Decomposed granite (DG) is a fine, granular material resulting from the natural weathering and erosion of solid granite rock. Resembling coarse sand or fine gravel, DG is widely used in landscaping for pathways, patios, and driveways. Understanding how DG handles water is a primary concern for managing stormwater runoff and drainage.
The Base Answer: Understanding Natural Drainage
Loose, untreated decomposed granite (DG) is inherently permeable, allowing water to drain through it effectively. This characteristic stems from its particle size and composition, which includes fine rock dust (“fines”) and slightly larger particles. The angular shape of the granite grains creates ample interstitial space, or tiny voids, when the material is laid down.
Water filters through these interconnected voids, allowing infiltration into the subsoil rather than surface runoff. This property makes natural DG desirable for reducing runoff and replenishing groundwater. While the drainage rate is slower than large gravel, it is steady and consistent, preventing immediate pooling. Untreated DG is often favored in arid climates where water conservation and slow absorption are priorities.
The Permeability Spectrum: Stabilized Versus Unstabilized
The degree of permeability depends heavily on whether the DG is left natural or modified with binders. Unstabilized DG offers the highest permeability but is vulnerable to erosion and displacement. Heavy rain or foot traffic can easily wash away the loose fines, leading to ruts and muddy conditions.
The most common modification is mixing DG with a stabilizing agent, creating stabilized decomposed granite. These stabilizers are typically organic polymers (like psyllium-based binders) or synthetic additives that coat the particles to lock them together. This binding process significantly increases the surface’s durability and resistance to erosion while maintaining some permeability.
Stabilized DG is often necessary for sloped areas or surfaces requiring compliance with the Americans with Disabilities Act (ADA), as it creates a firm, predictable surface. Its drainage rate is slower than unstabilized DG because the polymer matrix partially fills the natural voids. A less permeable option is DG mixed with a wax-polymer or resin coating, which essentially seals the surface. This type is designed for maximum durability in high-traffic commercial applications and is considered functionally non-permeable, diverting all water runoff.
How Installation and Compaction Impact Flow
The installation process determines the final drainage performance, even when using highly permeable DG grades. A proper foundation is necessary, often including a layer of coarse, permeable material like road base or gravel beneath the DG layer. This sub-base acts as a drainage reservoir, allowing water that passes through the DG to move away quickly and prevent saturation of the native soil.
Compaction is necessary to create a firm, stable surface, but the degree of compaction directly impacts flow. DG should be compacted in thin layers, or “lifts,” typically 1 to 2 inches at a time, using a plate compactor. Over-compaction, especially with stabilized mixes, can crush the fines together too tightly, eliminating air voids and creating a near-impermeable layer that causes pooling.
Correct grading is also important; the finished surface should be sloped slightly (a minimal 2% grade) to encourage surface water to move away from adjacent structures. Without this slope, even a permeable surface can struggle to shed excess water during heavy rain. The goal is to achieve maximum stability with sufficient porosity, ensuring a durable surface that manages water effectively.