A deck or balcony looks like a weekend project. Yet it's a full-fledged structural element: it supports people, furniture, sometimes a hot tub, and it must resist both frost and snow. Failures do happen — and they often occur at the same spot. Here's what not to overlook.

Attachment to the house: the classic weak point

The most common cause of deck collapse isn't the structure itself, but its attachment to the building. The ledger board fixed to the house wall must be properly bolted — not simply nailed — and protected by flashing to keep water from seeping in and rotting the assembly over time. A deficient attachment can hold for years… then let go suddenly under a normal load.

Footings below frost depth

Like any outdoor structure, a deck rests on supports that must be protected from frost heave. Piles or footings generally have to reach below the depth of frost penetration, otherwise the deck will move with the seasons (see our article on frost heave). A poorly founded deck distorts, pulls away from the wall and weakens that very attachment.

Loads, guardrails and the permit

A few other elements govern the design:

  • the loads to plan for: occupants, furniture, snow, and the very real weight of a filled hot tub, which completely changes the calculation;
  • the guardrails (called guards in the code): their height and load resistance are regulated, especially at elevation;
  • the materials: treated wood and galvanized or stainless hardware to resist moisture;
  • the municipal permit: often required, and certain configurations (elevated balcony, cantilever, attached structure) may require signed and sealed engineering plans, depending on municipal requirements.

When to consult an engineer

A modest ground-level deck is often a matter of sound construction sense. But as soon as there's height, a cantilever, a hot tub, a large span or any doubt about the attachment, a structural engineer's opinion is warranted — for safety and for the permit. Our residential structural expertise covers exactly this kind of question.

Planning a deck, or unsure about yours? Get in touch.

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