Geotextile Membrane: The Filter Fabric That Keeps Soakaways and Sub-Bases Working

A homeowner orders soakaway crates from an online drainage supplier, then walks into Wickes for the membrane because it's three minutes down the road. They pick up a 50gsm woven weed fabric labelled "heavy duty landscape membrane" and wrap their crates. Eighteen months later the kitchen extension's gutter starts overflowing in heavy rain. Excavate the soakaway and the crates are full of clay slurry. The fabric was never rated to filter soil under hydraulic load. Replacement runs to £800 – £2,000 in plant hire and groundworker time. The right product would have cost a small premium over the weed fabric on day one.

Geotextile is not damp-proof membrane

Before anything else, get this straight. Geotextile membrane is permeable. Damp-proof membrane (DPM) is impermeable. They do opposite jobs and you must not substitute one for the other.

Geotextile is a fabric. Water passes through it freely. Soil particles do not. It filters and separates.

DPM is a sheet of solid polythene (1200 gauge / 300 micron). Nothing passes through it. It blocks moisture rising from the ground into a concrete floor slab.

If you wrap a soakaway in DPM, the soakaway cannot soak away. Water has nowhere to go. The crates fill, back up to the inlet, and surcharge into your downpipes. If you lay geotextile under a kitchen floor as a moisture barrier, water rises straight through it and rots your floor finish.

The two products are sold from different aisles, by different suppliers, for different prices. They are not interchangeable. PavingExpert, the most authoritative UK paving site, is explicit: never substitute polythene, DPM, or building film where geotextile is specified. It traps water and causes saturation. The mistake is more common than it should be, usually because both come on a roll and look vaguely similar to a homeowner who hasn't been told the difference.

What geotextile actually is

Geotextile is a permeable engineering fabric, almost always made from polypropylene. The non-woven type (the kind you want for drainage) is manufactured by needle-punching short polypropylene fibres into a felt-like mat, then thermally bonding it to give the sheet structure. The result looks like a thick, white, slightly fuzzy industrial felt.

Two functions matter for domestic work:

Filtration. Water can pass through the fabric, but soil particles cannot. The pore size (approximately 120 microns O90 for 100gsm non-woven, dropping to around 70 microns at 200gsm) is small enough to block clay and silt fines while letting water flow at rates of 60-130 litres per square metre per second. That's the property that makes it the right product to wrap a soakaway.

Separation. A sheet of geotextile laid between two different materials stops them from mixing. Lay it between soft natural subgrade and a clean MOT Type 1 hardcore sub-base, and the hardcore stops sinking into the soil while the soil fines stop pumping up into the hardcore voids. Without separation, a clay subgrade will progressively contaminate your sub-base under traffic load until the whole structure goes spongy.

The relevant UK standard is BS EN 13252:2016 (Geotextiles for drainage systems). Reputable products carry CE marking referencing this standard. For soakaway design, the parent standard is BRE Digest 365, which the NHBC explicitly accepts.

The general principle is plain. Without a separation layer, the granular layer migrates into the soil, causing sinking, poor drainage, and structural movement. Without a filter layer around a soakaway, soil fines progressively clog the void space until the soakaway stops working.

The two domestic use cases

For a UK homeowner self-managing an extension, geotextile shows up in two places.

Wrapping soakaway crates. Modern soakaways are made from modular polypropylene crates (Aquavoid, EcoBloc, Polystorm, Aquacell). Stack them to the volume calculated from your percolation test, then wrap the entire stack in geotextile before backfilling. The membrane creates a filter envelope: rainwater from your roof flows in through the inlet, through the void space inside the crates, then permeates outward through the membrane into the surrounding soil. The fabric stops the soil flowing the other way. Without it, every winter washes more clay into the crates until the void space is gone.

Separating sub-base from soft subgrade. Under a new patio, a driveway, or a shed base, the standard build-up is hardcore sub-base over compacted natural ground. On clay or silt subgrades the soft material will progressively contaminate the hardcore unless something separates them. A layer of geotextile rolled out before the hardcore goes down does the job. MOT Type 1 at the standard 100-200mm depth over a geotextile separation layer is the textbook detail.

