DIY deck building
vs hiring a pro in Ottawa

A backyard deck is one of the most attempted DIY projects in Ottawa, and one of the most quietly dangerous to get wrong. The structure looks simple. Pressure-treated lumber is cheap. The cuts are mostly square. Anyone who can read a tape measure can figure out the geometry.

The reason deck failures are so common is that the parts that matter most are buried, hidden, or invisible until something gives way. A ledger that wasn't flashed correctly rots out behind a band board. Post holes that didn't reach frost depth heave 4 inches in their first winter and twist the entire structure. A guard railing nailed (not bolted) into the rim joist passes inspection and then fails when somebody leans on it three years later.

Here's the honest map of what's permit-required, what's load-rated, and where DIY decks actually fail in Ottawa.

When you need a permit (and almost everyone does)

The City of Ottawa requires a building permit for any deck that meets either of these conditions:

• The walking surface is more than 24 inches above grade at any point.
• The deck is attached to the house in any way (ledger-bolted to the band board, sharing a wall, or supporting any element of the building).

That covers about 85 percent of decks in Ottawa. Anything walk-out from a kitchen door, anything attached to the back of a townhouse, anything more than two steps off a backyard slope. All require permits, signed-off footing inspections, and final inspections.

Permit fees from the City of Ottawa start at $117 minimum and scale with the deck's footprint. The application is submitted through the MyServiceOttawa portal. Plans must show framing, footing depths, post spacing, ledger details, and guard heights.

What you can build without a permit: a free-standing deck under 24 inches off grade with no attachment to the house. Most ground-level patios and platforms qualify. Anything else does not.

Span tables: the math that decides what stays up

Every framing member in a deck has a maximum span based on its size, species, and load. The Ontario Building Code Part 9 publishes span tables. The City of Ottawa's deck guide is built on those tables.

The numbers you need to know:

• Joist span (5/4 decking, 16-inch joist spacing). 2x6 joists span up to 7 ft 6 in. 2x8 joists up to 10 ft 0 in. 2x10 joists up to 12 ft 6 in. 2x12 joists up to 15 ft 0 in.
• Beam span (built-up beam from doubled or tripled 2x material). A doubled 2x10 beam supports about 6 ft of joist span over a 6 ft beam span. A tripled 2x10 supports about 8 ft of joist span over an 8 ft beam span. The exact number is in the table.
• Post size. 6x6 posts are the standard for any deck over 4 ft tall. 4x4 posts are allowed only for decks under 4 ft tall and with light loads.

The mistake DIYers make: oversizing the joists and undersizing the beam. Or running joists at 24-inch spacing under 5/4 decking, which sags between joists. Or using a single 2x10 as a beam rather than a doubled or tripled built-up beam.

Read the span table. Sketch the framing on paper. Match every member to the table. If the framing looks tight against the table limits, go up a size.

Ledger flashing: the failure mode that rots houses

The ledger is the band of pressure-treated 2x10 or 2x12 lag-bolted to the side of the house. The deck joists hang off the ledger via joist hangers. The ledger transfers half the deck's weight to the house's framing.

What goes wrong: water runs down the back of the deck (from rain, snowmelt, deck washing) and gets behind the ledger. If there's no flashing between the ledger and the house's siding or sheathing, the water stays trapped. The sheathing rots. The band joist behind it rots. The entire connection weakens for years before the deck pulls away from the house, often catastrophically.

The right ledger detail in Ottawa:

1. Open the siding at the deck location.
2. Check the band joist for existing rot. Replace any soft wood.
3. Apply self-adhered membrane (ice-and-water shield or equivalent) to the band joist.
4. Bolt the ledger to the band joist with structural lag screws or through-bolts at code-required spacing (typically 16 inches o.c. staggered).
5. Install Z-flashing over the top of the ledger, tucked up under the siding above and lapping over the front face of the ledger.
6. Re-install siding above the flashing, leaving a gap between the siding and the deck surface.

