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For homeowners

Types of retaining wall: which one fits your slope?

Choose Modular block wall (interlocking concrete blocks) for low walls under four feet in total height, buried blocks included, that hold back no surcharge, with small store-bought blocks (Snohomish County, WA); Rockery (stacked boulders) for cut slopes, where it needs only a narrow base, on the order of 30% of the rockery height (WSDOT, 2024); Gabion wall (rock-filled baskets) for short walls (Snohomish County, WA); Poured concrete wall (cast in place) for tall, vertical faces, when engineered to support them (Snohomish County, WA).

The right choice depends on how tall the wall must be, counting the buried part, whether the wall holds back a surcharge such as fill, whether the slope is stable without the wall, and how water will drain from behind the wall.

How we know: By InstaBids editorial team with AI research agents. · Sources read: 22 October 2026 · Updated 27 October 2026

Snohomish County, WA: retaining walls may be built with modular blocks, angular boulders (a rockery), gabion baskets or poured concrete. Rows marked WSDOT come from the Washington State DOT highway design manual.
TypeBest forMain advantagesMain drawbacks
Modular block wall (interlocking concrete blocks)low walls under four feet in total height, buried blocks included, that hold back no surcharge, with small store-bought blocks (Snohomish County, WA)Snohomish County, WA: each layer of interlocking blocks slopes back ever so slightly, which increases the wall's stabilitySnohomish County, WA: small modular blocks typically sold at large home-improvement stores have limited applications; Snohomish County, WA: taller block walls will require additional design features for stability
Rockery (stacked boulders)cut slopes, where it needs only a narrow base, on the order of 30% of the rockery height (WSDOT, 2024)Snohomish County, WA: water can freely pass through a rockery, so a drain pipe is not neededSnohomish County, WA: "Most rockeries are not true retaining walls"; they tend to be a wall of boulders that cover freestanding soils; WSDOT (2024): rockeries cannot tolerate much settlement, as rocks can shift and fall out; WSDOT (2024): the slope needs to be at least marginally stable without the rockery
Gabion wall (rock-filled baskets)short walls (Snohomish County, WA)Snohomish County, WA: gabion walls have the look of rockeries with the ease of installation of modular block wallsSnohomish County, WA: the fill rock must be larger than the cage openings and less than 18 inches in diameter; WSDOT (2024): it is typically safe to build gabions two baskets high (6 feet) but only one basket deep, where soil conditions are reasonably good
Poured concrete wall (cast in place)tall, vertical faces, when engineered to support them (Snohomish County, WA)Snohomish County, WA: "Most soil retaining structures lean slightly into the hillslope, although some concrete walls can be vertical."Snohomish County, WA: poured concrete walls must be reinforced by steel and need special design for their foundations and drainage; CSU Extension: poured-in-place concrete ("structural") walls require weep holes so water can drain from behind the wall; WSDOT (2024): cantilever and rigid gravity walls, such as reinforced concrete, generally need total settlement held to approximately 1 inch or less

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