418 Landscape Design, Installation, and Management Copyright Goodheart-Willcox Co., Inc. As each layer is placed on top of another, smaller particles will fi ll in spaces between the larger particles. Compaction further forces the particles together to make a hard packed surface. The compacted aggregate is covered with a smaller aggregate, such as manufactured sand, to a specifi c depth (2″ to 4″). It may be necessary to install a deeper aggregate base in some areas of the coun- try and with other hardscape materials. Check local codes and manufacturer’s product recommendations. A screed board is used to screed (level) the material to a uniform surface, Figure 15-27. The top material, such as concrete pavers, is then laid and compacted with a hand or powered compactor. The designer should factor in the possibility of tree roots or excessive traffi c because they may alter the performance over a long period. R. Lee Ivy Figure 15-27. Screeding creates an even surface for laying the paving materials. Poles and a straight board are commonly used. Permeable joint material Geotextile Subgrade soil Curb/edge restraint Paver units Bedding course (permeable aggregate) Base course (permeable aggregate) ESB Professional/Shutterstock.com Figure 15-26. A properly installed subsurface is necessary for quality of installation and longevity of the hardscape, free from buckling, sagging, or shifting.