Hi,
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 2×4 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, 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 2×4 redwood.
Many thanks!
Replies
I like to see floor systems at a length/depth ratio of about 18 to 1. So a 4" member would be good for about 6'.
To span 10' or so I'd probably go with 2x8s.
I ran a quick calculation. I assumed 15 psf dead load for the full length (this may be a bit high if the ceiling below is drywall, but a bit low for plaster. I assummed 40 psf for the room half of the span and 10 psf for the other (behind the knee wall). A HEM-FIR No. 2 2x6 is strong enough but the deflection was about 5% high, which is no big deal. Assuming you are sistering the 2x6s to the original 2x4 this should be fine. BTW, double 2x4s will not make it. Also, load from the roof through the knee wall can cause the joists to sag. It would be best to leave a gap between the top of the knee wall framing and the rafters. You could use a rafter clip or other bracket to support the wall laterally.