Preparing the Ground for Artificial Grass

The level you cannot move

The call that brings somebody back to a finished lawn a year later is almost never about the grass. It is that the back door catches on the mat, or that the bottom two courses of brick behind the lawn have gone dark and stayed dark through a dry August. Both have the same cause: the build-up went in from the bottom, one comfortable layer at a time, and finished wherever it finished. Nobody set the top of it first.

So begin at the other end. Walk the perimeter and write down every level you are not allowed to change: the damp-proof course, which Approved Document C, Site Preparation and Resistance to Contaminants and Moisture requires to sit clear of the adjoining ground by a margin normally read as two brick courses; the threshold of any door onto the lawn; the patio and the line of its falls; the rim of every gully and inspection chamber; the top of the fence gravel boards; the nose of the garden steps; and the surface roots of anything you intend to keep alive.

Then subtract downward from the lowest of those constraints. A carpet with a 30 mm pile and a latex backing occupies a little over 30 mm; the screeded laying course takes another 25 to 40; the compacted sub-base is the biggest number and the only one still genuinely open to argument. Add them, measure down, and the spade depth falls out as a consequence rather than a habit. Crews who dig 100 mm because they always dig 100 mm find the mismatch on the day the grass is unrolled, which is the last day it can be corrected cheaply.

Everything you dig out has to go somewhere

An established lawn is not a thin skin of turf over builder's fill. It is turf, then a root zone that has been feeding on itself for decades, then whatever the developer left. Strip through the dark, crumbly, root-bound material until the spade meets something that stops giving, and expect that to be deeper than the drawing said. On a garden mown, fed and top-dressed since the house was built, the organic horizon can run well past 150 mm, and any of it left in place keeps decomposing and settling under a surface that shows every millimetre.

The volume that leaves is not the volume of the hole. Excavated topsoil and clay bulk substantially once broken up, and the gap between the loose barrow load and the in-situ measurement is wide enough to turn one grab lorry into two. Take the in-situ figure from plan area and strip depth, then decide how it leaves: a skip barrowed to the kerb through a side gate, a grab lorry needing a stockpile it can reach from the road, or a muck-away with a machine. That decision changes the day's labour more than anything else here.

Look at what comes out as it comes out. Shallow lighting cable, an irrigation lateral and, occasionally, a water service at nothing like its proper cover are all live in old gardens, and rubble backfill over a former outbuilding reads as firm ground while behaving nothing like it. Where roots larger than a wrist cross the dig, stop: severing structural roots of a mature tree is an arboricultural decision and possibly a legal one where a tree preservation order or conservation area applies.

The spoil is controlled waste from the moment it is in the skip. Section 34 of the Environmental Protection Act 1990 places a duty of care on the producer, which in practice means a registered carrier and a transfer note kept. Where Japanese knotweed, giant hogweed or the other listed species are present, spreading them is an offence under Schedule 9 of the Wildlife and Countryside Act 1981 and the soil goes to a permitted facility — inconvenient, but the alternative is a spread you have documented yourself by laying a lawn over it.

Fabric under the stone

Between formation and sub-base, the fabric is doing a slow job rather than an urgent one. Nothing heavy will cross this lawn again after the vibrating plate leaves, so the punching action that drives stone into soft ground under a wheel is not the threat. The threat is a decade of wetting and drying at the interface, during which stone works down into a fine-grained subgrade and its fines work up into the voids, until the layer specified at 100 mm is 80 mm of stone-flecked clay that holds water instead of passing it.

Match the product to that job rather than to the price list. Separation and filtration properties are what the classification systems describe: BS EN 13249, Geotextiles and Geotextile-Related Products: Characteristics Required for Use in the Construction of Roads and Other Trafficked Areas, in the European scheme, and AASHTO M 288, Standard Specification for Geosynthetic Specification for Highway Applications, which sets its classes against subgrade strength. A woven separator resists the tearing that happens when stone is tipped straight onto it; a light non-woven weed sheet does not, and a fabric torn during placement was never installed.

