How to specify geotextiles

How do you specify geotextiles?

To specify a geotextile, state its function first (separation, filtration, drainage, protection or reinforcement) and the application standard that matches the use, such as BS EN 13249 for roads and trafficked areas. Then give the declared values that function depends on: static puncture (CBR) and cone drop resistance for robustness, characteristic opening size and water permeability for filtration, tensile strength, and durability for the design life, rather than a weight in grams per square metre.

"Geotextile membrane" is how most drawings label it, and the label is part of the problem. A geotextile is a permeable fabric. Its job is to let water through while keeping soil and stone apart, or to protect something else from damage. Specifications that call it a membrane, or describe it only by weight, invite a contractor to supply whatever roll is on the van, which may be the wrong fabric for the ground or too weak to survive the stone placed on it.

This guide covers Uniclass Pr_15_57_35_61 Plastics geotextiles, the woven and non-woven polypropylene and polyester fabrics used under sub-bases, around drains and behind retaining walls. Its neighbours Pr_15_57_35_57 Needle-punched non-woven geotextiles and Pr_15_57_35_88 Thermally bonded non-woven geotextiles describe how the fabric is made; the properties to specify are the same.

What does a geotextile clause need to cover?

Some properties are specific to geotextiles. Others belong in every product clause. A clause that leaves either out gives the contractor room to substitute.

Which decisions does the clause depend on?

Most properties follow from the product you choose. These depend on the project, so they need a decision from the designer before the clause is written.

Which standards should the clause cite?

Cite the product standard the declaration of performance refers to, the code of practice for installation, and the test method behind each value you set. A value without its test method cannot be checked against a submittal.

BS EN 13249

Geotextiles and geotextile-related products. Characteristics required for use in the construction of roads and other trafficked areas (excluding railways and asphalt inclusion)

The application standard for geotextiles under roads, car parks and other trafficked areas, listing the characteristics to be declared for each function.

In the clause Name it with the function: "Declared to BS EN 13249 for separation and filtration".

BS EN 13250 to BS EN 13257 and BS EN 13265

Geotextiles and geotextile-related products. Characteristics required for use in ... (railways, earthworks, drainage, erosion control, reservoirs and dams, canals, tunnels, solid waste disposal, liquid waste containment)

The other application standards in the series, one per type of works. BS EN 13251 covers earthworks, foundations and retaining structures, BS EN 13252 drainage systems and BS EN 13253 erosion control.

In the clause Pick the one that matches the use: "Declared to BS EN 13252 for filtration" around a filter drain.

BS EN ISO 12236

Geosynthetics. Static puncture test (CBR test)

The plunger test that gives the CBR puncture resistance in newtons, the most common robustness value in UK specifications.

In the clause "Static puncture (CBR) resistance: not less than [value] N to BS EN ISO 12236."

BS EN ISO 13433

Geosynthetics. Dynamic perforation test (cone drop test)

A falling cone test that measures the hole made in the fabric, as a proxy for stones dropped during placing.

In the clause "Dynamic perforation: hole diameter not more than [value] mm to BS EN ISO 13433."

BS EN ISO 12956

Geotextiles and geotextile-related products. Determination of the characteristic opening size

The test for O90, the opening size that controls which soil particles the fabric retains.

In the clause "Characteristic opening size O90: not more than [value] mm to BS EN ISO 12956."

BS EN ISO 11058

Geotextiles and geotextile-related products. Determination of water permeability characteristics normal to the plane, without load

The flow rate of water through the fabric, needed wherever the geotextile acts as a filter.

In the clause "Water permeability normal to the plane: not less than [value] to BS EN ISO 11058."

BS EN ISO 10319

Geosynthetics. Wide-width tensile test

Tensile strength and elongation at maximum load, in both machine and cross-machine directions.

In the clause "Tensile strength: not less than [value] kN/m in both directions to BS EN ISO 10319."

BS EN 12224

Geotextiles and geotextile-related products. Determination of the resistance to weathering

The weathering test behind the declared time a geotextile can be left uncovered on site.

In the clause Use it in the exposure line: "Cover within the period declared from testing to BS EN 12224".

Confirm the current edition of each standard at every issue. Avoice's State of UK Specifications 2026 found a withdrawn standard cited in 40% of specifications at contract issue.

