The bag at the builders’ merchant says “landscape fabric” and that is where the trouble starts. Two very different materials get sold under that name, they do close to opposite jobs, and swapping one for the other is one of the more expensive quiet mistakes you can make in a yard. One of them is woven. The other is not. Getting that right costs nothing at the point of purchase and saves digging the job up later.
Woven fabric separates. Non-woven fabric filters.
A woven geotextile is made of flat tapes or monofilaments crossed over each other like a basket. It is strong in tension and it holds its shape under a load, but all that water can do is squeeze through the small gaps between the threads. A non-woven geotextile is a mat of fibres needle-punched or heat-bonded together, more like felt. It is weaker in tension and far more porous.
The numbers in the highway specifications make the gap plain. In the AASHTO M288 specification used across state DOTs, a fabric sold for separation only has to pass water at a permittivity of 0.02 per second, with an apparent opening size around 0.60 mm. A fabric sold for subsurface drainage has to hit 0.1 to 0.5 per second, with openings tightened to 0.22–0.43 mm depending on how fine the surrounding soil is. In other words, the drainage fabric moves something like five to twenty-five times more water through itself, through smaller holes. That combination is the whole trick, and it is why the two are not interchangeable.
Around a French drain, use non-woven
A French drain only works if groundwater can cross into the stone from every direction. The fabric is there to let water in and keep soil out, which is a filtration job, so it wants the non-woven mat. We line the whole trench with it, bring it up the sides, lay the stone and pipe, then fold the top over with a generous overlap rather than a two-inch lap that opens up the first time somebody walks on it.
The failure we are called out to most often is a trench wrapped in the cheap black woven sheeting sold as weed barrier. The drain runs for a season, the water starts standing over the line instead of disappearing into it, and when we open it up the stone is still clean and the fabric is holding water out like a tarp. Worth knowing too: the NCHRP guidance that Virginia’s highway department publishes points out that a fabric meeting both the soil retention and the permeability criteria can still fail by clogging, as fine particles work into the matrix. That is an argument for keeping mud out of the stone while you backfill, and for not skimping on the depth of clean stone around the pipe.
Under a paver base on soft clay, use woven
Here the job changes. Nothing needs to pass through the fabric in a hurry; what matters is that the clay subgrade underneath stays down there and the crushed stone above stays up here. Drive a loaded wheelbarrow over wet clay with stone sitting on it and the two migrate into each other, the base loses its structure, and the patio settles in dips a couple of summers later. That is separation, and a woven monofilament is what the specification asks for, because it has the strength to take the load without stretching.
We do not put it under every patio. On firm, well-drained subgrade it is money spent on nothing. On the low corner of a yard, or anywhere the subgrade turns to grease after rain, it is the cheapest insurance in the build.
Behind a retaining wall, fabric is not the reinforcement
These two get confused constantly, so it is worth saying plainly: geotextile is a fabric and geogrid is a structural grid, and one will not do the other’s work. The grid is the ribbed plastic mesh that runs back into the hill between courses of block and ties the wall to the soil behind it. Where a wall is tall enough to need it, that is an engineer’s decision, not a choice at the counter. The fabric’s job on the same wall is smaller and still worth doing: a non-woven layer between the drainage stone behind the block and the native backfill, so the clay does not slowly silt the stone up and turn a drained wall into a dam. Our page on retaining wall materials goes into where that water pressure comes from.
Where fabric does not belong: under mulch
This is the one we pull out of yards rather than put in. Fabric under a mulch bed sounds like less weeding and delivers the opposite within about three years. Mulch is supposed to break down into the soil underneath it, feeding it and keeping it open; a sheet of plastic between the two stops that happening, so the bed stays hungry and compacted. Meanwhile windblown seed germinates happily in the decomposed mulch sitting on top, roots straight through the weave, and now every weed has to be pulled with a sheet of torn fabric attached to it. What we do in our own beds is simpler: two to three inches of mulch or pine straw, topped up when it thins, and ten minutes of hand weeding when we are there anyway. Pine straw or mulch is a separate question and mostly comes down to slope.
What our soil actually needs, and what it does not
It is tempting to blame Georgia red clay for everything, and the official record argues otherwise. Most of the yards we work in around Dunwoody, Sandy Springs and Brookhaven sit on the Cecil series or one of its neighbours, and the USDA soil series description classes Cecil as well drained, with moderately high saturated conductivity and a seasonal water table more than sixty inches down. The subsoil is a kaolinitic clay, which is a low-activity clay: it does not swell and shrink the way the heavy clays of central Texas do.
So when a yard here stays wet, the soil is usually not the culprit. It is grading that runs toward the house instead of away from it, subgrade that was compacted flat by machinery while the house was built, and four downspouts emptying within a foot of the foundation. No fabric fixes any of those. We wrote about the last one in where your downspouts put the water, and about the first two on our drainage and grading page. Fabric belongs in the detail of a drain or a base, not in place of fixing the fall of the land.
What to ask for at the counter
Name the function, not the product. “Non-woven drainage fabric, four ounce or heavier” and “woven separation fabric” both get you the right roll; “landscape fabric” gets you whatever is nearest the door. If the label quotes a flow rate and an apparent opening size, it is a real geotextile. If the only claim on the bag is about weeds, it is a weed barrier, and the only place that belongs is nowhere we would use it.
If you are putting in a drain, a patio base or a wall around north Atlanta and you want a second opinion on what goes where, we are glad to come and look at the ground. Most of these decisions are made in an hour on site, and they are much cheaper to make before the stone goes in. More in our guides.
Sources: AASHTO M288 geotextile property requirements as summarised in NCHRP Report 544 guidance published by the Virginia Department of Transportation; USDA official series description for the Cecil soil series.
