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Unread 11-24-2020, 11:00 PM   #5
Veteran DIYer- Schluterville Graduate

STAR Senior Contributor

Join Date: Jul 2004
Location: Nashua, NH
Posts: 14,225
The core or rib of a I-joist is mostly to help hold the top and bottom pieces in their proper orientation. Blocking can help limit twist when a load is applied, that helps maintain the vertical deflection and share the load between adjacent runs. You can get similar results by adding strapping, along the bottoms, tying them together. Gluing and properly attaching the subflooring on top of the joists keeps each of them aligned and working in conjunction with each other.
Adding drywall to the bottoms of the I-joists can perform a similar action, but may be better done with the strapping first, as the screws will just eventually elongate the holes through the drywall slightly.

The I-joists work by the top cord being compressed, and the bottom one being stretched. The middle just holds them in position, so sistering doesn't do all that much. That's one reason why you can cut holes in the webbing more easily in an's not providing all that much strength to the assembly.
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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