Chapter 10 discussed the various aspects of form composition, the first step of the preliminary design phase. When completed, form composition provides the basic structure and visible skeleton on which the remainder of the design will be built.
An essential point to realize about form composition is that it is only the beginning step of preliminary design and by itself does not create a complete residential site design. Form composition is essentially only a two-dimensional study of the design and does not fully consider the desired total spatial experience of the outdoor environment. The next step in preliminary design, spatial composition, proceeds beyond the two-dimensional form composition to establish the spatial shell or envelope of a residential site design. Spatial composition builds on the form composition’s framework by adding the third dimension, very much as the walls and ceilings of indoor rooms build on the underlying floor plan of a house. It considers how the overall space is formed and develops ideas for vertical and overhead planes of enclosure. As indicated before, spatial composition usually occurs simultaneously with form composition. However, the two steps are being separated here for the sake of explanation.
This chapter discusses the different aspects of spatial composition including preliminary grading, planting design, use of vertical planes such as fences and walls, and use of overhead structures in residential site design.
PRELIMINARY GRADING DESIGN
Spatial composition in residential site design should start with the ground plane. There are several reasons for this. First, the three-dimensional design of the ground can and should be done in close association with form composition. A second reason is that the ground plane is the foundation for every other design element, such as plant materials, pavement, walls, fences, and overhead structures. Thus, the elevation of the ground has a direct influence on the function and appearance of other elements. And finally, the ground plane is the surface on which we walk, run, sit, drive, and so on. It receives the most direct use and wear in the outdoor environment. Therefore, its three-dimensional composition is critical.
The term grading is commonly used to refer to the manipulation of the ground’s third dimension and is defined as shaping or molding the ground’s surface for both functional and aesthetic purposes. Grading involves physical movement of soil from
one area of the site to another. When earth is added to an area, it is called fill. When earth is removed or excavated from an area, it is called cut. Generally, there is an attempt to balance the quantity of cut and fill on a given project to eliminate the need for transporting earth to or from the site.
Two general purposes for grading on a residential site are necessity and aesthetics. For necessity, grading is undertaken to properly drain surface water and to accommodate circulation or other uses on the site. For aesthetics, grading is done to create space, screen or direct views, and provide visual interest. Grading for necessity is a utilitarian and engineering process, whereas grading for enhancement is an aesthetic and artistic endeavor. Both types of grading should be undertaken together so that all grading is both functional and appealing to the eye. Each specific purpose for grading is discussed more in the following sections.
One utilitarian purpose for grading is to provide proper drainage across the ground’s surface. There are a number of places on the residential site where special effort should be made to correctly drain surface water.
1. Surface water should be drained away from the house and other structures on the site to reduce problems. There are several general existing slope conditions the designer may encounter on residential sites (Figure 11—1). In the first condition, where the ground slopes naturally away from the house, the designer should maintain the slope so water continues to drain away from the house. This situation typically requires little or no regrading of the existing site. In the second situation, where the house is located on level ground, the surface must be regraded to slope gradually away from the house. It is usually recommended that the ground’s surface be sloped away from the house or other structures at a rate of 1 percent to 10 percent (Figure 11—2).
A slope of 1 percent is approximately equal to 1/8-inch vertical elevation change for every 1 horizontal foot across the surface (Figure 11—3). Another way to understand this is by applying the formula for percent of slope:
Rise divided by run = percent of slope
Rise is the vertical elevation change of the slope and run is the horizontal distance across the slope (Figure 11—4). Therefore, a slope of 1 percent rises or falls 1 foot for every 100 horizontal feet (1 divided by 100 = 0.01 or 1 percent). A slope of 10 percent rises or falls at a rate of10 feet for every 100 horizontal feet or 1 foot for every 10 horizontal feet.
The third situation for existing slope conditions is where the house is located on a sloped site (bottom of Figure 11—1). Here it will be necessary to create a swale or shallow valleylike landform on the uphill side to collect surface drainage and direct it around the house.
2. Water should be drained as quickly as possible from paved walks and driveways so they can be used safely during and immediately after a rainstorm. It is also desirable to prevent water from accumulating during the winter season in northern climates because wet areas are apt to become covered with ice, causing a real safety problem. In addition, it is important to drain surface runoff from paved outdoor use areas such as the outdoor living and entertaining space so they can be used as soon as possible after it rains. Pools of standing water on any paved surface reduce its safety and usefulness. For adequate drainage, paved surfaces of concrete or asphalt should
have a minimum slope of 1 percent. Exposed aggregate concrete, brick, stone, or other rough pavement materials should have a minimum slope of 1.5 percent (a 1-1/2-foot vertical change for every 100 horizontal feet). At the other extreme, paved surfaces in outdoor spaces where people stand or sit for any length of time should not exceed a maximum of 3 percent. A paved surface that is steeper than 3 percent is perceived as having a definite slope and gives a space an uncomfortable or unstable feeling. Paved walks should not exceed a slope of 5 percent and driveways and parking areas should not be steeper than 8 percent.
3. Water should be properly drained from lawn surfaces to prevent standing water or soggy, wet areas. For positive drainage, it is recommended that lawn surfaces slope at 2 percent or 2 feet fall in every 100 horizontal feet (Figure 11—5). However, lawn surfaces should not exceed 25 percent (a 1-foot vertical elevation change for every 4 horizontal feet). Above this maximum, it becomes dangerous to operate a lawn mower. It should be noted that in some locations, such as arid regions or areas with problems of too much runoff from urban development, it may actually be desirable to temporarily hold water on a site during and after a storm. In this situation, lawns and planted areas may be designed to retain water until it can seep into the ground or be slowly drained away.
4. Planting beds or other vegetative surfaces should be drained to prevent damage to plant materials. For plant beds, it is recommended that the ground slope at a rate of at least 2 percent, but not more than 10 percent. A planting bed that is steeper than this is susceptible to erosion unless protected by ground cover.