What size sonotube do I need for a deck in New Brunswick?

For most residential decks in New Brunswick, 10-inch or 12-inch sonotubes are standard, with 12-inch used for higher-load posts and corner posts on larger decks. The appropriate diameter depends on the tributary area each post supports, the deck load, and the soil bearing capacity at your site — it is not a one-size-fits-all answer.

How engineers and code officials determine sonotube size: Each footing must support the weight above it — dead loads (deck framing, decking, railings, snow load) plus live loads (people, furniture, hot tubs). A corner post of a large deck can be carrying several thousand pounds of combined load. The footing base area must spread that load over enough soil that the footing doesn't punch down (exceed the soil's bearing capacity). Standard residential soils have bearing capacity of 75–150 kPa, and typical residential deck footings sized to 10–12 inches handle most residential applications safely.

For small decks under 100 square feet with light framing and no hot tub, 8-inch sonotubes are sometimes used and may be acceptable with your building inspector's approval. For medium-to-large decks (100–400 sq ft) with standard loads, 10-inch is the common minimum for interior posts and 12-inch for corner posts. For decks supporting a hot tub (which can add 4,000–8,000 lbs of load when full), 14-inch or 16-inch sonotubes and engineered footing designs are typically required — always get a structural engineer involved for hot tub decks.

Depth trumps diameter for NB frost protection. A 12-inch sonotube at 3 feet deep will still frost-heave in Fredericton, Moncton, or Saint John because it's above the frost line. A 10-inch tube at 4.5 feet will stay put. Diameter handles load capacity; depth handles frost stability. Both matter.

Sonotubes are available at NB building supply stores in diameters from 8 to 16 inches and run $8–$20 per tube depending on diameter. They're filled with standard concrete (bagged for one or two footings, ready-mix for larger numbers). Always check with your local building inspector on the required diameter for your specific deck design — they'll confirm based on your deck size and soil conditions.