Dear James: I plan to build a deck against my house on my sloping backyard lot. Part of the deck is going to rest on the ground.
What is the best design?
— Bonnie T.
Dear Bonnie: With a sloping backyard close to the same height as the first floor as yours, it is trickier to build the support structure for the deck. Another option is a slightly raised deck, but you will have to step up as you come out the door to it.
The support lumber for the deck, other than the heavy posts, should all be above the ground. Even though you will use pressure treated lumber, it must all still be above the ground so that it lasts a long time. It will be difficult to repair is if ever deteriorates.
If the lumber does touch the ground, it may not rot, but ground moisture will make it unstable causing warpage and movement. The only movements you want on your deck are friends dancing at your next party.
The proper solution to your dilemma is a grade beam made from poured concrete. This is usually poured in an 8-inch-wide trench that is about one foot deep.
This portion of the job should be left up to a professional. Once the grade beams are ready, you can do the rest of the work yourself.
You will need a minimum of two concrete grade beams and more for a large deck. If you live in an area with freezing winter temperatures, they should be set on concrete piers that extend down below the freeze line. The piers should be spaced about every five feet and flared out at the bottom.
Since your yard is sloped, one side of your deck will be above the ground level. Continue the grade beams across the entire deck area anyway. It can get difficult trying to match up a wood post to it.
For a better appearance, position the outer grade beam in from the deck edge so it is hidden from view.
When you select the contractor to dig and pour the grade beams, make sure that they plan to use steel reinforcing rods in the beams. They should also install vertical anchor bolts at least every six feet. Eight- to 10-inch long, one-half-inch diameter bolts are commonly used and they should extend up at least two inches.
The tops of beams should be smooth, level and all beam top surfaces should be in the same plane. Check this with a string and a level before proceeding. If it was not done properly, you will have nothing but headaches with the building of the rest of your deck.
Lay 2-by-6-inch or 2-by-8-inch treated lumber over the anchor screws on the top of the beams and secure them in place. The remainder of the deck construction will be similar to any other deck.
Since your deck is relatively close to the moist ground, definitely use stainless steel screws to assemble it. Do not lay plastic film on the ground under it. This will likely allow rainwater to stand, which creates more moisture and breeding locations for mosquitoes.
While you are purchasing your fasteners, consider using special decking screws. They are knurled above the screw threads, so they bore their own pilot hole. This design nearly eliminates the problem of snapping a screw when you install it.
It is extremely important to seal all the wood with a good synthetic type of wood sealer before you begin to build the deck. With its being so close to the ground, you will not be able to treat the underside again.
Without sealing underneath, too, the decking with surely cup if it is exposed to the hot sun.