Landscape Boulders

Boulders are graded by size class rather than by a precise engineering spec, which is the key difference from riprap. Small boulders run 12 to 18 inches and 50 to 300 pounds, medium boulders run 18 to 36 inches and 300 to 1,500 pounds, and large boulders run 36 to 60-plus inches and 1,500 to 4,000-plus pounds. Anything larger than roughly 5 to 6 feet or 4,000 pounds is usually quoted as an individual specimen rather than sold as bulk stock. Rock type affects both weight and durability. Granite and quartzite are hardest, at Mohs 6 to 7, and hold up best against decades of freeze-thaw cycling. Limestone and sandstone are softer and lighter, with more surface erosion or flaking over time. Solid rock weighs 150 to 175 pounds per cubic foot, so a quick mental check, an 18 inch stone lands around 150 to 300 pounds, a 3 foot stone around 1,000 to 1,800 pounds, and a 4 to 5 foot stone can run 2,500 to 5,000-plus pounds.

The single biggest placement mistake is setting a boulder fully on top of the ground. Burying roughly a quarter to a third of the stone anchors it against rolling and mimics how boulders actually sit in nature, having settled into soil over time. For walls or any load-bearing arrangement, excavate a shallow seat and compact a gravel base a few inches deep before setting stones, and batter each course slightly back into the slope. Purely decorative single boulders on stable native soil usually skip the gravel base, but still need a dug seat and backfilled soil around the buried portion. Anything over roughly 200 to 300 pounds needs mechanical help, a skid steer, mini excavator with forks or a thumb attachment, or in some cases a small crane. Hand placement is realistic only up to about one or two person lift weight. Plan for equipment before delivery day, not after.

Compared with riprap. Riprap is graded, angular quarry rock sized to an engineering spec for dissipating water flow energy on slopes and channels, typically 6 inches to 2 feet and sold by the ton. Landscape boulders are chosen for size, shape, and appearance rather than flow calculations, and are often more rounded field or river stone used for accents rather than mass armor.

Compared with retaining wall block. Manufactured concrete block is uniform, interlocking, and rated for specific wall heights with predictable structural performance. Boulder walls rely on the mass and friction of irregular stones, generally topping out safely at lower heights without engineering, but they give a natural look that manufactured block cannot replicate.

How many boulders do I need. Because boulders are placed individually rather than spread, sizing is about counting stones, not covering square footage. As a rough placement estimate, one ton covers 55 to 80 square feet laid one boulder deep, with smaller stones covering more area per ton and a single large specimen sometimes being the entire ton. For a wall, estimate the linear footage and height, then work backward from average per-stone weight in your chosen size class to figure tonnage. For an accent, you are simply choosing individual pieces by size and shape.

How boulders are sold and delivered. Most suppliers sell loose or mixed boulder stock by the ton, since weight is far more predictable than volume for irregular shapes, unlike mulch or gravel which sell by the cubic yard for depth coverage. Large individual specimens are often quoted and priced per piece rather than by weight. Delivery is bulk by truck, and machine placement, using a skid steer or excavator, is commonly offered as an add-on for anything too heavy to hand-place.