I am helping my son remodel the 70 year old house that he just purchased. He is planning on expanding on the attic on his second floor. The current ceiling joist are 12'- 2"x6" with the old plaster lathe in the home. Since that won't support a load of 2 bedrooms and bath - a general contractor close friend suggested rather than putting a 2x8 or 2x10 to sister another 12' 2"x6" to the existing ceiling joists. He said this would be strong enough to support the load. This sounds like a great idea since the ceiling height is tight. Will this work? I would really appreciate a response so I can continue helping my son with this project. Thanks for your help.
Yes, you can sister 2x6 joists, but you have a few problems:
1) Code requires 40 lbs. per square foot live load plus dead load. 2x6 joists at 16” will span 12’ and support a floor live load of 40 lbs. per square foot plus a dead load of 10 lbs. per square foot. However, they need to be “Select Structural “ grade or better.
2) The existing plaster ceiling will crack with very little deflection from the weight of the floor. So, I’d isolate the floor load from the joists that support the ceiling plaster. In order to do that, the new joists need to be about 1/2” above the plaster, not attached to the existing joists and the top of the new joists needs to be 1/2” above the top of the old joists so when the new joist deflect the floor sheathing won’t touch the top of the old joists. (New 2x6 joists are going to be about 1/8” smaller so make sure you account for their size...)
Btw, the minimum ceiling height in a room is 7’, unless you have a sloping ceiling. Then a portion of the room can be under the 7’.
I’d add a couple of extra joists under the tub/shower too.
Btw, no waterbeds.