Where should joists be doubled under bearing walls?

Doubling joists under load-bearing walls that run parallel to them is crucial for maintaining structural integrity. Load-bearing walls transfer the weight of the structure above down into the foundation. By doubling the joists in these areas, they can better support the additional load without bending or failing.

This practice reinforces the strength of the floor system where there is significant weight concentrated from walls and upper floors. Adequately supporting these walls with doubled joists helps to distribute the weight evenly and reduces the risk of sagging or structural damage over time.

In contrast, non-load-bearing walls do not carry this kind of weight, so it is unnecessary to provide the same level of support. This makes it clear that not all walls require doubled joists, only those that are load-bearing and positioned parallel to the joists themselves.