A deck that is attached to your house and stands above the ground is a structure, and the footings and the ledger are what hold it up. So this guide starts with the parts you cannot see once the boards go down: the permit, the footings and the ledger. Then it covers the framing and the decking.
Before you start
- Ask your building office first. The 20×20 deck in the This Old House video had a building permit and a property survey, and an inspector had to measure every footing hole before concrete was poured. The 8×12 ground-level deck video says its deck is not permitted where the builder lives because it floats, but advises always contacting the local building office. Rules differ by city, so the permit office has the final word. See Do You Need a Permit?.
- Find out what is buried before you dig. Ask the permit office and your utilities how to have lines located. We have no video on how that works where you live.
- Use stamped structural fasteners only. The 8×12 video says structural fasteners carry a code stamp on the head.
- Seal the cut ends of pressure-treated lumber, because the cut wood is no longer treated (same video).
- Wear safety glasses and hearing protection when you cut, and a respirator when you pour powdered concrete or thinset.
Time, skill and cost
| Skill | A ground-level floating deck is a reasonable first deck build. A raised or attached deck is intermediate to advanced work |
| Time | No time is quoted in the videos. The 20×20 build was filmed in two parts, framing and then decking and railing |
| Cost | Only what the videos state. See the table near the end |
Step 1: Choose the type of deck
- Attached deck. Fastened to the house by a ledger board. The footings and ledger carry the load. Inspected in most places.
- Freestanding deck. Its own posts and beams, nothing fastened to the house.
- Floating deck. Sits on blocks or footers at ground level. The Mr. Build It floating deck builder says concrete blocks on the ground may work for a season or two but the ground eventually settles and moves, and so he set anchors in concrete past the frost line instead. The low-profile deck used adjustable plastic footers on cinder blocks, and anchor stakes at the corners.
Step 2: Lay out and square the deck
Set batter boards, which are stakes with a horizontal board, outside the deck perimeter, and run string along them. Square the corner with the 3-4-5 method: both the 20×20 and the 8×12 videos do it, in the 20×20 doubled to 6 ft, 8 ft and a 10 ft diagonal. Check the two diagonals as well. The 8×12 builder used diagonals of 174 in. and nudged the frame about an eighth of an inch to bring it square.
Step 3: Footings
Footings go below the frost line, which varies a lot across North America. In the 20×20 video the holes are 48 in. deep because frost goes four feet deep there and the code sets a four-foot minimum. The frost line where you live is a number the building office will give you. A cabin build in another video put its footings 36 in. past the frost line (see Planning an A-Frame Cabin).
The 20×20 deck steps:
- Dig 18 in. holes to depth. A small skid steer with an auger cost $400 delivered and picked up.
- Set a 12 in. paper form tube in each hole and backfill evenly.
- Wait for the inspector to measure every hole.
- Pour concrete. The couple used a little over five bags per column, about 42 in total.
- Set an expansion bolt and a metal post base in each column, with a plate that leaves about an inch so the wood does not sit in water.
Step 4: The ledger
The ledger is the board that ties an attached deck to the house. In the 20×20 video, the siding’s first course was removed, self-sealing membrane flashing was tucked under the siding, and a pressure-treated 2×8 ledger was held off with a gauge block to leave an air space. It was then fastened with 6-inch lag bolts, pre-drilled with a 3/8 in. bit and staggered high and low so the wood does not split.
This is the one step to get an inspector or a pro to look at before you cover it. The fasteners, the flashing and the framing behind the siding must all be right. We have no video on how to check what is behind your siding, so ask.
Step 5: Posts, beams and joists
- Posts and beams. The 20×20 deck uses two 2×12 beams on 6×6 posts. The beams span 10 ft, so not every column was needed, which Tom Silva showed saves digging and concrete. Post tops are cut at an angle so water does not sit on them. A laser level marks the posts when the columns aren’t level.
- Joist spacing. 16 in. on centre in the 20×20 video, with hangers set using a gauge block. The 8×12 video says composite decking needs joists 12 in. on centre, and that 16 in. voids the warranty. Always follow the decking maker’s span table.
- Joists in a hanger need a screw in every hole, with structural fasteners. A corner bracket is used where a fastener must not stick out the side.
- Joist tape goes on top of the joists to protect them from debris and rot, according to the 8×12 builder, who found it awkward to apply.
Step 6: Decking
- Snap a chalk line parallel to the house and pre-drill and face-screw the first board.
- Hidden fastener systems pull the boards tight. Butt joints must land on the centre of a joist, not the middle of a bay, and fresh cuts should be sealed (ipe video).
- Order extra for waste: the 20×20 deck, about 400 square feet, was allotted 450 square feet of decking.
- Leave a gap for air under the deck edge. The 8×12 builder left a quarter inch.
Step 7: Railings
If the deck is above the height your code requires a guard for, the railing is a safety item. The 20×20 video bolts 4×4 posts with 5 in. lag bolts and builds 2×4 rails on 2×2 balusters. Your local code decides the height and baluster spacing. Ask the permit office, and don’t copy a video.
Tools and materials
- Level, tape measure, string and a chalk line
- Circular saw, cordless drill and impact driver
- Hammer drill for masonry anchors (the Mr. Build It video used an SDS hammer drill, and warns an impact driver on expansion anchors kept them from setting properly)
- Pressure-treated lumber, concrete, form tubes, post bases, hangers, lag bolts, decking, hidden fasteners
Costs the videos state
| Item | Cost | Video |
|---|---|---|
| Rental skid steer with auger, delivered | $400 | 20×20 deck |
| 28 composite boards, 16 ft, at $35 each | about $1,000 | Floating deck |
| Hidden hardware kit | about $40 to $50 a box | Floating deck |
| 2×4 joists, 1×4 decking, plastic footers, blocks, stain and fasteners (16×6 ft) | $189, $630, under $200, $75, $100 and $100 | Low-profile deck |
These are one builder’s prices in USD at the time. Get local quotes and ask for your permit fee.
Mistakes to avoid
- Pouring before the inspector has seen the holes.
- Fastening the ledger to siding without flashing.
- Using non-structural screws where the load is.
- Setting joists at 16 in. under composite decking whose maker asks for 12 in.
- Buying exactly the board count. Order extra for waste.
Watch it done
- The Build a Deck and How to Build a Deck series, the 20×20 project in two parts
- Every video for Build a deck, and the Deck and patio room
- The Abandoned House Transformation project for the floating deck
- The Framing and Carpentry systems
Call a pro if
- The deck is attached to a house with brick, stucco or an unusual wall, or the framing behind the siding looks rotted.
- The deck will be a second storey, or people will be underneath it. The 8×12 video flags this for the building office.
- Your plans need engineering, or the inspector asks for it.
- You find buried lines, or water in the footing holes.
- You are not sure how your local code treats the railing, the stairs or the footings.
Sources
- This Old House on YouTube, shown on this site. Building a 20×20 Two-Level Deck: Layout, Footings and Framing (video, This Old House) Source
- This Old House on YouTube, shown on this site. Ipe Deck Boards and a Dovetail Railing on a 20×20 Deck (video, This Old House) Source
- Shown on this site, source of the composite joist spacing and the permit advice. Building an 8×12 Ground-Level Deck on CAMO Blocks with Trex (video) Source
- Shown on this site, source of the board prices. Floating Deck Build: Footings, Framing and Composite Decking (video, Mr. Build It) Source
- Shown on this site, source of the itemised costs. Low-Profile Floating Deck on 2×4 Joists and Plastic Footers (video) Source