BUILD A DECK FOUNDATION

Level of Difficulty: Intermediate Do-It-Yourselfer – Moderate
Completion Time: Week-end Project

A sound deck foundation is crucial for the construction of a one or two level patio so it is important that this is decided before any details are written down. This also includes where the deck will be built and the type of ground where the foundation will sit.
There are four types of foundations for your deck: concrete pillars with footings; concrete pillars without footings; concrete deck blocks: and screwable posts However, from the standpoint of construction stability, we suggest you build a foundation with footing. One weekend is all it takes to get a secure deck foundation in place and ready for the deck installation.

TOOLS

  • Carpenter's level
  • Chalk line
  • Circular saw
  • Drill bit
  • Electric drill
  • Gardener's hoe
  • Hammer
  • Measuring tape
  • Pickets
  • Plumb line
  • Shovel
  • Square
  • Powered compactor if using deck blocks
  • Post hole digger (for concrete pillars without footings)
  • Geotextile

MATERIALS

  • Anchoring screws
  • Bags of pre-mixed concrete or ready-made concrete
  • Cylindrical forms (Sonotubes®) or 4" deck blocks
  • Galvanized 4" post saddles (if using concrete)
  • Carriage bolts, nuts and washers
  • Galvanized joist hangers and brackets
  • Galvanized screws and nails
  • Lag screws and washers
  • Reinforcing mesh (if using concrete)
  • Reinforcing wire (If using concrete)
  • Treated wood beams and posts
BEFORE ASSEMBLY
Always wear gloves and glasses.

Never burn treated wood scraps as the fumes are noxious.

Calculate the amount of concrete needed to ascertain if it is more cost effective to buy bags of concrete mix or book a truck with pre-mixed concrete.

Always put footings below the frost line or follow the guidelines of the area where the deck will be constructed.

Check with municipal regulations regarding how close deck footings can be placed to the property line.

BUILDING CODE REQUIREMENTS

The National Building Code of Canada establishes guidelines for the construction of buildings and various structures, including decks. However, these standards can vary depending on the local building requirements in effect in a particular province or municipality.

STEPS
Concrete pillars with footings are the most stable type of foundation you can use. The advantage of sills is that they solidly stabilize the concrete piers that are attached to them. However, this technique is rarely used because of the extensive excavation work involved. A large area must be excavated in order to place the formwork below the frost line. This work requires the use of a mechanical digger.

Use prefabricated circular bases that adjust to 8", 10" or 12" diameter pier tubes into which concrete is poured. These may be inserted into the holes without excavating the entire perimeter of the deck, but since the diameter of the hole to be dug must be at least 18" a mechanical digger would nonetheless be required.

1.1. Dig out 4" of soil inside the perimeter.

1.2. Calculate the weight of the terrace in order to decide on the number of pillars required. If you have drawn your own plan, you may need to consult an expert to determine that. Then, determine the location of the pillars. If plans call for the terrace to be built at the same time as the house, have both foundations dug at the same time. That will make it easier to install the formwork (54" deep) and pour the concrete.

1.3. Dig 54"-deep holes using a posthole digger (In such cases use pre-fabricated circular footings which adapt to circular forms of varying diameters and in which you will pour the concrete or dig holes large enough across to house the 24" forms that will be built.

1.4. Build 24" square forms using 2" × 8" boards. Lay in a reinforcing rod (diagonally) and add wire mesh. Forms are ready for concrete

1.5. Stand an L-shaped reinforcing rod in the centre of the frame placing the short end of it under the diagonal reinforcing rod. The L-shaped rod will serve as the link between the footing and the pillar.

1.6. Pour concrete and hold L-shaped rod erect as the concrete is spread. Be sure to comply with the concrete manufacturer's instructions on curing time.

1.7. Place tubular forms on the centre of the footings when concrete has cured and hold them up with pieces of wood. Use a level to ensure the tubes are vertically leveled and the tops are even.

1.8. Nail the tube to the footing at a 45° from the outside of the tube into the footing.

1.9. Backfill the footing using the original soil. Use a long narrow object to slowly stir the concrete to the bottom of the tube to make sure the concrete remains homogenous and free of air bubbles.

1.10. Insert in the centre of the pillar the post saddles. These are for the treated posts which will be attached later. Do this before the concrete sets.

CONCRETE PILLARS WITH FOOTINGS

CONCRETE PILLARS WITH FOOTINGS

CONCRETE PILLARS WITH FOOTINGS

CONCRETE PILLARS WITH FOOTINGS

Concrete pillars without footings are the most common type of deck foundation. They also require digging below the frost line to insert the tube except that the hole diameter required being smaller, you can simply use a mechanical post hole digger. Some people pour the concrete directly into the hole without using tubular forms, but that method is not recommended. Although this method is easier than the previous one, it offers less stability for the pillars.

2.1. Dig out 4" of soil inside the proposed perimeter of the deck.

2.2. Determine the location of the pillars.

2.3. Dig holes 54" below the surface of the soil using a posthole digger.

2.4. Insert tubular forms in each hole. Use a mason's line and level to ensure the tubes are on the level and at the same height so as to minimize adjustments later.

2.5. Pour the concrete in the frames.

2.6. Insert a steel rod in each frame to reinforce the pillar and minimize damage caused to the structure by frost.

2.7. Install post saddles making sure to comply with the concrete manufacturer's instructions on curing time.

CONCRETE PILLARS WITHOUT FOOTINGS

CONCRETE PILLARS WITHOUT FOOTINGS

CONCRETE PILLARS WITHOUT FOOTINGS

If your deck is close to the ground. concrete deck base blocks involve no pouring of concrete and only minimal digging. Perfect for people in a hurry! On the other hand, their stability is compromised by the frost and thaw cycles and, for that reason, they are not recommended for a deck that will be anchored to the house. Non-anchored decks will often require a few adjustments in the spring to straighten the structure. If the soil in your area is sandy, this type of foundation will be more appropriate.

3.1. Dig out 4" of soil inside the proposed perimeter of the deck.

3.2. Determine the location of the deck base blocks and dig holes 24" square X 12" deep.

3.3. Fill the holes with ¾" gravel in 3 successive coats of 4" which will be compacted using a vibrating plate or mechanical compactor.

3.4. Spread a geotextile sheet over the entire excavated surface.

3.5. Lay the deck base blocks in place. For greater stability the blocks can be placed on 12" X 12" patio stones placed on the excavated surface.

3.6. Backfill the entire perimeter using ¾" gravel.

CONCRETE DECK BASE BLOCKS

CONCRETE DECK BASE BLOCKS

CONCRETE DECK BASE BLOCKS

Screwable foundation posts require no excavation whatsoever. Under the best conditions they are easily installed. However, in a rocky or clayey soil, the operation will no doubt be very difficult and, at the least obstacle, the pile could warp or shift its angle or the bracket used in screwing in the pile could be damaged.

4.1. Determine the location for the posts.

4.2. Insert a beam in the pillar-supporting bracket at the top of the stake to screw the posts into the ground below the frost line and attach a pillar to the bracket. Twist the post until the saddle is at ground level.

4.3. Check for level and attach beams.

SCREWABLE FOUNDATION POSTS