How We Build Decks That Last

Sarnia-Lambton sits on the St. Clair clay plain, and St. Clair Region Conservation Authority watershed reports put local soils at around 80% silt and clay. Fine soils like that hold water. Water expands about 9% when it freezes, and frost-prone soils keep drawing in more water to form ice layers that push upward. Every winter, frost works its way down into the ground and lifts whatever it can grab.

The National Research Council’s research on frost heave describes the mechanism: frozen soil bonds to the sides of a foundation — “adfreezing” — and drags it upward as the frozen layer expands. A footing that stops above the frost line gets lifted from below. In practice, a footing that goes deeper can still get gripped on the sides if the concrete is rough and irregular.

So Ontario building departments apply the Building Code’s minimum of 1.2 m (4 ft) below grade for deck footings. We dig every footing to that depth, set a smooth Sonotube form — smooth sides give frost less to grip — and fill it solid with concrete. Where the soil or the load calls for more bearing, a flared footing base spreads the load at the bottom and anchors the pier against uplift.

The code allows one exception: a surface deck under 600 mm (about 2 ft) high, not attached to any building, not supporting a roof (building departments commonly treat a pergola the same way) and no bigger than 55 m² (592 sq ft) can sit on surface footings like deck blocks. They’re fast and cheap, and for a small low platform they can make sense — but they move with the ground. We’ll always tell you which you’re getting and why.

Wood decay needs three things: moisture, oxygen and wood. The USDA Forest Products Laboratory points out that the ground line is the one place on a post where all three meet — and it’s also the point of maximum bending stress. That’s why rotten posts snap right at grade.

A deck post buried in concrete has the same problem in a different spot: water runs down the post, collects at the bottom of the hole and can’t escape. You won’t know it’s rotting until the deck starts to move.

We don’t bury deck posts. Every post sits on a galvanized standoff post base anchored to the top of the footing. The standoff holds the end grain about an inch off the concrete so it can dry out after every rain. And if a post ever needs replacing, it can be swapped without touching the footing.

Size matters too. The Ontario Building Code’s minimum for most deck columns is 140 × 140 mm — a 6x6 — with 4x4s allowed only in limited cases. Beams sit in notches or approved post caps that carry the load straight down, not hung off the side of the post on a couple of bolts.

The frame is what you’re actually walking on — the boards just cover it. Ontario decks are designed for a live load of 1.9 kPa (about 40 lb per square foot), and every joist and beam is sized for its span at that load.

The ledger is the board that attaches a deck to your house. The North American Deck and Railing Association estimates 90% of deck collapses involve the ledger connection — a figure that comes from a Virginia Tech engineering professor’s expert estimate rather than hard national data, but it’s widely regarded as the most critical connection on a deck.

The failures look the same every time: a ledger held on with nails that slowly pull out; missing or felt-paper “flashing” that lets water rot the house’s rim joist behind it; or a ledger fastened to brick veneer, stone veneer or a cantilevered floor that was never meant to carry a deck.

Permits factor in too. In Sarnia, attaching a deck to the house requires a building permit at any height, while a low freestanding deck (under 2 ft) generally doesn’t. We’ll lay out the options honestly and build whichever suits you — to the same standard either way.

Since 2004, residential treated lumber in Canada has used copper-based preservatives (ACQ, copper azole and micronized copper) instead of the old arsenic-based CCA. They’re safer, but they corrode ordinary steel much faster.

Simpson Strong-Tie’s guidance for treated wood outdoors is ZMAX (G185) or hot-dip galvanized at minimum, and stainless steel for severe exposure. Electroplated “galvanized” fasteners don’t count, and galvanized and stainless shouldn’t be mixed. Home inspectors routinely find rusted standard hangers on treated decks — hardware that was never rated for the job.

Then there’s how hardware gets fastened. Joist hangers usually have the required nail or listed structural screw stamped on them, and every hole is meant to be filled. Ordinary deck screws aren’t a substitute — the Journal of Light Construction calls them “too brittle,” and under load a brittle screw snaps instead of bending. Only manufacturer-listed structural screws can replace the nails.

We use galvanized structural connectors and fasteners throughout — hangers, post bases, ties and structural screws — installed the way the manufacturer specifies.

Brown or green treated lumber is the same idea with different colourant. What actually matters is the CSA O80 use category printed on the end tag:

Treatment in spruce often doesn’t reach the core, which is why Canadian treated lumber has those small slits on the surface — incising, which helps the preservative soak deeper. It also means cutting a board exposes untreated wood. The CSA standard calls for two brush coats of copper-naphthenate preservative on every field cut, and lab testing has found decay entering through treated cut ends while the factory-treated shell stayed sound. Simple rule: factory ends go in the ground, cut ends go up and get treated.

Most treated lumber in Ontario is sold wet from the plant, so it shrinks as it dries. That affects how boards are gapped and when they can be stained — see when to stain new wood.

Wet pressure-treated boards go down tight because they’ll shrink open as they dry; kiln-dried boards get a small gap from day one. Composite moves differently — it expands and contracts along its length with temperature — so manufacturers spell out end gaps by installation temperature. Trex, for example, calls for 1/8" end-to-end above about 4.5°C (40°F) and 3/16" below it, and at least 3/16" side to side.

Fastening follows the same rules: two screws per joist for face-fastened boards, a clip at every joist for hidden fasteners, and airflow underneath so the frame can dry. Composite warranties run 25 to 50 years on many lines — but they cover replacement material, not labour, and they’re void if the boards weren’t installed to spec.

The Ontario Building Code requires a guard on any deck more than 600 mm above the ground — 900 mm tall for decks up to 1.8 m high and 1,070 mm above that, with openings that won’t pass a 100 mm ball and nothing climbable between 140 mm and 900 mm off the deck. That climbability rule is why horizontal cable railings don’t pass on raised decks in Ontario. A guard also has to resist about 1 kN (roughly 225 lb) pushed at any point along the top.

Most railings fail that test at the post. A railing post is a lever, and Virginia Tech testing found most commonly used post connections couldn’t meet even the US code’s guard load, which is lower than Ontario’s — the post pries the rim board off the joists. Railing makers like Fortress require solid blocking under every post and through-bolts, never lag screws, and never fastening to the deck boards alone. That’s how we mount every railing post.

One more detail people miss: a built-in bench or planter against a railing is something a child can climb. Building-code guidance (the City of Kitchener’s, based on the National Building Code) calls for keeping it 760 mm away from the guard, or measuring the full guard height from the top of the bench. We design built-ins with that in mind.

Here’s when a deck needs a permit across our service area:

When a permit is required, the drawings show the footings, framing, ledger connection, guards and stairs, and the municipality inspects the work as it goes. Sarnia targets about 10 business days for review and needs 48 hours’ notice for each inspection.