Assemble an attractive deck
Q: I am planning some landscaping that will include a deck. My backyard is level, so the deck will not be up on posts. Is it wise and possible to build a deck which is flush against the ground? -- Suzi H.
A: Adding a deck is probably the least expensive living area you can add to your home and it can be an attractive addition to your landscaping. Building a deck is a fairly simple do-it-yourself job and you can find deck building plans at most home center stores.
It really is not wise to place deck lumber, even pressure-treated lumber, directly on the ground. It will most likely not rot, but the constant moisture from the ground can causes warping and it just makes the entire deck less stable.
Before you worry about this problem, inspect the ground around your house. You may have enough vertical height to build a standard deck on posts. The top of a house foundation is almost always above the ground level. The soil may have been sloped up to the house wall for drainage or landscaping purposes and you may be able to remove it for adequate free height.
Assuming you still do not have clearance above the ground for the deck framing, pouring a concrete grade beam is your best option. This is a mini-foundation for the deck, similar to your house, so the decking lumber is not in contact with the damp ground. Check your local building codes for the proper spacing of deck supports to determine how many beams will be required.
To build a grade beam, you just have to dig a trench, usually eight to ten inches wide and about one foot deep. If you live in a cold climate, dig deeper (below the frost line) holes every five feet for support piers. They should be tapered out at the bottom to create a wedge shape. This resists any upward motion of the piers.
Install reinforcing steel rods in the beams for added strength. When the concrete just begins to set, insert one-half inch threaded rods up vertically from the top of the beam. You will be able to tell when the concrete is stiff enough to support them. These rods will be the anchor bolts used to attach the deck framing to the beams.
Place 2x8 pressuretreated lumber on top of the beam and attach it to the beam using the anchor bolts. This is very similar to the sill plate on top your house foundation. From this point, you will build the deck as you would any other deck on posts.
Even with the deck built up on the concrete beams, it will still be close to the ground with little air flow beneath it. Make sure to treat the ends of any lumber you cut with approved preservatives.
With this higher moisture situation, it would be wise to spend a little extra and use stainless steel screws and fasteners instead of galvanized ones. A good source for special screws for this application is Design House, (800) 558-8700. These screws have a knurled shank to create its own pilot hole so there is less chance of pullout over time.