Before you build, know how to prepare the area under a freestanding deck and form the footings for the posts.

January 26, 2019

When building a deck, 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 in 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.

What You Need

  • Posthole digger or power auger
  • Spade
  • Level
  • Layout square
  • Trowel
  • Sand or flour for marking the ground
  • Plastic sheeting
  • Gravel
  • Concrete
  • Tube forms
  • J-bolts

Step 1: Mark Perimeter and Dig

Remove the mason's lines, but keep the batterboards in place. Take care not to disturb the batterboards as you work. Mark the perimeter of the deck with sand or upside-down spray paint. Dig away the sod from the deck area.

Step 2: Roll Sod

It may be possible to slice lines in the sod with a straight shovel or spade and roll up the sod for use elsewhere. Leave the stakes that mark the post locations in place, or pull them up and replace them as soon as you have rolled up the sod around them.

Step 3: Dig Hole

At each posthole, mark a large X with flour, sand, or spray paint. Remove the stake and dig a hole wide enough to accommodate the tube form that you will use. Shovel a few inches of gravel into the hole and tamp it down.

Step 4: Cut and Anchor Form

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.

Step 5: Fill Form with Concrete

Don't fill the tubes with concrete until the inspector has signed off on the holes. Fill any spaces around the form with well-packed soil. Mix the concrete and pour it into the form. Poke a pole or a piece of 1x2 into the concrete several times to remove any air bubbles.

Refresher Course

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.

Step 7: Insert J-Bolt

Reattach the mason's lines and check that they are square. Directly below intersecting lines, insert a J-bolt into the concrete. Wiggle it downward until about an inch sticks up. Drop a plumb bob from the intersection to help center the J-bolt.

Step 8: Smooth Concrete

Use a layout square to check that the bolt is pointing straight up. Smooth the concrete with a small trowel and check the bolt again.

Step 9: Level and Cover with Sheeting

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.

Step 10: Cover With Gravel

Pour 1 or 2 inches of gravel on top of the sheeting and rake it smooth. The gravel will poke small holes in the plastic, which will allow water to seep through. However, avoid large holes or tears that may permit weeds to grow.

Groundcover Options

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.


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