A third use, French drains and land drainage, uses the same product wrapped around a perforated pipe in a gravel-filled trench. Same physics, smaller scale.

Types and grades

There are two structural types of geotextile and several weight grades. For soakaway and driveway work on a domestic extension, you want non-woven, and you want a specific weight depending on the load.

Two things from that table are worth understanding properly.

Non-woven beats woven for drainage. Non-woven geotextile is the felt-like product. Its randomly-oriented fibres give high cross-plane permeability (water flows through it easily). Woven geotextile is a basket-weave of plastic strips. It has higher tensile strength but much lower permeability. For a soakaway wrap or a French drain, you want non-woven. Pick woven only where you genuinely need the load-bearing reinforcement.

GSM (grams per square metre) is the headline spec, but it's not everything. GSM tells you the fabric weight. Higher GSM generally means more durable and more puncture-resistant, but it also slightly reduces permeability. The other numbers that matter on a datasheet are tensile strength (kN/m), CBR puncture resistance (newtons), and water flow rate (L/m²/s). For domestic work, 100gsm with a CBR around 1500N is the standard reference point.

The brand names you'll see at UK merchants:

• Terram (the market leader, Berry Plastics). Terram T1000 is the 100gsm classic. Carries a 30-60% brand premium over equivalent products.
• TCS Geotechnics TNW range (TNW1000 is 80gsm, TNW1100 is 100gsm, TNW2000 is 200gsm). Same spec as Terram at lower prices.
• Drivetex (sub-base specific, around 90gsm). Common at Jewson and Travis Perkins.
• Draintex (drainage specific, around 80gsm). Sister product to Drivetex.
• Wallbarn, Lotrak, Bidim, Groundtex are all reputable. For a one-off domestic job, brand barely matters as long as the GSM and the standard reference (BS EN 13252) are correct.

How to work with it

Buying the right size and format

Geotextile sells either as a full roll (typically 4.5m x 100m, 450m²) or as a pre-cut "homeowner" pack (commonly 2m x 25m or 2.25m x 25m, around 50-56m²). Full rolls work out cheaper per m² but are heavy and unwieldy (40-55kg of plastic on a cardboard tube). For a single soakaway, a pre-cut pack is the practical choice.

For a typical domestic soakaway crate stack of 3 crates, you need around 15m² of membrane to cover the box plus 5-10m² for overlap and the awkward inlet sealing. Buying a 2.25m x 25m roll gives you 56m² (priced at £70 – £85) with plenty of waste allowance. Don't try to scrape by with the absolute minimum; you'll regret it during installation.

Cutting

Cut geotextile with a sharp utility knife (a Stanley knife) or strong scissors. The non-woven fabric blunts blades quickly, so have a few spare blades on hand. Cut on a flat surface (a sheet of plywood is ideal), measure twice, and allow 300mm extra on every dimension for overlap. There is no chamfer or edge prep. The cut edge is the cut edge.

Overlap is non-negotiable

Wherever two pieces of geotextile meet, they must overlap by at least 300mm. For soakaway wrap, lay the membrane out so the box of crates can be fully enclosed by a single sheet folded over itself, with the seam landing on top of the stack and overlapping by 300mm minimum. For sub-base separation under a patio, run successive strips with 300mm overlap along every join and at the perimeter.

Some basic guides quote 150mm overlap. That figure pre-dates the current BS EN 13252 guidance and isn't enough. The Civil Engineering and Aquacell installation guides both specify 300mm minimum, and CivilWeb's technical detail sheet states the same. For heavy-duty applications or weak subgrades, 500mm.

Securing during install

Wind catches geotextile easily. On a windy day a roll of 100gsm fabric will travel half the garden if you leave it unweighted. Cut to size, then pin with steel U-shaped landscape pegs every 1-2 metres around the perimeter while you backfill. For soakaway wrapping, hold the membrane in place with bricks or shovel-loads of pea shingle as you build up the crate stack. Don't trust gravity on its own.

