Slopes, Soil and Drainage
Sloping sites, such as hillsides, banks, ravines, and drainage swales, often pose problems, but they can offer you more design opportunities than you might think. If you ignored a slope when you started assessing your site, take a second look.
Land sloping away from a high spot offers a view to the area below. Land that slants uphill generally creates privacy and shelter from harsh winds. A seemingly problematic area might turn out to be the best spot in your yard for a new deck. So even where there’s a slope, there’s hope—you can usually find a way to grade it level.
Leveling the Soil
To level a slope, either cut into it to remove soil and form a flat area, or fill in a low point, or both.
Both methods will create a level surface suitable for your deck. But untamped fill dirt is not stable and will settle unevenly, causing a ground-level deck to crack. You’ll need to tamp and firm the loose surface of a filled area before installing such a design. Use footings, not piers, to support the posts, and dig the holes deep enough so the foundations are in firm, ungraded soil.
If the soil sloughs off when you cut into a slope, you should build a retaining wall to hold it in place. Even if a retaining wall isn’t necessary, building one will provide you with a cozy deck nestled into the side of a hill.
Types of soil
All soil is not created equal. There are several kinds of soil, and each will have a different effect on your site and how you prepare it.
- Loose, sandy loam absorbs water and drains well, is good for plantings, and is easy to grade. It erodes easily, however, and does not compact well; you’ll need to set posts in concrete.
- Silted soil is easy to dig and to compact, but posts for overheads and fences will need to be set in concrete.
- Clay is compact and sheds water so easily that runoff can prove to be a problem. Fix it with grading or with drains that terminate in storm sewers or catch basins.
Each type of soil has a different angle of repose—the steepest angle at which soil on a slope will stay in place and not slough off. In general, soils with a high clay content have a steeper angle of repose and hold together better than loose, sandy soils, which readily give way
What to Do with Excess Fill Dirt
Put the soil removed from grading to the best use by leveling other parts of your lawn. This procedure is called cut and fill, and it works best when the grading removes an amount equal to the areas that need additional soil. Cut and fill also eliminates the expense of disposing of excess soil, as well as the cost of purchasing fill dirt.
If cutting into a hillside results in more fill dirt than you need, use the excess topsoil in your landscape—in planting beds, raised or flush with the rest of the lawn. Because you’ll probably remove more than just topsoil, not all of the excavated dirt will be suitable for planting beds. Use it instead to construct berms—low mounds of earth in a landscape. Avoid spreading excess soil around trees, even temporarily. Just a few extra inches of dirt over tree roots can suffocate delicate feeder roots and kill the tree.
Runoff and plants
Runoff from a deck can wreak havoc away from the deck site. Water moving across impermeable surfaces flows quickly. As it gathers speed and runs off the hardscape, it can cut channels through beds, washing away seedlings and topsoil.
Established plants with fibrous roots, such as lawns and ornamental grasses, will probably be able to stand up to this wash of water. But shallow-rooted plants, such as those in recently installed flower beds, will usually wash out of the ground. Runoff flowing into planting areas can also become trapped in puddles, and the overly wet soil drowns plants by suffocating their roots.
You have several options for dealing with excess water:
- Redirecting the water
- Planting annuals in raised beds or containers
- Choosing plants that thrive in wet soil
If you have water in your basement, it may be caused by ground sloping toward the foundation. Here’s an easy solution:
- Slope the soil next to the foundation away from the house for a distance of at least 4 feet.
- Bring in new soil as necessary.
- Lay landscape fabric over the sloped surface.
- If you’re going to plant the area, cut holes in the landscape fabric for the plants. Spread decorative rock or wood chips to camouflage the fabric.
Trees, Rocks, and Other Obstacles
Many existing features of your landscape, such as flower beds, foundation plantings, fences, walls, and walkways, will affect the location of your deck. If you can’t part with them, and are certain of your favorite spot, integrate them into your design. The same goes for trees, rocks, and other obstacles. Building trees into your design, however, calls for careful planning.
ENCLOSING A TREE: Decks built around a tree can appear as though nature put them there. Their height and mass will balance the horizontal expanse of your deck surface. But take care to avoid damage to the root system. Your tree can be the life of your deck. Don’t let your deck be the death of the tree.
Avoid hemming in trees too tightly with decking. Allow room for trunks to grow, or you’ll end up cutting through planks later on to enlarge tree holes.
Retrofitting for drainage
After heavy rains, water sometimes puddles on decks and other walking surfaces in your landscape. Perpetually damp spots provide ideal conditions for slick moss to grow and for mosquitoes to breed. Even if you’ve already built your deck or have inherited it as part of the property purchase, it’s not too late to remedy these problems.
To stop puddling, divert water away from low areas. During a heavy rain, observe the water’s path. Almost anything—debris, excessive mulch, or a poorly placed plant—can block the flow of water and divert it where you don’t want it. If the water originates from beyond the surface, change the direction of the flow at its source. For example, relocate downspouts so they don’t dump water onto the deck. Or install extensions to the downspouts to carry the water elsewhere.
Use curbing to block the unwelcome flow of water from the lawn or a planting area. Or divert the flow with a swale—a slight depression made to channel water. When installing a curb to redirect storm water, dig a swale along the back side of it. Often, a shallow swale alone keeps water flowing in the right direction. Avoid building a walkway across a swale; it will form a dam and impede drainage.
Removing unused portions of paving is another solution because it reduces the amount of impermeable surface. Removing a section of paving can also pleasantly alter the shape of a patio or path, and create room for low plants within the space. This solution works best if the sections removed aren’t in the part of a surface that you use regularly.