Lay it in full widths, lap generously, and turn it up the excavation faces rather than cutting it flush — the perimeter is where the lawn meets undisturbed soil, where roots arrive laterally and where the sub-base has least confinement. Do not, however, sell this sheet as weed control. It is a separator. Perennial rhizomes — horsetail, bindweed, ground elder — travel through, around and along a buried geotextile, and the only thing at this depth that reliably beats them is having removed the ground they were living in.

Type 1, Type 3, or nothing at all

MOT Type 1 is a specific thing, not a synonym for hardcore. It is a graded unbound mixture whose grading envelope and constituents are set out in BS EN 13285, Unbound Mixtures: Specifications, with the aggregate described by BS EN 13242, Aggregates for Unbound and Hydraulically Bound Materials for Use in Civil Engineering Work and Road Construction, and with the Type designations familiar on UK sites coming from the Series 800 pavement clauses of the Manual of Contract Documents for Highway Works, Volume 1, Specification for Highway Works. What makes it work is the full spread of sizes down to fines, which fill the voids between coarse particles so the layer densifies into something behaving as one body.

Notice what that means for a lawn. Type 1 was developed to spread wheel loads across weak ground, and a person standing on a garden is not a wheel load. The reason to specify it here is dimensional stability: a compacted graded layer holds a level through a wet winter and a shrinkage summer on ground that would otherwise move, and a synthetic surface reports movement mercilessly, being a taut uniform sheet with no joints and no texture to hide behind. You are buying flatness in five years, not bearing capacity today.

Against that sits the drainage problem, and it is the argument most often got wrong. This surface has no falls to a gully; it drains vertically, through perforations in the backing, into whatever is underneath. A well-compacted Type 1 with its full complement of fines is close to impermeable, so on a clay garden the classic build-up makes a shallow tray with a plastic lid — water reaches the laying course, cannot leave, and the lawn smells and grows moss on the shaded half. Where vertical drainage has to work, the reduced-fines Type 3 grading, or clean single-size stone with a blinding, keeps a path open at some cost to that stability. The trade-off is genuine and there is no universally right answer.

So decide it from the formation rather than from a standard detail. Firm, undisturbed, free-draining sandy ground under a small pedestrian area can reasonably take a laying course straight onto a separated formation with no sub-base at all — an honest answer many specifications will not admit to — while clay, made ground and root-zone influence are the opposite case, where skimping is the failure that cannot be repaired from the top.

Place it in lifts the plate can reach through, not in one deep tip, and keep it evenly damp: bone-dry graded stone rearranges under the machine without locking, and saturated stone shoves. Then order it correctly. The layer is specified in compacted millimetres and sold by the tonne, and loose delivered material occupies appreciably more room than the same material once down. Get that allowance the wrong way round and either the last barrow is short or a spare tonne sits on the drive.

The only layer you screed

You cannot screed a sub-base flat enough for this surface. The top of a well-compacted graded layer is a stone surface with 40 mm particles standing proud of it in places, and every one of them will telegraph through 30 mm of pile as a hard spot underfoot and a shine in raking light. The laying course exists to convert that into a plane, and it is the layer against which the whole job will be judged, because a synthetic lawn is the flattest thing most gardens will ever contain and the eye has nothing else to look at.

The choice is between a fine crushed dust — granite, limestone or whatever the local quarry runs, typically a nought-to-six material — and a washed sharp sand. Dust carries fines, so it screeds to a crisp arris, binds slightly when damp, and sets firm enough to walk on without leaving a heel print. Sharp sand drains more freely and never goes pasty, but nothing binds it: it will not hold a screeded edge at an open perimeter, and ants, worms and running water all move it. Where the sub-base was left open for vertical drainage, sand keeps that path open; where the priority is a surface that stays flat, dust wins. Silica sand, incidentally, is the kiln-dried infill and not a laying-course material at all.

