Last Updated on October 20, 2026 by Umar Farooq
Paver base washout is what happens when moving water carries the bedding sand, and sometimes the fines from the stone base, out from under the pavers. The pavers above lose their support, drop into the void, and the patio develops a dip. Water gets in through open joints, around drains and steps, or from a downspout aimed at the edge. The fix is to find and stop that water first, then lift the sunken area, rebuild the base and relay the same pavers.
Most "my pavers are sinking" posts are really "my pavers lost their sand" posts, and the difference matters. For the repair itself, see how to fix sunken pavers. This article is about why the sand left.
What a washed-out base looks like from above
Washout rarely announces itself. Rochester Concrete Products' sunken paver guide lists the warning signs that point to water:
- puddles that form after rain
- fine soil or sediment collecting in low areas
- joints that repeatedly lose sand
- pavement edges that spread
- damp foundation walls or pooling near structures
It also lists what an inspection tends to find underneath: soft or saturated soil, voids or erosion, insufficient or poorly compacted base, and repeated water infiltration. Sandy soils are especially vulnerable, the guide says, when concentrated water flows through them.
The joint-sand clue is the easiest to read. Re-filling joints that keep emptying is treating the symptom, and I'd stop buying bags of sand until you know where the last bag went. (For that specific problem, see polymeric sand washed out.)
Paver base washout: four escape routes for the sand
Sand needs water to carry it and somewhere to go. The sources point to four exits.
- Through the joints. Once the joint sand has gone, rain pours into the bedding layer instead of running off.
- Down into the base. ICPI Tech Spec 2 says the finished surface of the compacted base should not allow bedding sand to migrate into it. If it does, the spec's fixes are a choke course of fine material compacted into the surface, or a bitumen tack coat.
- Past the edge. A restraint that is loose, short or missing gives sand a way out sideways. Rochester lists failed or missing edge restraints among the causes of settlement.
- Beside drains, curbs and utilities. ICPI says to inspect the base surface and its perimeter for places where sand could migrate, including joints in curbs and the area around utility structures and catch basins, and to cover those spots with geotextile fabric to prevent loss of the bedding sand.
Agrio's explainer on pavers sinking behind steps shows route one in action: the riser acts as a dam, rain and snowmelt pool against it, the bedding sand saturates, and the water gradually washes the fine particles away, leaving voids the bricks sink into. It is one site's explanation, not a test, but it matches ICPI's mechanism.
Is it washout, or something else?
Not every dip is washout. A rough guide, built from Rochester's cause list:
| Clue | Likely cause |
|---|---|
| Dip is near a downspout, a low edge or a step, and joints are empty | Water washout |
| Dip is broad, with sand still in the joints | Thin base or poor compaction |
| Surface rises in spring and drops later | Frost heave or expansive soil |
| Ridge or lift near a tree | Roots displacing the base |
| Pavers spreading apart at one edge | Failed edge restraint |
Washout is local and tied to a water source; a patio that sags evenly has a different problem.
Find the water before you touch a paver
Here is the one opinion I'll defend: washout repairs are mostly plumbing. If you lift pavers, rebuild the base and leave the downspout pointing at the edge, you have built a very neat place for the next hole.
The cheap checks come first:
- Run the hose. Spray the patio for ten minutes on a dry day and watch where the water travels, and where it disappears into joints.
- Trace the sources. Rochester's common water sources are downspouts discharging near the pavement edge, poor grading toward the paving, irrigation that repeatedly saturates the same area, water entering through open or deteriorated joints, and runoff collecting against walls, steps or foundations.
- Lift one paver at the dip. If the sand under it is soaked, or there is a void, you are looking at washout. Pavers are individual units, so putting it back is easy.
If a buried downspout pipe crosses the area, treat a leaking joint as a suspect. See downspout drain under patio for how that line should be built.
Building washout out of a new patio
- Close the base surface. Per ICPI, bedding sand shouldn't be able to work into the stone. Where it can, use a choke course or geotextile at the risky spots.
- Fabric where sand could escape. Cover joints in curbs and the base around utility covers and drains with geotextile, as ICPI describes. Never put fabric under the whole bedding layer: see landscape fabric for paver patio.
- Keep the base slope. ICPI sets a minimum surface slope of 1.5%, so water leaves the surface instead of soaking in.
- Protect the open edge from rain. ICPI says the remaining uncompacted edge can be covered with a waterproof covering if there is a threat of rain, which prevents saturation of the bedding sand and saves removing and replacing sand and pavers.
- Restrain every edge and send roof water elsewhere. See patio drainage for options.
Repairing a washed-out patch
The steps follow the standard repair, with washout-specific points:
- Lift the dip plus a border around it. Hunker's repair guide pulls up the sunken area and a 16-inch border.
- Dig out the loose or wet material. Agrio notes that filling the hole with sand alone won't hold, since sand moves when water hits it.
- Rebuild with crushed stone in compacted lifts. Agrio uses lifts of about 2 inches. Rochester adds that bedding sand should not be used to correct depressions in the base, which is its citation of CMHA guidance.
- Lay non-woven geotextile where soil is washing into the stone, per Agrio.
- Re-screed 1 inch of bedding sand, relay the pavers and refill the joints.
If erosion, voids or unstable soil are widespread, Rochester suggests a professional assessment.
Plug the leak, then rebuild
A washed-out base is a water problem that shows up in the pavers. Find the source with a hose, stop the sand leaving by every route, and only then relay the patch on a compacted base. Do it in reverse and you've just given the water fresh sand to carry off.
FAQ
What causes pavers to wash out?
Moving water carrying away the bedding sand and fines from under the pavers. Common sources are downspouts discharging near the edge, grading that slopes toward the paving, irrigation that saturates one area, and open or deteriorated joints, per Rochester Concrete Products.
How do I know if my paver base has washed out?
Look for localized dips, joints that keep losing sand, puddles after rain and fine sediment in low spots. Lifting one paver in the dip shows soaked sand or a void if washout is the cause.
Can I fix washed-out pavers by adding sand?
No. Sand alone moves when water hits it, and CMHA guidance cited by Rochester says bedding sand should not be used to correct depressions in the base. Rebuild the base with compacted stone, then re-screed 1 inch of sand.
Does polymeric sand stop washout?
It can slow the loss of joint sand, but it doesn't fix water that is entering from a downspout, an open edge or a gap beside a drain. Stop the water source first.
Can geotextile fabric stop base erosion?
It helps in specific places. ICPI says to cover openings where bedding sand could migrate, such as curb joints and the area around catch basins and utility structures, with geotextile fabric.
