What is the maximum gap allowed between deck railing balusters under the Alberta Building Code for residential decks?

The maximum gap between deck railing balusters is 100mm (4 inches) under the Alberta Building Code. This is measured as the largest opening that can exist anywhere in the railing system — a 4-inch diameter sphere must not be able to pass through any opening.

This 100mm sphere rule applies to all types of railing infill systems, not just traditional vertical balusters. Horizontal cable railings, wire mesh panels, glass panels, and decorative metal infill must all meet the same 4-inch maximum opening requirement. The measurement is taken at the point where the opening is largest, which is often at the top or bottom of the railing where cables may sag slightly or where panels meet posts.

For traditional vertical balusters, this typically means spacing them no more than 3.5 to 3.75 inches on center to account for the baluster thickness and ensure the gap between them stays under 4 inches. Many Calgary deck builders use 3.5-inch spacing as a safe standard. Horizontal cable systems require careful tensioning and intermediate support posts to prevent sagging that could create openings larger than 100mm between cables.

Calgary's chinook wind cycles can affect railing stability over time. The rapid temperature swings cause expansion and contraction in metal components, potentially loosening cable tension or shifting panel connections. Annual inspection of cable tension and fastener tightness is recommended, especially after severe chinook events. Glass panels should be tempered safety glass and properly secured in their frames to handle wind loads and potential hail impact.

The 100mm rule exists for child safety — preventing small children from slipping through railing openings and falling from elevated decks. This applies to any deck more than 600mm (24 inches) above grade, which covers most attached decks in Calgary built off the main floor level. Ground-level decks under 600mm above grade don't require railings under the Alberta Building Code, though many homeowners choose to install them for aesthetics or to define the deck perimeter.

When installing cable or wire railings, pay special attention to the end connections and corner details where openings can inadvertently exceed 100mm if not properly planned. Professional installation is recommended for cable systems to ensure proper tensioning and code compliance throughout the railing run.

Need help finding a deck builder familiar with Alberta Building Code requirements? Calgary Deck Contractors can match you with experienced professionals who understand proper railing installation and local permit requirements.

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Looking for experienced contractors? The Calgary Construction Network connects homeowners with qualified professionals:

• Mr & Mrs Paintastic Inc
• Dealtwith.
• Bracha Concrete & Coatings Inc.
• Jk Stucco
• Turnbull masonry