Sealing around the inlet pipe

The single most common installation defect, flagged by NHBC inspectors and recurring across BuildHub forum threads, is the inlet pipe sealing. Where the 110mm soil pipe from your downpipe enters the soakaway, the membrane must form a continuous seal around the pipe with no gap for soil to bypass. Cut a cross-shaped slit in the membrane just slightly smaller than the pipe outside diameter, push the pipe through, then tape the membrane to the pipe with proprietary geotextile tape (priced at £8 – £15). Heavy-duty duct tape works as a backup but it's not specified.

UV and weather

Geotextile loses tensile strength under prolonged UV exposure. CivilWeb's guidance is to avoid leaving the fabric exposed for more than a few weeks. In practice, install and backfill within a week of laying the membrane. If a roll has been stored outdoors in sunlight for a season, treat it as compromised.

The fabric handles rain fine while it's being installed. Wet geotextile is heavy and slightly harder to manoeuvre, but the material itself is unaffected.

Click to enlarge

How much do you need

For a soakaway, the simple rule: surface area of the crate stack (top, bottom, four sides) plus 300mm overlap on every edge plus 20% for installation waste.

For a typical 1m³ domestic soakaway built from three 190-litre standard crates stacked 1m x 1m x 1m, that's roughly:

• 6m² for the six faces
• ~6m² for overlap allowance (300mm extra around all edges plus a generous fold-over on top)
• ~3m² for waste, awkward cuts, and spares

So 15m² minimum for the soakaway itself. Buy a 2m x 25m or 2.25m x 25m pre-cut pack (50-56m²) and you have margin for error and offcuts for any French drain runs you didn't plan for. Total geotextile material cost for the soakaway portion of a typical extension is £15 – £40.

For driveway or patio sub-base separation, calculate the area of the build-up plus 300mm overlap at every join. A 30m² patio needs roughly 35m² of fabric allowing for joins. A 50m² driveway needs 60-65m². At £1 – £2, the membrane is genuinely a small line item against the cost of the hardcore, the bedding, and the surface finish.

Cost and where to buy

Pricing splits cleanly into two markets: drainage and civils specialists, and the wrong shops.

The right places to buy: dedicated drainage and civils suppliers like Drainage Superstore, EasyMerchant, EcoTradeCounter, United Civils Supplies, AVS Fencing, Spudulica, and TradeGeos. Plus any builders' merchant with a civils desk: Travis Perkins, Jewson, MKM, Buildbase. Expect £1 – £2 for 100gsm in a bulk roll; pre-cut homeowner packs (2m or 2.25m x 25m) typically run £70 – £85 inc VAT.

£15 – £40 is the typical total membrane bill for a single domestic extension's soakaway. Cheap insurance against a four-figure excavation in five years' time.

The wrong places: Wickes, B&Q, Homebase. Their entire "landscape membrane" range (typically Apollo branded at Wickes) is horticultural weed fabric in the 50-110gsm range. The 95gsm "heavy-duty" Apollo product, at lightweight-non-woven pricing similar to £1 – £1, looks like a great deal. It is not rated to BS EN 13252, has no published CBR puncture resistance, and isn't suitable for soakaway filtration or sub-base separation. The forum verdict on this product, repeated across BuildHub threads, is consistent: do not use it for engineering work, even if it's three minutes down the road.

Screwfix and Toolstation sit in a middle ground. They stock some bagged or boxed lengths of proper drainage membrane (typically own-brand 100gsm) but the range is narrow. For full-roll buying or specific weights, a drainage specialist is better. For one off-cut roll to wrap a single soakaway, Toolstation is fine if they have it in stock.

Watch the delivery cost

Online geotextile suppliers sometimes advertise free shipping and then surcharge based on delivery postcode. Check your basket before paying. BuildHub forum threads document substantial delivery surcharges on rolls advertised as free shipping. Pre-cut packs travel by parcel courier. Bulk rolls go by pallet, which is where the surcharge tends to land.

Common mistakes

Using DPM instead of geotextile. The headline mistake. Polythene damp-proof membrane is impermeable and will kill a soakaway dead. They look superficially similar (both come on rolls, both are sheet products), but they do opposite jobs. Buy from a drainage supplier and you can't make this mistake. Buy from your shed and you can.

