Permeable pavers are a type of precast paving block designed to reduce water runoff by allowing water to drain through them. Typical paving surfaces such as concrete, asphalt, or stone do not allow water to seep into the ground. Instead, such solid surfaces redirect water that is contaminated with chemicals like motor oil or herbicides into nearby streams and rivers. The porous surface of permeable pavers helps water pass through natural filters like the soil. Permeable pavers are not functionally different from other pavers and can be found in the same durable, interlocking forms.
Like other paving bricks, permeable pavers can be made from a number of different materials. Traditional paving materials, such as concrete and clay, are available in special mixes that allow the mater to let water seep through it. Solid stone bricks can be made into a semi-permeable pavement if they are combined with wider spaces between bricks. These spaces can be filled with grass, ornamental ground cover plants, or gravel. Other types of pavers may be made from synthetic materials, which can maximize the amount of water seeping into the soil but may not be as attractive as others.
Porous pavement designs have some variations. A narrowly spaced pattern of pavers made of non-permeable concrete or other materials offers the least natural drainage into the soil. More permeable paving can be achieved by increasing the space between non-porous pavers. If solid bricks are used, the space between them should be at least a quarter of an inch wide (about half a cm) or preferably wider. Narrower spaces have less benefit in reducing runoff. Some types of permeable pavers can increase drainage into the soil if a layer of sand is laid beneath them.
The most effective method of reducing runoff combines pavers made with porous materials laid in a pattern with space between the individual bricks. This design offers the best method of reducing harmful runoff. It will also remain more effective in periods of heavy or prolonged rainfall.
The most common approach to storm water management has been to let storm sewers collect water and transport it, untreated, into local bodies of water. Storm water can collect atmospheric pollution, automobile chemicals from roadways and parking lots, as well as toxins from other sources. Permeable pavers are designed to allow the homeowner some influence over how rainwater is treated. Porous surfaces make use of the filtering capability of soil microorganisms, which help to purify runoff by breaking down some chemicals.