Structural Safety of Wood Decks and Deck Guards: Multi-family Balconies

by Joseph R. Loferski, PhD, and Frank E. Woeste, PE, PhD

Multi-family balconies are often framed with cantilevered wood, with a concrete covering and gypsum or vinyl ceiling. Structural drawings for a project typically contain a note describing the quality of lumber materials assumed and used by the structural engineer in the structural design process. For example, a materials note may read:

All framing lumber shall be No. 2 (minimum) D-FIR (or HEM-FIR) S-Dry.

S-Dry means at the time the lumber was surfaced (or planed) at the sawmill, the maximum moisture content (MC) of the individual lumber pieces was less than 19 percent. When structural engineers use a typical lumber note as presented, the intent is more inclusive than simple stating the MC of the product when manufactured, but it includes the assumed maximum MC of the lumber for the service life of the project (e.g. 50 years or more). The validity of their structural designs is conditioned on the assumption the lumber components will be protected from high MC conditions above 19 percent and liquid water.

‘Dry’ lumber that remains dry in-service is known to perform for centuries. It is a biological fact that manufactured ‘dry’ lumber that is exposed to liquid water, either continuously or intermittently, will absorb water and trigger the decay process. When lumber and lumber connections (i.e. nails, screws, and bolts) experience decay, they can no longer be relied on to function in-service as expected.

For the technical reasons cited, it is absolutely critical for the entire balcony and interface with the primary structure to be protected by a waterproofing system that protects all water exposed surfaces. For example, a guard system design detail may show a connection of the guard post to the outside balcony side(s). If the balcony sides are not protected from water entry, the guard system connections to balcony framing may be compromised due to hidden decay of the wood products.

In conclusion, waterproofing design/detailing by a professional is critical to the likely in-service performance of a balcony and balcony-structure interface. Further, field inspection of the waterproofing installations for strict conformance with the waterproofing details by the responsible design professional is recommended.

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