Expert: Dan Griffin - 8/28/2010

Question
I am in the process of replacing 2x4 joist with 2x8 joist.The room is 15ft wide X 22ft long.I am placing them 16" on center with 3 sets of doubles,one in the middle two half the distant on each side of that.with 5/8 sheeting on top.I am placing 2x8 on both sides of the room with lag bolts to the studs to hang my joist with hangers.
My question is do I need any other support to the ceiling as I am turning the attic space above into A bedroom?

Answer
Glen, I am mildly uncomfortable with the lag bolt install to carry the entire load.  It would be so much better to have the rim joists bearing on good structural uprights.

Are the walls in the downstairs room stripped so you have good access to the framing?

Why do you have doubled up joists? - concentrated load?  I am surprised you decided to loose so much head room in the downstairs room.  It would have been more normal to install the 2x8's bearing on the top plate, the way I suspect your 2x4's were doing.

Rim hoists used as load carrying members are very prone to problems.  Lags along a continuous line create a weakness that can allow the rim to split.  At the very least make sure that all fasteners are staggered.  The loads are all in the tension side of the rim joists (the bottom), so lags in the upper 2/3 do little good.

The size and spacing of lags to carry a 40#/SF load is substantial.