Retaining wall block vs stone: choosing a material for walls 4 feet and under

Why 4 feet is the working limit

Every wall we build is 4 feet tall or less. At that height a well-built gravity wall holds the soil behind it with its own weight and setback, without buried reinforcement grid or an engineered design. The Concrete Masonry and Hardscapes Association notes that unreinforced segmental walls can typically be built up to 3 to 4 feet high, and less on poor soil or where a driveway or slope sits above the wall.

Taller walls usually need a licensed engineer's design and a building permit, and they are a different kind of project. We do not build engineered walls. When a yard needs more than 4 feet of grade change, we split it into two or more walls, each 4 feet or under. The permit and engineering side is covered in our retaining wall permits and engineering guide.

Height is measured from the top of the base, including the buried course, to the top of the cap. A wall that shows 42 inches above grade with 6 inches buried is a 4 foot wall.

Segmental concrete block

Segmental retaining wall block is a dry-stacked concrete unit with a lip or pin that sets each course back slightly from the one below. That built-in setback, usually a few degrees, leans the wall into the hill so the weight of the wall works against the soil. The units are made to consistent dimensions, so the wall goes up level and the courses stay straight.

Block is the most common choice for walls under 4 feet because it is predictable. It comes in many colors and textures, it matches paver patios and steps from the same manufacturers, and it curves easily. The concrete is dense and rated for freeze-thaw use, and caps are glued on with concrete adhesive.

The downside is that block looks manufactured up close, and cheaper units can fade or chip at the corners over time. Buying a better unit from a major manufacturer costs more in material but keeps its color and edges longer.

Natural stone and boulders

A dry laid stone wall is built from fieldstone or quarried stone stacked without mortar. Each stone is set so it bears on the stones below and slopes back into the hill. Dry laid walls drain through their joints and flex slightly with frost instead of cracking. A good dry laid wall takes more labor than block, because every stone is selected and fitted.

Boulder walls use large stones, often 2 to 4 feet across, set with a machine. A single row of boulders can hold a short slope, and two rows can build a wall close to 4 feet. Boulder walls look natural along a wooded edge or a driveway cut, and they go up quickly once the stone is on site. They need room: the base of a boulder wall is wider than a block wall, and the stones need equipment access to set.

Natural stone costs more per face foot than block in most cases, and the wall face is irregular, which makes it harder to run a straight cap or tie into steps. It is the better choice where the wall is meant to blend into the landscape rather than match a patio.

Timber walls and why they rot

Landscape timber and railroad tie walls were common on older properties, and many of them are now failing. Wood in constant contact with wet soil rots, even when it is pressure treated. The back face of a timber wall sits against damp soil all year, and the bottom course sits in water in clay yards. The treatment slows the rot but does not stop it.

A timber wall usually fails from the back and the bottom, where it cannot be seen. The first sign is the wall leaning forward or the top course pulling away from the tiebacks. By the time the face looks soft, the wood behind it is gone. Old railroad ties also carry creosote, which is why we haul them out rather than reusing them.

We do not recommend timber for a new retaining wall. Replacing a failing timber wall with block or stone usually includes regrading and adding the drainage the original wall never had.

Base, drainage stone, and pipe

Every wall material needs the same three things underneath and behind it. The base is a trench dug below the first course and filled with compacted crushed stone, at least 6 inches thick and wider than the wall. Techo-Bloc's installation guide calls for a 6 inch base and burying the first course about 6 inches, so the bottom of the wall cannot kick out.

Behind the wall goes a column of clean, angular drainage stone, at least 12 inches wide for the full height of the wall. The stone lets water in the soil drop straight down instead of building up pressure against the back of the wall. Saturated soil pushes much harder than dry soil, and water pressure is the main reason walls bulge or tip.

At the bottom of the drainage stone we lay a 4 inch perforated drain pipe, pitched at about 1 percent to an outlet where it can daylight. Filter fabric separates the drainage stone from the native clay so fines do not wash in and clog it. On clay lots this drain is not optional, because clay does not let water drain away on its own. Finding the outlet is part of our drainage and excavation work.

Backfill behind the drainage stone is placed and compacted in lifts as the wall goes up, not dumped in after it is finished. Loose backfill settles, and a settled slope behind the wall sends surface water straight down the back of the wall.

Terracing larger grades into two walls

When a yard needs more than 4 feet of grade change, we build two or more shorter walls with a flat terrace between them. Each wall stays 4 feet or under, and each one has its own base, drainage stone, and pipe. The terrace between them can be lawn, a planting bed, or a path.

The spacing matters. Industry guidance, including the Concrete Masonry and Hardscapes Association, says the distance between terraced walls should be at least twice the height of the lower wall for each wall to act on its own. A 3 foot lower wall needs at least 6 feet of flat terrace before the upper wall. If the walls are closer than that, the upper wall loads the lower one and the pair behaves like one taller wall.

That spacing takes yard depth, so terracing does not fit every lot. Where there is not enough room for two properly spaced walls, the options are to regrade part of the slope, accept a planted slope for part of the change, or reduce how much flat yard is created. We lay out the terraces on site before any digging so the owner can see how much flat space each option gives.

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