How do you write it in CAWS?

Most geotextiles on building projects sit under paving, so the usual home in CAWS is Q20, with a product clause 170 Geotextile filter/ separator membrane. The paving type clauses in Q22 to Q25 call up the Q20 sub-base, and the sub-base clause in turn calls up the geotextile, so one geotextile clause can serve every paving type on the project.

Where the geotextile belongs to earthworks or drainage rather than paving, NBS has equivalent clauses elsewhere: D20 Excavating and filling has a geotextile sheet clause, and R12 and R13 cover geotextiles around pipes and filter drains. Whichever section holds it, the clause relies on the subgrade preparation and sub-base placing clauses for the work around it.

Clauses the type clause relies on

Q20/150

Subgrades for vehicular areas Preparing and proof-rolling the formation the geotextile is laid on.

Q20/211

Granular material The sub-base material whose grading drives the puncture requirement.

Q20/230

Placing granular material generally Tipping and spreading stone without tracking plant on bare fabric.

Q20/240

Laying granular sub-bases for vehicular areas Layer thickness and compaction over the geotextile.

Q20/340

Protection Covering subgrades and sub-bases promptly and keeping traffic off.

Clause numbers vary between NBS versions and practice masters; check that the paving clauses in Q22 to Q25 point to the Q20 geotextile clause you actually keep.

How do you write it in Uniclass?

In Uniclass the geotextile is a product inside a system. Under paving that system is the sub-base layer, Ss_30_14_65_82 Sub-base layer systems, which describes the subgrade, the geotextile, the granular layers and the finished bearing and drainage it has to give.

The geotextile itself is written once as Pr_15_57_35_61 Plastics geotextiles, with its function, standard and declared values. The same product clause can then be pointed to from a filter drain or soakaway system, provided its filtration values suit that use too; if not, it is a second product with its own type mark.

There is no NBS Chorus reference against this product type in our data, so the Uniclass code carries the identification on its own; do not mix CAWS and Uniclass references in one specification.

What goes in the spec, and what goes in the schedule?

The specification defines each type once. The schedule and drawings say where each type goes.

What are the common mistakes?

Most problems are a missing property, a property filled with the wrong kind of value, or a decision left silently open.

• !
  Specified by weight alone. "200 g/m² geotextile" says nothing about puncture or filtration. Two fabrics of the same weight can differ widely in both.
• !
  Called a membrane. The word leads some contractors to supply polythene or a geomembrane, which stops water passing. Say geotextile and say it must be permeable.
• !
  No function named. Without a function the declaration of performance cannot be checked, because declared values only apply to the functions listed.
• !
  Left in the sun. Rolls laid out weeks ahead of the sub-base go brittle. State the exposure limit and programme the cover.
• !
  Plant on bare fabric. Tracking over uncovered geotextile tears it and moves the laps. Require end-tipping and spreading from the advancing layer.
• !
  Laps too short on soft ground. A standard lap pulls apart when soft subgrade deforms under the first layer. Increase laps, or sew or overlap as the manufacturer requires.

How Avoice helps

Avoice's spec writer drafts geotextile clauses from the build-ups on your drawings, and keeps the function and values tied to each use.

1. 1
   Drafts from your details

   It picks up geotextile type marks from paving, drainage and retaining wall details and drafts the Q20, D20 or Uniclass clause with the function each location needs.

2. 2
   Leaves a TBC where the design is missing

   If the puncture value depends on a pavement design that has not arrived, the line says TBC with the reason and joins the project TBC register to close before tender.

3. 3
   Writes in your format and your style

   Choose CAWS or Uniclass. Avoice adapts your practice's own Q20 and D20 masters rather than replacing them with generic text.

4. 4
   Checks the clause against everything else

   QA/QC flags a detail note calling for a "membrane" where the specification has a geotextile, a GT02 with no clause, or a paving clause pointing at a Q20 clause that has been deleted.

5. 5
   Keeps product data in one place

   Declarations of performance live in the Product Room, linked to the clause, so a substitute roll can be checked against the declared functions and values the clause asks for.

Related guides: How to specify soakaway crates, How to specify block paving, How to specify gabions.

Software can draft and check. The designer still chooses the product and is responsible for the specification.