Chapter 15 Hardscapes 407 Copyright Goodheart-Willcox Co., Inc. of aggregate, such as pea gravel, over a porous sub- drainage system, Figure 15-12. (An aggregate is a collection of units, such as gravel, used together.) This course aggregate can be adhered to itself with a liquid adhesive-like substance that keeps the aggre- gate in place but also allows water to permeate into groundwater supplies. When adjacent to a structure, an impermeable hardscape must have at least a 1% slope away from the structure. This will direct water away from the structure and prevent water from fl owing under the structure and hardscape. If water remains under the hardscape, it will expand and possibly shift or crack the hardscape as it freezes and thaws. The proper slope is achieved by determining the vertical change across a defi ned horizontal distance. The units must be calculated using the same units of measurement. A simple way to ensure that slope is a 1″ drop over 8′ can be achieved by adding a level to the top of an 8′ board or string with level and measuring the distance from the ground to the board or string. Hardscape Materials A wide variety of materials, such as stone or concrete, may be used to cre- ate hardscapes. Hardscape materials must meet the client’s expectations, have reasonable longevity, and accentuate the architectural features of the existing structure. Many designers choose materials based on what is available locally to simplify transport and reduce shipping costs. Unit pavers, stone, wood or wood composites, concrete, local or upcycled materials, asphalt, and gravel are some of the most common hardscape materials. See Figure 15-13. Precipitation Filter fabric lines the subsurface bed Non-compacted subgrade is critical for proper infiltration Stones allow water to flow into the recharge bed Uniformly graded stone aggregate with 35% to 40% void space for stormwater storage and recharge Porous asphalt pavement Goodheart-Willcox Publisher Figure 15-12. Hardscape materials can provide solid walking and driving surfaces and still allow water permeability. Little Perfect Stock/Shutterstock.com Figure 15-13. A designer may use natural materials, such as wood and stone, with manufactured products, such as brick and concrete pavers, to create unique landscapes.