Balconies, Stairs, and Landings April 4, 2015
Of the various types of balconies, the overhanging or cantilever types are potentially the most dangerous. Balconies can be found on single family dwellings, but most frequently are found in condominiums or apartments. A balcony can totally collapse if it becomes structurally unsound. Many deaths and serious injuries have resulted from collapse. Balcony live loadings are required to be higher than other parts of residential structures by building codes. The typical interior of a residential structure is designed for 30 to 40 pounds per square foot live loads, but 60 pounds a square foot is specified for balconies and landings.
Balconies have protective railings and such railing is subject to failure. Failure can be a result of the uprights being ripped from the supports, screws pulling out of the wall or the railing simply coming apart. In addition, the balcony can have verticals that are too far apart. The current code requires a four inch clear space max. This has been found by test to be the maximum spacing to prevent a small child from squeezing through.
A good load test is whether or not a balcony railing can withstand a 350 pound lateral force. Think of a group of young (or old) men horsing around during a party and 350 pounds does not seem like enough. Deaths and serious injury can result from the failure of railings on balconies.
Frequently, balconies partially exposed to the weather have been seriously weakened by damp rot of the wooden framing. Damp rot is particularly insidious since it is often not detected from the outside. Sometimes the only clue is a slight staining or mildew appearance on the outside of the soffits. Moisture typically penetrates cementitious decks because waterproofing has failed or the flashing has been improperly installed. The cementitious material can hold moisture next to the wood for extended time periods. Dampness on the interior of an enclosed framed area can cause damp rot to destroy the framing in as little as six months time.
Experience has shown that exterior wood posts should not be set into concrete, as this is a sure recipe for damp rot of the base. Concrete depressions will always hold rain water and rot follows. In many cases just having wood in contact with concrete on the exterior leads to unhappy results. An air gap above the concrete surface is preferable. All the preceding comments can apply to exterior stairs and landings as well.
In one unusual case, a number of cementitious decks for stair landings and balconies completely self-destructed over a four year period because of expansion of the concrete surfacing. The cause was discovered to be a layer of gypsum based floor leveler compound which had been placed on the balconies. This was in turn overlayed with 3/4 inch of pea-gravel concrete. The gyp material was intended for internal use only as a level-up under carpet, and specifically not recommended for exterior use. When moisture reached this layer, it began to grow. In addition, the cement of the over-lying concrete was found to chemically react with the gypsum. The result was a major lateral expansion of the balconies and landings of up to 6 inches. This horizontal movement displaced the railing supports, opened the waterproofing protecting the plywood deck and framing, and caused large pieces of deck to fall. A complete re-build of hundreds of decks and landings was required.
If you are responsible for building or own a structure with a balcony, exterior stairs and landings, I would counsel extreme caution and be certain that they are structurally sound and safe in all regards.
Kirby T. Meyer, P.E.
MLAW Consultants & Engineers
Originally published in MLAW Newletter, February 2003