Expert: Dan Griffin - 1/7/2008

Question
I want to convert my attic into usable living space. My attic have 2 X 8 ceiling joist for the second floor ceiling, with 1 X 9-inch tongue and grove plank flooring attached to the top of the joists for attic flooring. Will the 2 X 8-inch ceiling/floor joist need reinforcing to support the additional weight? The present roof framing is 2 X 4-inch lumber with a "A Frame shape". I will be adding knee walls to this space. The area is 24' X 44'3"

Answer
I assume you have a center bearing wall under this attic.  This wall would need to divide the house down the center so that your attic 2x8's are 2 separate 12 to 14 foot boards spiked together at mid point being carried by the wall below.

It seems a bit unusual to have 2x8 joists unless the original builder planned on finishing the attic some day.You need to double check the span on the existing 2x8 joists.  This requires the distance from the outside wall to the center bearing wall.   The other thing to double check is how close together the joists are.  Normal patterns would be 16", 19.2", or 24".  You should be able to look at the rows of nails in the attic flooring to see.  The one other thing we wish we knew would be the grade of lumber.  There may happen to be some grading stamps on the exposed roof rafters.  Example SYP (southern yellow pine) #2.

Span tables and code books use L/360 deflection for joists.  YOu have an existing ceiling load and I assume you are looking at this as bed room type space which will require at least 30 PSF live load.

#2 SYP, 40 PSF live load, 10 dead load at
16" o.c. can span 12'- 10"  
24" o.c. can span 11' - 0"