Decomposed granite (DG) is an aggregate material resulting from natural geological processes that break down solid granite rock into a granular, soil-like substance. DG offers an attractive, natural aesthetic combined with beneficial properties, making it an alternative to traditional paving materials like concrete or gravel.
Composition and Characteristics
Decomposed granite is a naturally occurring material, formed in situ through the prolonged weathering and erosion of granitic bedrock. This slow process involves both chemical alteration, where minerals like feldspar break down into soft clay, and physical disintegration from forces like freeze-thaw cycles and root wedging. The resulting material is a mix of sand-sized particles derived from the granite’s original minerals, primarily quartz, feldspar, and mica.
The particle size of DG ranges from fine dust to small gravel, typically not exceeding three-eighths of an inch. This grading allows the material to compact effectively, creating a relatively firm surface. DG has excellent permeability, allowing water to drain through easily, which helps manage runoff. Its coloring is dictated by the parent rock, resulting in natural, earthy tones such as gold, buff, gray, and reddish-brown.
Common Uses in Outdoor Design
Decomposed granite is favored in outdoor design for its rustic appearance and ability to blend into natural environments. It is frequently employed to create permeable pathways and walking trails, offering a softer, quieter surface than crushed rock. Its natural look and cost-effectiveness make it a choice for covering large areas, such as a durable, drought-tolerant substitute for turf grass in xeriscaping projects.
The material is also used extensively as a base layer for patios and is sometimes applied to create natural-looking driveways, particularly in areas with lighter vehicular traffic. When spread around plants, DG functions effectively as a groundcover, suppressing weeds and retaining soil moisture much like an organic mulch, but with far greater longevity. Its stability, once compacted, also makes it a viable material for fire pit areas, as it is non-combustible and helps define the space with a soft border.
Stabilized vs. Unstabilized DG
Decomposed granite is generally available in two major forms: unstabilized (or natural) and stabilized. Unstabilized DG is the loose, raw material with no binders, offering maximum permeability and the lowest initial cost. However, this form is prone to erosion from rain and can easily track indoors on shoes and pet paws, leading to material loss and a dusty surface.
Stabilized DG incorporates a binding agent, often a polymer or natural compound like psyllium, which is mixed directly with the granite fines. This modification significantly improves the material’s durability and resistance to erosion and shifting. Once wetted and compacted, the stabilizer creates a firmer, more resilient surface that reduces dust and material washout, making it suitable for high-traffic areas, sloped landscapes, and surfaces requiring ADA compliance.
Installation and Upkeep Guidelines
Proper installation of decomposed granite begins with site preparation, including clearing the area and grading the sub-base for proper drainage. For most applications, a sub-base of crushed stone or gravel is laid and compacted to a depth of several inches to provide stability and drainage, especially in high-traffic zones. Edging, typically made from metal or wood, must be installed at the perimeter to contain the DG and prevent lateral shifting.
The DG is then spread in thin layers, generally no more than two inches thick at a time, before being moistened and thoroughly compacted. This layering and compaction process is repeated until the total desired depth, usually three to four inches for a pathway, is achieved. Maintenance is relatively simple, involving occasional raking to level minor depressions and adding a fresh layer of material every few years to refresh the surface.