Using woven weed fabric instead of engineering geotextile. The Wickes/B&Q trap. Weed membrane is rated for blocking light from weed seeds, not for filtering soil particles under hydraulic pressure or supporting hardcore loads. It clogs faster, tears more easily, and has no documented filter pore size. It will fail. Just because both are labelled "membrane" doesn't make them the same product.

Insufficient overlap at seams. 150mm isn't enough. 300mm is the minimum specified by BS EN 13252 and the major installation guides. Soil migrates through small gaps under sustained pressure. A poorly-overlapped seam is a slow leak of fines into the void space, and you won't see it until the soakaway stops working.

Skipping the seal around the inlet pipe. A tear or unsealed cut around the soil pipe is functionally identical to no membrane at all in that location. Soil pours straight into the void. Use a cross-cut just smaller than the pipe diameter and tape with proper geotextile joining tape.

Leaving membrane exposed to UV for months. A roll left out in sun across a build season degrades. Order it close to install date, store under cover, install promptly, backfill within a week.

Installation FAQ

Does geotextile go above or below the hardcore? For sub-base separation, the membrane goes under the hardcore, directly on the prepared natural subgrade. The hardcore is then laid and compacted on top of the fabric. The fabric never goes above a finished hardcore layer in a sub-base detail. For a soakaway, the fabric forms a complete envelope around the crates, so it sits on every face including the bottom of the pit.

Do I need geotextile if I'm laying a patio on a small area of firm clay? Best practice on any sub-base is to use a separation layer regardless of subgrade firmness, but on hard, well-drained natural ground a competent installer might skip it for a small area. On any clay subgrade, on any vehicle-loaded surface, and on any soakaway, the membrane is mandatory. Cost is so low (£1 – £2) that there is no real reason to omit it.

Will the membrane block tree roots? Standard 100gsm non-woven geotextile slows root penetration but is not designed as a root barrier. Aggressive species (willow, poplar, bamboo) will eventually grow through it. If a soakaway is sited near such a tree, either move the soakaway or use a dedicated root barrier in addition to the geotextile.

Can I overlap two different brands of geotextile together? Yes, provided both are rated to BS EN 13252 and both are non-woven of equivalent or higher GSM. Mixing a leftover roll of TCS TNW1100 with a fresh Terram T1000 around the same soakaway is fine. Don't mix non-woven and woven products on the same wrap.

What does building control actually inspect? The building control officer attending at groundwork stage will look for visible geotextile around the soakaway, confirm there is no DPM substitution, and check the seal around the inlet pipe. They will normally want sight of percolation test results before approving the infiltration drainage design under NHBC Chapter 5.3 / BRE Digest 365. Photograph the wrapped soakaway before backfilling; it makes the inspection conversation easier and gives you evidence if a future claim ever needs it.

Can I install geotextile in winter? Yes. Cold weather has no effect on the fabric. Frozen ground is the actual problem: you can't excavate or compact it, so groundwork stops in hard frosts. The membrane itself is fine down to well below zero. Snow on a partially-installed wrap is not a quality issue but does add weight and slipping risk on the working surface.

Where you'll need this

• Drainage - the soakaway construction stage of any extension that drains its roof to ground rather than to mains; geotextile wraps the crate stack and seals around the inlet pipe before backfilling
• Foundations and footings - a separation layer below any hardcore sub-base over soft natural subgrade keeps the granular fill working as designed
• Plastering - not a direct dependency, but the same supplier mailing list often carries the floor membranes (DPM) that show up here, which is exactly the confusion this page exists to head off

Geotextile membrane shows up wherever an extension's surface water drains to a soakaway, wherever a new patio or driveway is laid over soft natural ground, and wherever a French drain is needed to manage groundwater. It also turns up under shed bases, raised-bed paths, and stabilised gravel driveways on any kind of construction or renovation project - not just kitchen extensions. The same product, the same 100gsm specification, covers all those uses. Buy once from a drainage supplier and any offcuts find a use elsewhere on site.