This detail is on every City of Ottawa deck plan. Inspectors look for it. DIYers skip it because the siding work intimidates them. The result is decks that look fine for 8 to 12 years and then suddenly the band joist behind them is gone.

Post holes: 1.2 metres or you're heaving

The frost depth in Ottawa is 1.2 metres (4 feet) below grade. Any concrete footing that supports a structure has to extend below that depth, or the freeze-thaw cycle will heave the footing every winter and lower it (sometimes) in summer.

For a deck, that means every post hole has to be a minimum of 1.2 metres deep. The right way:

1. Dig (or auger) a hole at least 1.2 m deep and 12 to 14 inches wide.
2. Pour a 6-inch concrete pad at the bottom for bearing.
3. Set a Sonotube (cardboard form) from the pad to about 4 inches above grade.
4. Fill with concrete and embed a galvanized post anchor (Simpson AB or BC series) before the concrete sets.
5. The treated wood post bolts to the anchor, which holds the post 2 to 3 inches off the concrete to prevent rot.

The DIY shortcut that fails: deck blocks (precast concrete pyramids set on grade) instead of full footings. Deck blocks heave with frost, twist the structure, and turn the deck into a wavy uneven platform within 3 winters. They're allowed only for free-standing platforms under 24 inches that don't need a permit.

Helical (screw) piles are an alternative to concrete footings, especially in tight access yards. They're driven by a rotating hydraulic head to a torque-rated depth, certified by an installer, and don't require digging. They cost more per point than poured footings but avoid the excavation, and pricing depends on soil conditions and access. For decks where concrete trucks can't reach, helicals are the right call.

Where DIY decks actually fail

The five failures we get called to repair, in order of frequency:

1. Ledger rot. No flashing, water trapped, band joist rotted. Usually discovered 8-15 years in. Fix is to remove the deck connection, reframe the band joist, re-flash, re-attach. The repair scope, and what it costs, scales with how far the rot has spread into the framing.
2. Frost heave. Posts on too-shallow footings heaved unevenly. Deck is no longer flat, joists twisted, fasteners pulled. Fix is to dig new footings and reset posts, with the price driven by how many footings have to be redone and site access.
3. Guard rail failure. Posts nailed (not bolted) to rim joists, popped loose under load. Code requires lag bolts or carriage bolts through the rim, not nails or screws. Fix is to retrofit bolts, a contained repair compared with the others on this list.
4. Decking rot at the perimeter. Deck boards butted tight to the house with no gap, water trapped at the edge, board ends rotted. Fix is to cut back perimeter boards and add a 1/4 inch drainage gap. Easy fix if caught early.
5. Stair stringer failure. Stringers cut from 2x10 with too much wood removed at the notches, weakened, eventually cracked. Fix is to replace with 2x12 stringers properly proportioned.

A deck stays up because of details you cover with decking and stop thinking about. Get the ledger, the footings, and the bolted connections right and the deck outlasts the house.

The honest DIY-vs-pro line

If you've read this far and the span tables, ledger flashing, and frost-depth footings sound manageable, a DIY deck is realistic. Plan for:

• 2 to 3 weekends for a 12x16 deck if you're new to framing.
• A materials budget for pressure-treated lumber, fasteners, footings, and railing, which moves with the species and grade of lumber and the deck's footprint.
• City permit, inspection, and document fees, starting at the $117 City of Ottawa minimum and scaling with footprint.
• One or two evenings on the permit application and drawings.

If span tables and ledger flashing already sound like work you'd rather not do, hiring a deck builder is the right call. Built-deck pricing tracks the deck's size, height, footing type, railing and decking selections, and site access, and a contractor gives you a written breakdown before any work starts. The deck is permitted, inspected, and warrantied. For larger projects, multi-level decks, or decks integrated into a fuller home addition, this is where DIY usually breaks down and a contractor's permit, drawings, and trades coordination earn back what they cost.