A deck or balcony looks like a weekend project, but it's a structural element: it carries people, furniture and sometimes a hot tub, and it has to withstand frost and snow. When a deck fails, it often fails in the same place.

Attachment to the house

Deck collapses are most often caused by the attachment to the building rather than by the deck structure itself. The ledger board fixed to the house wall must be bolted, not just nailed. It also needs flashing so water doesn't get in and rot the connection over the years. A poor attachment can hold for a long time, then give way all at once under a perfectly normal load.

Footings below frost depth

Like any outdoor structure, a deck sits on supports that need protection from frost heave. Piles or footings generally have to go below the depth of frost penetration. Otherwise the deck will move with the seasons (see our article on frost heave). A deck that shifts eventually pulls away from the wall, which weakens the attachment.

Loads, guards and permits

Other factors shape the design:

  • loads: occupants, furniture, snow and, where applicable, the weight of a filled hot tub, which changes the calculation completely;
  • guards (guardrails), whose height and strength are regulated, especially on elevated decks;
  • materials: treated wood and galvanized or stainless steel hardware to resist moisture;
  • the municipal permit, which is often required. Depending on municipal requirements, some configurations, such as an elevated, cantilevered or attached balcony, may require plans signed and sealed by an engineer.

When to consult an engineer

For a small ground-level deck, good building practice is often enough. A structural engineer's opinion is warranted as soon as there is height, a cantilever, a hot tub, a long span or any doubt about the attachment, both for safety and for the permit. Our residential structural assessment covers this kind of question.

For a deck project or a check on an existing one, get in touch.

This article is provided for general information. Applicable requirements (construction code, guards, municipal permit) vary by project and municipality.