This project covers preparing the area under a freestanding deck and forming footings for the posts.
Check with a building inspector for advice on how to excavate your site and prevent weed growth. In some locations, there is no need to excavate -- building the deck will kill all foliage under it. More likely, however, you must remove the sod, lay down landscape fabric, and cover it with gravel to keep things from growing. You can remove the sod before or after you have laid out the footing locations.
Local codes will specify posthole depth and width, as well as how the concrete should be formed and how the post will attach to the concrete. These steps show the most common method, using a tube form and a post anchor that attaches to a J-bolt.
Digging is back-straining work, especially if you are not used to doing it. Don't rush it. When possible, work with your back straight rather than bent. Consider renting a power auger and hiring some help. Or call fencing or landscaping contractors and ask how much they would charge just to dig the holes.
To dig a posthole, from 15 minutes to two hours, depending on soil conditions
Posthole digger or power auger, spade, level, layout square, trowel
Using a spade or a posthole digger
Have utility companies mark locations of water, gas, electric, and phone lines. Double-check that the locations for the postholes are correct.
Sand or flour for marking the ground, plastic sheeting, gravel, concrete, tube forms, J-bolts
Set a tube form into the hole and mark it about 2 inches above grade. Cut the form with a handsaw or a knife. Check that the top of the form is close to level; recut the form if necessary. Anchor the form with screws driven into 2xs and add braces, if necessary, to keep the top of the form level.
A 60-pound bag of dry-mix will fill an 8-inch tube form about 20 inches deep; a 10-inch form, 13 inches; or a 12-inch form, 9 inches. If you need more than 40 bags, consider having concrete delivered.
To mix concrete, pour a bag of dry-mix into a wheelbarrow or tub and dig out a small depression in the middle. Pour or spray water into the hole, taking care not to add too much. Mix with a shovel or a hoe. Scrape the tub or wheelbarrow as you mix so there is no dry powder left at the bottom.
A good mix is just loose enough to be pourable but not too runny. It should hold its shape and cling for at least 1 second to a shovel turned on edge.
Once the concrete has set, smooth the landscaped area and tamp it firm by walking on it. Fill any indentations that would create puddles. Make sure that the footings stick up at least 2 inches above the ground. Spread heavy-duty plastic sheeting or landscape fabric over the deck area to prevent the growth of weeds. Use gravel or stones -- not soil -- to hold the sheeting in place. If you need two or more sheets, overlap them at least 1 foot.
For long-lasting insurance against weed growth, lay down black plastic sheeting that's 6 mils thick. Landscaping fabric protects against weeds but does not last as long as plastic. Purchase landscape staples to hold the sheeting in position.
You'll be walking on top of the gravel while you build the framing, so choose gravel that is free of sharp edges, which could tear the plastic. Pea gravel or lava stone are two good choices.