If you search for geotextile fabric for a french drain, retaining wall, or dry well and wonder why most recommendations point to 4oz — this guide explains exactly why, what makes it the standard residential weight, and precisely when you should consider going to 6oz instead.

What "4oz" Means

The number refers to the fabric weight in ounces per square yard (oz/sy). A square yard of 4oz fabric weighs 4 ounces. This weight is directly related to the density of polypropylene fibers in the needle-punch mat:

• More ounces per square yard = more fibers per unit area
• More fibers = smaller effective pore openings (finer AOS)
• More fibers = higher tensile strength and puncture resistance
• More fibers = slightly lower permittivity (but still above minimums for all residential drainage)

The typical 4oz non-woven geotextile has an AOS of approximately #40–#60 US sieve (0.297–0.425mm), permittivity of 1.0–2.0 sec¹, and grab tensile of 90–120 lbs.

What 4oz Handles Well

4oz non-woven geotextile is the correct choice for the following applications in sandy to loamy soils:

Residential French Drains

The most common application. 4oz provides adequate AOS to retain medium sand and coarser particles from sandy or loamy soils while allowing excellent water flow through its high permittivity. It is the industry-standard specification for residential french drain installations in non-clay soils.

Retaining Wall Drainage Layers

4oz is the standard weight for the separation/filtration layer behind retaining wall drainage aggregate, when the retained soil is granular or sandy-loam. It prevents fine particles from migrating into the drain zone over the wall's service life.

Dry Wells and Seepage Pits

4oz lines the gravel-filled pit to prevent native soil from infiltrating into the stone void space. The high permittivity allows water to drain freely while the fabric maintains the clean stone-soil interface.

Pavement Edge Drains

4oz is often adequate for residential pavement edge drain applications, though 6oz is specified for DOT or commercial edge drains in fine-grained soils.

Green Roof Drainage Layers

4oz placed between growing medium and drainage aggregate prevents fine growing medium from washing into the drainage layer over time.

Under Gravel Paths and Patios

4oz provides separation between native soil and gravel, preventing the gravel from sinking into soft subgrade over time. Less demanding than driveway stabilization (which needs woven geotextile); 4oz non-woven provides adequate separation for foot-traffic applications.

Where 4oz Falls Short

Clay or Silty-Clay Soils

Clay particles are finer than the AOS of most 4oz geotextile. In heavy clay soils, fine clay particles can migrate through 4oz fabric over time, eventually clogging the drainage system. For clay soils, 6oz (typical AOS #50–#80) provides better retention.

High Silt Load Environments

Sites with very high suspended silt content in runoff — construction sites, agricultural drainage, near disturbed soil — may clog even properly-specified fabric more quickly. Use 6oz and accept that some maintenance may be needed over a long service life.

DOT Applications Requiring AASHTO M288 Class 2

4oz typically meets AASHTO M288 Class 3 requirements (90 lb min grab tensile). Class 2 requires 157 lb min grab tensile, which generally requires 6oz or heavier. For DOT submittals, always verify the Class 2 grab tensile requirement.

Heavy Aggregate Placement with Equipment

When 3/4-inch or larger angular crushed stone is placed by heavy equipment directly on the fabric, 6oz provides greater puncture resistance than 4oz. For hand-placed aggregate or rounded stone, 4oz is typically fine.

4oz vs 3oz vs 6oz Comparison

Weight	Typical AOS	Permittivity	Grab Tensile	Best For
3 oz	#30–#50	2.0–3.0 sec¹	70–90 lbs	Light drainage, pipe sock, sandy soil
4 oz	#40–#60	1.0–2.0 sec¹	90–120 lbs	Residential drainage, sandy/loamy soil — most common choice
6 oz	#50–#80	0.8–1.5 sec¹	120–160 lbs	Clay soils, higher silt loads, commercial drainage

Available Roll Sizes

Width	Common Lengths	Best Use
4 ft	100, 300 ft	Pipe wrapping, narrow drainage trenches
6 ft	100, 300 ft	Typical residential french drain trench wrap
12.5 ft	360 ft	Retaining walls, large area drainage
15 ft	360 ft	Wide commercial applications

Applications Table by Soil Type

Soil Type	Is 4oz Adequate?	Better Choice If Not
Gravel or coarse sand	Yes	—
Medium sand	Yes	—
Sandy loam	Yes	—
Loam	Typically yes	6oz if high silt content
Silt loam	Marginal	6oz recommended
Clay loam	No	6oz
Heavy clay	No	6oz or 8oz

Frequently Asked Questions

Is 4oz geotextile adequate for a french drain?

Yes, 4oz is the standard specification for residential french drains in sandy to loamy soil. If your soil is heavy clay or contains significant fine silt, use 6oz for finer particle retention.

What is the difference between 4oz and 4.2oz geotextile?

4oz and 4.2oz refer to the same product category. The slight weight variation is a manufacturing tolerance. Products labeled either way are functionally equivalent with similar AOS, permittivity, and tensile specifications.

When should I use 6oz instead of 4oz?

Use 6oz when your soil is clay or silty clay (requires smaller AOS), when the project uses heavier aggregate, when silt loads are high, or when a DOT specification requires AASHTO M288 Class 2 performance.

What roll sizes does 4oz geotextile come in?

Available in widths of 4ft, 6ft, 12.5ft, and 15ft. For french drains, 6ft wide is common for typical residential trenches. Buy by the foot for small quantities.