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Flooring - Deflection and Vibration

Overloading of Wood-I Joists

Problem: Overloading Of Wood-I Joists At Cantilevers


Wood-I joists have limited capacity over bearing walls.

When wood-I joists are cantilevered and carry roof loads, there is considerable stress over the bearing wall or beam. The wood-panel webs can become overstressed and fail under severe loading. The high concentrated loads at the bearing support are more than the wood-I joists can handle.


Add web stiffeners:

  • For some loading situations, refer to manufacturer's recommendations, cantilevered wood-I joists need web stiffeners installed over the support wall. This strengthens the joists at the location where stresses are the highest. Web stiffeners are pieces of blocking, usually plywood or sawn lumber that are attached to each side of the web between the top and bottom flanges. A ± 1/4″ (6 mm) gap is left between the blocking and the top flange as shown in Figure 20.
Floor Joist Web Stiffeners
Figure 20 - Floor Joist Web Stiffeners
  • Fasten the web stiffeners securely to the web to ensure that the stiffeners and the web act as one unit. This effectively increases the capacity of the web and thus of the structural member over the support, allowing it to carry high loads.
  • The type and length of fasteners that are required will vary from one manufacturer to another. Always check the manufacturer's requirements, as the fasteners are critical in getting the web stiffeners and the web to act as one unit.

Add reinforcement:

  • When supporting roof loads, it may be necessary to install reinforcement to the sides of the wood-I joists as per the design requirements. Reinforcement is normally made from sheathing cut to the full depth of the joist and fastened to both the top and bottom flanges. This reinforcement is generally three times the length of the cantilever and extends from the outside of the cantilever into the house. It may be required on one or both sides of the joist.
  • Another option for reinforcement is to add another section of wood-I joist to the side of the cantilevered joists. The length of the added section is usually three times the length of the cantilever. A filler block is installed between the two webs, and the two sections are fastened securely through the webs and filler block as shown in Figure 21a and 21b.
Reinforced Cantilevered Wood-I Joists - Filler Blocks
Figure 21a - Reinforced Cantilevered Wood-I Joists - Filler Blocks
Reinforced Cantilevered Wood-I Joists - Blocking Panels - Sheathing Support
Figure 21b - Reinforced Cantilevered Wood-I Joists - Blocking Panels - Sheathing Support
  • Reinforcement sheathing or joists must be installed when the floor is being framed so that loads imposed by walls, upper floors and roofs can be handled properly. Once the load has been applied to a joist, adding reinforcement does not necessarily do the job properly. For reinforcement to be useful, it must assume the load at the same time the joist does.