Timber Decks and Balconies: Design and Construction

About this course.

Timber decks and balconies fail principally in three ways: the structure collapses because a connection or member is inadequate, the timber decays where it stays wet, or people fall because a barrier is missing, too low or not strong enough. What makes these failures so serious is that decks and balconies are elevated, are used by groups of people, and are often loaded most heavily at exactly the moment a failure is most dangerous — a gathering, a party, people leaning on a rail. A deck that quietly rots at a hidden connection can give way years after it was built, without warning, when it is full of people. The stakes are therefore measured in injuries and lives, not just rectification cost.

The first failure mode is structural collapse. A deck is a chain of load paths: decking spans to joists, joists to bearers, bearers to posts or to the building, posts to footings, and the whole assembly must also be braced against sway and, where attached to the house, tied back so it cannot pull away. A weakness anywhere in that chain can bring down the whole structure. The most notorious weak link is the connection of the deck to the building — the ledger — which is addressed in detail in Module 2, but collapse can equally originate in undersized bearers or joists, inadequate footings that settle or pull out, missing bracing that lets an elevated deck rack sideways, or connections that were never designed to carry the loads applied to them. Because timber structures are forgiving up to a point, an inadequate deck may perform acceptably under light everyday use and only reveal its deficiency when fully loaded.