Do You Need Filter Fabric Under Riprap?

What filter fabric actually does

Filter fabric sits between the soil and the riprap, doing one specific job: letting water pass through freely while holding soil particles back. Properly selected geotextile typically achieves 85-95% fines retention while still maintaining enough permeability to avoid trapping water pressure behind it. Without it, water moving through the voids in the riprap layer gradually carries fine soil particles out with it — a process called piping — hollowing out the ground beneath the rock until the riprap above has nothing left to sit on and settles or collapses into the void.

Woven vs. nonwoven: which one

Nonwoven’s needle-punched, felt-like structure lets it conform to an uneven, rocky subgrade far better than a woven fabric can, while still filtering effectively — which is exactly why it’s the default recommendation for the great majority of riprap projects.

The three specs that actually matter

• AOS (Apparent Opening Size)The effective size of the fabric’s openings. Too large, and soil pipes straight through. Too small, and the fabric clogs and blocks drainage. It has to be matched to your specific soil’s particle size.
• PermittivityHow fast water passes through the fabric vertically. Higher permittivity matters more in tidal or rapid-drawdown conditions, where trapped water pressure behind the fabric can otherwise blow it off the slope.
• Puncture resistance (CBR)Whether the fabric survives a heavy, angular stone being dropped directly onto it during placement. This is non-negotiable — a torn fabric loses its filtration function instantly, right where the stone landed.

Matching AOS to your soil type

Fine soils have small particles that can pass through a loose weave easily, so they need a fabric with smaller openings to hold them back. Coarser soils are naturally harder to pipe through the fabric, giving more flexibility on AOS without risking soil loss.

Fabric vs. a granular filter layer — the alternative approach

Geotextile fabric isn’t the only way to filter soil from riprap — some specifications call for a granular filter layer instead: a graded sand or fine gravel bed that performs the same filtering function through particle size grading rather than a synthetic membrane. Geotextile fabric is generally faster and more consistent to install, which is why it’s become the more common choice, but a granular filter remains a valid engineered alternative, and some specs (particularly for very high-energy sites) call for both a granular layer and fabric together.

Installation practices that prevent puncture

• A sand cushion layer over the fabricA few inches of sand between the fabric and the riprap absorbs some of the impact from stone placement, reducing puncture risk — especially valuable on delicate applications like a pond liner installation.
• Careful, low-drop placement rather than dumping from heightThe higher a heavy stone drops before hitting the fabric, the greater the puncture risk — placing rather than dumping matters most for the first layer.
• Proper overlaps at every seamAdjoining fabric sections need enough overlap that soil can’t migrate through a gap at the joint.
• Choosing a heavier fabric for sharp or irregular stoneAngular, jagged riprap punctures more easily than rounder stone at the same weight — size the fabric’s puncture resistance to the actual stone being used.

What to check before selecting a fabric

• Soil type at the siteDetermines the AOS requirement.
• Water condition (tidal, rapid drawdown, steady flow)Affects how much permittivity you need.
• Stone size and angularity being placedThe bigger and sharper the stone, the more puncture resistance the fabric needs.
• Whether a specific spec (like AASHTO M288) governs the projectIf so, select a fabric that’s certified to meet that class’s exact property requirements.

Three scenarios, three fabric choices

Scenario 1 — fine silty soil, moderate flow, medium riprap

A tighter AOS is needed to hold the fine soil back. A mid-weight nonwoven fabric (roughly 200-300 gsm) with a smaller AOS rating and good general puncture resistance is the typical match.

Scenario 2 — coarse gravelly soil, tidal shoreline, medium riprap

The coarse soil allows a somewhat larger AOS, but tidal rapid-drawdown conditions call for high permittivity specifically, to avoid trapped water pressure pushing the fabric loose during a fast tide change.

Scenario 3 — sharp angular heavy riprap, high drop-height placement

Puncture resistance is the dominant concern here. A heavyweight nonwoven fabric (300+ gsm, high CBR puncture rating) is worth specifying, along with careful, low-drop placement rather than dumping the stone directly.

Mistakes with filter fabric under riprap

• Skipping fabric entirely to save costLeaves the site vulnerable to piping — soil erosion underneath the riprap that no amount of correct stone sizing above it can prevent.
• Choosing woven fabric for a standard riprap applicationNonwoven’s superior filtration and puncture performance make it the right default for the vast majority of projects.
• Ignoring AOS and picking any available fabricWrong-sized openings either let soil pipe through or clog and block drainage entirely.
• Dumping heavy stone directly onto unprotected fabricRisks puncturing it right where the stone lands, especially with sharp, angular riprap.
• Skipping proper overlap at fabric seamsCreates a gap where soil can migrate through even with correctly-selected fabric everywhere else.
• Using a lightweight fabric under large, angular heavy riprapPuncture resistance needs to scale with the actual stone size and shape being placed.

Quick reference: fabric selection by condition

What most affects the right fabric choice

• Soil particle sizeDirectly sets the required AOS.
• Water flow conditionsTidal and rapid-drawdown sites need higher permittivity than steady, moderate flow.
• Stone size and angularityBigger, sharper riprap demands a heavier, more puncture-resistant fabric.
• Governing specificationA required AASHTO M288 class or agency spec narrows the acceptable fabric properties directly.

Select your filter fabric in five steps

1. Identify your soil type to set the required AOS.
2. Assess water conditions — tidal or rapid-drawdown sites need higher permittivity.
3. Match puncture resistance to your actual stone size and shape.
4. Default to nonwoven unless a specific reason calls for woven.
5. Install carefully — proper overlaps, and a sand cushion where puncture risk is high.

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