The design of sloped sites should be undertaken with care and understanding for the unique conditions that exist. The design guidelines that follow will help to accomplish this objective.
Fit Uses to Slope Extra study is typically needed to mold proposed site uses to a sloped site. This should start with the preparation of a slope analysis, a map depicting the different categories of slope on the site. A slope analysis will show which areas of the site are steepest and which are the most gentle (Figure 13—19). Then, the designer should attempt to match the proposed uses to slope conditions where they will fit the site with minimal grading (Figure 13—20). For example, a recreational lawn area should ideally be placed in a location that has a slope between 2 and 4 percent. A lawn that is not for recreation can be placed on an area that is up to 25 percent slope. Above this, it is too steep to safely mow. An outdoor entertaining space, on the other hand, could be placed on a slope that is between 5 and 15 percent by terracing it on different levels. The reader is referred to Chapter 11, where slope standards for other uses are outlined.
Outdoor use areas can also be properly tailored to a sloped site by orienting them on the site to minimize grading. This is frequently accomplished by placing the long dimension of outdoor spaces parallel to the contours (Figure 13—21). This stretches the space out along the slope rather than into the slope. Cut (soil that is excavated) and fill (soil that is added to existing ground) and costs are reduced by the approach.
On steeper site areas, outdoor uses may need to be molded to the site by creating terraces that are cut into the slope at different elevations. This creates a series of large “stair-steps” on which outdoor uses are placed (Figure 13—22). Planted slopes that do not exceed a 50 percent or 2:1 grade can serve as a transition between the elevation of the individual spaces. This approach gives a soft appearance to the landscape and separates spaces by the horizontal distance across the slopes. Retaining walls, sometimes located on both the uphill and downhill sides of spaces, can also be employed as a means of accommodating the different elevation between spaces. Retaining walls give a landscape a more architectural appearance and allow spaces to be placed closer together (Figure 13—23). They likewise can be designed as visual extensions of the house by extending materials and edges of the house into the adjacent landscape. Retaining walls should not exceed 3 or 4 feet in height without requiring special engineering and cost.
To locate outdoor use areas on sloped areas in excess of 15 percent most often requires a deck. A deck is simply built above a slope, allowing the existing grade underneath to remain essentially as is (Figure 13—24). Decks work well for spaces of limited size such as outdoor sitting, entertaining, and eating and many times can serve as architectural extensions of the house (see the section, “Take Advantage of Views”).
Some outdoor uses may not be possible on steeply sloped sites. Outdoor areas that are large in size and/or require a gentle ground surface may need to be eliminated from a design program for a steep site. There is a point where it is simply best not to force a use onto a site if it does not easily fit. The steepest areas of a site are often best left alone. This is especially so where existing trees or other forms of natural vegetation cover the site. The designer might reserve the steepest areas for revegetation on disturbed or regraded lots as well.
Accommodate Movement Special attention should be given to accommodating movement on a steep site. This is required because movement, particularly foot traffic, is frequently difficult and restricted on a sloped site. Walks or paths, as suggested in Chapter 11, should not exceed a 5 percent grade. Walks that are between 5 and 8.33 percent are considered to be ramps and must adhere to ADA (American with
Disabilities Act) standards. To maintain this standard, walks may need to take a more indirect route between two points. In other words, the elevation difference between the top and bottom of the walk should be spread out over a greater distance in order to reduce the walk gradient. In extreme situations, walks or paths may need to “switch back” to avoid being too steep.
Steps are also a common necessity on sloped sites to provide access between nearby spaces. Where possible, extreme elevation differences between adjoining spaces should be avoided to minimize the number of steps that are required. Steps should follow the guidelines provided in Chapter 11 when they are incorporated into a design. In addition, they should visually fit into the site context in terms of form and materials. Steps between adjoining spaces might also be wider than necessary so that the spaces feel more connected. Wide steps allow adjoining spaces to visually flow together.
The one disadvantage of steps is that they form barriers to universal accessibility. Therefore, it may also be necessary to incorporate ramps, especially in the public areas such as the approach to the front of the house.
Take Advantage of Views Everything possible should be done to take advantage of the inherent views from a sloped site, assuming they are worth capturing. During site analysis, the designer should determine what locations on the site have the best views, both toward other areas of the site and to the landscape beyond. Then, selected uses should be consciously placed in these locations to utilize the views (Figure 13—25).
Some sitting or gathering spaces might even be located on the front or public side of the house if the views there are worth savoring. Portions of the site that lie downhill from the remainder of the site should likewise be studied and enhanced if necessary. Remember, these low areas will definitely be looked at and so they should be worthy of the attention they will receive.
The spaces themselves should also be designed to take advantage of the views. Reducing the height of the vertical plane on the side with the best view can accomplish this (left side, Figure 13—26). Vertical planes that must extend above eye level should be as transparent as possible. Even glass or Plexiglas might be used for vertical enclosure along the downhill side of a space (right side, Figure 13—26). In some instances, it may be desirable to frame views by locating vertical objects on either side of the view as well as placing an overhead plane above. Again, decks should be used to take advantage of views on especially steep ground. On dramatically sloped sites, decks may be at the level of or higher than surrounding trees, thus providing a panoramic view into the distance.
Control Runoff and Erosion As indicated previously, care must be taken to drain surface runoff around the house and drain outdoor use areas from portions of the site that are located uphill. This is necessary on all sloped sites, but becomes more difficult on steeper sites because of the potential for erosion. Swales, valley-like excavations into the earth, that are cut into the site to catch and direct water should be designed so they visually fit into the topography of the site. Swales that look like gashes because of overly steep side slopes should be avoided. The low side of the site, on the other hand, may be wetter because of the water that drains to it. This location is usually not good for many outdoor uses and may be best set aside as a planted area or place where native vegetation is allowed to grow. Finally, all slopes that are over 50 percent also should be left untouched to minimize erosion on a sloped site.