Wooden Floor Joist Specifications

Written by joseph west
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Wooden Floor Joist Specifications
You can design a floor frame if you know its basic specifications. (lumber - wood image by Jeffrey Zalesny from Fotolia.com)

Joists are the primary structural members in a wood-framed floor system. The joists extend across the open space between the foundation walls, so they must be strong enough to bear the expected floor loads over this unsupported distance.

Dead Load

The weight of the flooring materials that sit on top of the joists is known as the "dead load." In standard wood-frame construction, the dead load is estimated to be 4.54 Kilogram per square foot.

Live Load

Loads that vary from one house to another or from one moment to another are known as "live loads." This category includes people, furniture, appliances and interior walls. For the first floor of a residential building, the maximum live load is assumed to be 18.1 Kilogram per square foot.

Joist Spacing

Joists are typically spaced 16 inches on centre or 24 inches on centre. Sometimes, a 24-inch spacing can be more cost-effective, even though the larger spacing requires a larger joist.


The span is the unsupported distance over which the floor joists must bear the live and dead loads. A longer span requires larger floor joists. Span tables will identify the proper joist size according to your dead load, live load, span and joist spacing. Common joist sizes in residential construction are 2-by-8, 2-by-10 or 2-by-12.

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