Expert: Dan Griffin - 3/8/2011

Question
I have a lot of broken ceiling joists directly above a knee wall in the attic. The Ceiling joists are on 2 x 4's on 16 inch centers and the span is a bit over 12 feet. To bring it up to code, I would like to sister 2 .x 6's how do I join them to the joists. The existing joist are just sitting on top of the main supporting walls. The top of the wall has 2 2x4's lieing on their side one on top of each other. I was looking at all different types of joist hangers but could not find anything that would seem to work.

Answer
Sonia, not quite sure I understand the dilemma.  A knee wall implies that the joists (?) in question are actually roof rafters that may well also function as ceiling joists.  The fact that they are broken tells me they have had an excessive load imposed on them.  Sistering is a very legitimate modification to rectify the problem.

I would prefer calling these things rafters to simplify some of the terminology.  I assume you need the tops of the new 2x6's to be at the same elevation as the tops of the 2x4's.  You will need to make a plumb cut at the top of the new rafters that matches the slope of the existing.  You may need to add another board directly under the ridge rafter to receive the full bearing of the new 2x6's.  All new pieces need to be crowned up.  YOu may need to use a mud sill and jack to raise the broken 2x4's to remove any belly in them - they will need to be straight or even crowned up slightly before sistering.  There will be some type of seat cut involved on the new rafters at the knee wall.

There is a chance that some of the original problem had to do with cutting out too much of the 2x4 for the seat cut which allowed them to break.  If this is the case, you may want to consider moving the knee wall or shortening the knee wall to allow a heavier section of rafter across the knee wall.

Without being able to see the problem, I can't tell much more.  I hope this has helped.