Screed to a fall, not to level. A slight consistent fall away from the house — enough to measure, too little to see — gives surface water somewhere to go on the day the backing perforations are blocked by a season's debris, which eventually they will be. Work off rails or off the edge timbers with a straightedge, compact the screeded layer, then re-skim the low spots the compaction reveals. Finish by walking it: anywhere a heel sinks will sink again under a table leg.

Order it by weight. You have measured a depth over an area, which gives a volume, and the yard sells dust and sand by the tonne, loose or in bulk bags rated by weight rather than volume. That conversion is the step that decides whether you are one bag short at four o'clock.

Weeds arrive from above now

Once the topsoil is in the skip, the soil seed bank went with it. Whatever germinates from here on arrives on the wind, off a bird, or out of the tread of a boot, and it roots in the organic debris that accumulates on top of the pile — leaf litter, moss, blossom, grass clippings blown in from next door. That is a completely different enemy from the one the buried separator was fighting, which is why treating the two sheets as interchangeable products makes no sense in either direction.

A light permeable membrane over the finished laying course does two useful things: it stops seed that works down through the pile rooting into the screed, and it discourages worms from drawing organic material up into a layer you spent the afternoon making flat. It also has real costs. A low-permeability sheet holds a film of water above the screed, which is the last thing a clay garden needs; it wrinkles under a heavy roll and telegraphs; and on a dog run it becomes the layer that holds the smell. Fixing pins pass through it without difficulty, so that particular objection is not a real one.

The honest position is that the upper membrane is optional, and that maintenance rather than fabric is what keeps a synthetic lawn clean. Where you do use one, choose a genuinely permeable non-woven, lap the sheets, and carry it out to the edge timbers instead of stopping short — an uncovered strip round the perimeter is precisely where seed collects.

Nap, batch, and the seam you will always see

The pile leans. It is stitched and finished with a direction, marked on the backing by most makers and findable in ten seconds by running a hand across it — one way the fibres part and stand, the other they lie down. Looked at into the lean, the surface reads dark, dense and convincingly grassy; looked at along it, the same material reads pale, flat and obviously plastic. So the direction is a design decision made before the roll is cut: point the lean towards where the lawn is looked at from, usually the back of the house or the seating area, and accept what that costs in material.

Every roll then runs the same way. Turning one end-for-end to save a metre is the most expensive mistake available on this job, because the two panels read as two different colours from every angle and nothing reconciles them afterwards. The same logic extends to supply: check that every roll carries the same batch reference, the way tiles are checked for dyelot, and refuse a part-delivery from a second run rather than finding the mismatch after it is glued.

Set out the cutting plan before ordering. Rolls are stocked in a few widths — two and four metres commonly, some ranges offering five — and the width chosen against the shape of the garden decides both the waste and where the seam lands. Put the join where it is least walked and least looked at, keep any strip wider than about half a metre so it has edge enough to be held, and remember that an awkward 4.3 m garden across a 4 m roll buys a whole second roll to produce a 300 mm ribbon. Turning the layout ninety degrees and taking the hit on nap is sometimes cheaper; decide that on paper.

The join itself is butted, never overlapped. Trim two or three stitch rows off each selvedge from the back, cutting along a straight stitch line rather than the factory edge. Bring the sheets together with a hairline gap — closed tight is worse than a couple of millimetres, because a tight seam bulges as it warms — then roll the edges back, centre the joining tape face up under the line, and lay adhesive in beads rather than flooding it. Fold the fibres clear as the sheets close onto the tape: glue that reaches the pile is visible forever. Weight the seam while it cures.

Conditions govern more of this than technique does. Roll the sheets out and let them relax before trimming; polyethylene backing moves with temperature, and a cold roll cut tight will grow and ridge in July while a hot roll fixed tight will shrink and open the seam in January. Two-part polyurethane adhesives have a pot life and a minimum application temperature, and self-adhesive tapes have a floor below which they do not grab at all. Both figures come from the manufacturer's installation instructions, and on a cold damp day those instructions decide whether the job happens.