them.
Green algae or moss on the surface
Cause: A shaded, damp area with organic debris left on the grass. — Fix: Brush the debris off regularly and treat with an artificial grass cleaner. It is a surface problem on the debris, not on the grass itself, but persistent ponding underneath makes it much worse.
It smells
Cause: Almost always pet urine, which needs to drain through and be flushed rather than sitting on an impermeable layer. — Fix: Check the drainage first, then flush regularly and use an enzymatic cleaner. A grass with a fully permeable backing and a free-draining base is what makes this manageable.
Questions people ask
How deep does the base need to be?
For a domestic garden, commonly 75 to 100mm of compacted sub-base plus 20 to 30mm of screeded laying course. Deeper on soft ground, on clay, or where the area will take heavy or repeated use.
Can I lay artificial grass on an existing patio or concrete?
Yes, if it drains. On a sound falling slab, a shockpad or a thin foam underlay over the concrete, then the grass, works well and uses very little height. On a slab that ponds, you will have a permanently wet grass.
Does it need infill sand?
It depends on the product. Many modern grasses are non-infill and specify none; others need a stated weight per square metre of kiln-dried silica sand to hold the pile up and stabilise the backing. Follow the manufacturer, because over-filling a non-infill product spoils it.