Rubber mulch is a popular, durable landscaping alternative that repurposes millions of old vehicle tires. Homeowners often choose this material for its low maintenance and aesthetic appeal, but a frequent concern arises during heavy rainfall: does it float away? The answer is generally yes, and understanding the physical principles behind this buoyancy is key to preventing washout in your yard.
The Science of Buoyancy
Whether rubber mulch floats is fundamentally a question of density, which measures mass per unit volume. Any object floats if its density is less than the density of the fluid it is placed in, a concept known as buoyancy. Since water has a specific gravity of 1, any material with a specific gravity less than 1 will float.
While the vulcanized rubber material itself can have a specific gravity ranging from 0.9 up to 1.5, the mulch product behaves differently. The individual pieces are not solid blocks; instead, rubber mulch is a collection of shredded, irregularly shaped particles with significant void space between them.
The collection of particles and the air trapped within the voids create a much lower bulk density for the overall mulch layer. When heavy rain saturates the area, water flows beneath and around the particles, encountering this low bulk density. The trapped air and the light weight of the loose layer relative to the water volume are often enough to lift and displace the mulch, causing it to float and wash away.
Material Structure and Composition
Rubber mulch is sourced from discarded vehicle tires, which undergo a rigorous shredding and grinding process. This manufacturing method yields small, irregular fragments, nuggets, or fine crumbs.
The sharp, uneven edges and variable particle sizes introduce air pockets. Even when spread, these pieces cannot perfectly interlock to eliminate all the space between them. This structural arrangement, characterized by significant void space, drastically lowers the product’s bulk density compared to a single, solid piece of rubber.
Low bulk density allows water to easily penetrate the layer and exert buoyant force. While the material is designed to be water-permeable, allowing rain to pass through to the soil below, a sudden downpour creates powerful runoff that can lift the entire lightweight mass and carry it along the flow path.
Strategies for Preventing Washout
Homeowners can employ several techniques to keep their rubber mulch in place, even during intense weather events. The first step is to install a physical barrier, or edging, around the mulched area. This border, which can be made of plastic, metal, stone, or wood, must extend several inches above the mulch line to act as a retaining wall against flowing water.
Applying the mulch at a sufficient depth is also important for stabilization. A layer between two and four inches deep allows the individual particles to settle and interlock, creating a heavier, more cohesive mat. This increased weight and friction make the entire layer more resistant to displacement by runoff.
Addressing the water flow itself is another solution to minimize washout. If the mulched area is located near a roof downspout or on a natural slope, the water flow must be redirected. Installing features like swales, which are shallow drainage channels, or French drains can divert the bulk of the runoff away from the mulch beds.
Finally, consider using specialized products designed to bind the mulch particles. Landscape adhesives, often called mulch lock sprays, can be applied to the top layer to create a light crust that holds the shreds together. This adhesive layer maintains the porous nature of the mulch while increasing its resistance to both wind and water-induced displacement.