Sistering joists means beefing them up with additional material.
Sistered sistering floor joists.
Properly sistering floor joists is very important and should not be rushed to save time.
Since this is a repair of a faulty or damaged floor joist it is imperative to do this project correctly and there are a few big mistakes to avoid.
The section being inserted and sistered should be twice as long as the cantilever.
Doing so strengthens weak joists and can help straighten sagging joists.
Sistering joists for a cantilever requires removing the rim joist and sliding the new joist in beside the existing one.
So a 2 foot extension will be 4 feet sistered to the existing joist with appropriate fasteners 3 nails in a w pattern every 12.
This article provides step by step instructions and pictures on how to make structural repairs by sistering floor joists alongside weak joists.