Another project on this 1912 Victorian is to consider turning the very spacious attic into a room, and possible small bathroom. Currently, the joists are full 2x4 redwood in great shape, 16" on center.

The center of the room would be the center of the house, which is a load bearing wall that runs down the middle of the house. The room would be about 10-12' wide, so the edges of the room would be in the middle of a 10 foot span, or 5-6 feet from the load bearing wall, since the house is 21' wide.

Are the current joists strong enough for this? What is the minimum size beam I would need to sister in order to make the floor strong enough to be safely used as a bedroom/playroom? The ceiling height is already marginal, so I'm trying to maximize headroom here.

I did look at a joist span calculator, but couldn't find specs for full-dimension 2x4 redwood.

Many thanks in advance.