Decomposed granite (DG) is a popular, versatile landscaping material derived from granite rock that has naturally weathered into fine, gravel-like material. It is used extensively for creating natural-looking pathways, patios, and garden beds. DG’s drainage performance depends heavily on three main factors: the material’s original composition, whether it has been chemically stabilized, and the techniques used during installation. Understanding these distinctions is necessary to achieve the desired drainage outcome for any outdoor project.
Inherent Drainage Properties of Raw Decomposed Granite
The natural drainage capability of decomposed granite is tied directly to its physical characteristics before any modification or installation process. DG consists of a range of particle sizes, from fine, dust-like “fines” to small, angular pieces of gravel, typically up to 3/8 of an inch. This variation in size and the irregular, angular shape of the particles create numerous void spaces within the material.
In its raw, unstabilized state, DG generally possesses excellent permeability, allowing water to filter through the layer rather than running off the surface. This structure allows for efficient water infiltration, making it a superior choice compared to dense clay or concrete. However, if the material contains too many fines, it can compact excessively, naturally reducing the flow of water. Raw DG is often recommended for areas that require quick runoff, such as low-traffic garden paths.
Stabilized Versus Non-Stabilized Material
The distinction between stabilized and non-stabilized decomposed granite significantly affects its drainage performance. Non-stabilized DG is the loose, raw material containing no binders or additives. This type offers the highest permeability and water infiltration, but it is prone to erosion, easily washes away during heavy rain, and can create dust.
Stabilized DG is blended with a binding agent, often a polymer or resin, to improve durability and stability. This modification creates a firm, low-maintenance surface that resists erosion and is suitable for high-traffic areas like driveways and patios. While some modern stabilizers maintain a degree of permeability, the binding process significantly reduces the material’s ability to drain water vertically compared to its raw form. Materials with resin binders, in particular, form a highly durable, almost impermeable surface, meaning stabilized DG is often unsuitable if the primary goal is rapid, vertical drainage.
Installation Techniques That Maximize Drainage
Even with permeable raw material, drainage performance relies heavily on correct installation practices. Proper preparation of the area beneath the DG layer is essential to ensure water has a place to go once it permeates the surface.
Sub-Base Preparation
Preparation includes excavating the area and installing a permeable sub-base, such as a two to three-inch layer of crushed stone or gravel. The larger aggregate in the sub-base creates a void network. This network allows water to move horizontally and vertically away from the finished surface, preventing saturation of the DG layer.
Compaction Control
Compaction must be applied carefully to balance stability and permeability. While some compaction is necessary to create a firm surface, over-compaction seals the material, especially the fine particles, which severely impedes water flow. The DG should be spread in thin layers, typically 1.5 to 2 inches at a time, and lightly compacted, rather than aggressively compressed.
Surface Grading
The finished surface should be graded with a slight slope, ideally at least 2%, to encourage surface water to run off the edges. This grading is particularly important with stabilized DG, which has reduced permeability. Grading directs water away from the area, preventing pooling and erosion.