A splash block is a piece of concrete or plastic of a roughly rectangular shape that is used to absorb the force of water being diverted by a downspout from the roof of a house during rainfall, and to divert that water away from the structure's foundation. Splash blocks are fabricated in a wide range of sizes, but for residential purposes, most fall with a range of 12 to 18 inches (30 to 46 cm) long by 6 to 9 inches (15 to 23 cm) wide. Simple splash blocks are relatively inexpensive, although more elaborate and complex splash blocks are available for many times the price of the basic unit.
Most structures are built with systems to handle rainwater. Structures with flat roofs have drains or other ways for rainwater to escape the confines of the roof, because of the many hazards posed by even a few inches of standing water on a flat roof. Houses generally have sloped roofs, eliminating the problem of standing water accumulating, and the ground around the house gently slopes away from the house, facilitating the runoff of water away from the house. Problems arise, though, if the water is allowed simply to fall off the roof onto the ground underneath.
Water dripping off a roof to the ground over a period of time will gradually erode the soil it hits, ultimately resulting in the soil being washed away. This will happen even if grass or shrubbery is planted. Once this process starts and the soil itself begins to erode and wash out, the foundation of the house is jeopardized because the protective slope has been washed away, allowing large amounts of water to stand and be absorbed into the ground immediately adjacent to the foundation.
To avoid these problems, most houses are built with systems for diverting rainwater from the house. Gutters line the edges of the roof, diverting water to downspouts, which direct the water to the ground. However, during a heavy rainfall, a downspout will discharge large quantities of water at a significant velocity, multiplying the effect of individual raindrops falling off the roof. If the ground where the water hits is not protected, erosion and washout could happen during the course of a single rainfall.
Positioning a splash block under each downspout accomplishes two objectives. First, the splash block will protect the ground underneath the downspout from the force of the falling water, preventing erosion and washout. Second, when positioned correctly, the splash block will divert water away from the structure. To facilitate this, splash blocks generally are built with one end higher than the other, and “lips” on the high end and the two sides. When water hits the splash block, it will run down the slope and over the lipless edge. When a splash block is properly placed, the lipless edge is placed away from the house in the direction the water's intended to flow.
There are other, sometimes very elaborate, systems available for the diffusion of rainwater and its deflection away from a structure. However, these alternatives to traditional downspout and splash block systems usually require a great deal more routine maintenance without accomplishing anything more than the traditional system, which requires minimal maintenance and attention once properly positioned.
On the other hand, the maintenance involved in systems designed to capture rainwater for later use in irrigation or other applications is often justified by the economic and environmental benefits derived. Among these systems are things like rain barrels and cisterns, with which homeowners can capture hundreds or even thousands of gallons of rainwater.