Expert: Dan Griffin - 1/5/2011

Question
Dan,

I have a 2100 sqft home built in 2000.  It's post tension slab, brick, hip and ridge roof.  I'd like to convert the wide open attic space to family/game room.  I have a good place on an inside garage wall to build a staircase with entry from the main living area.  The ceiling joists are 2x10 throughout and I'm pretty sure it's #2 SYP.  The roof is fairly steep so there's plenty of headroom for a span of about 14'-16' across the middle.  For the most part, I'll be able to put the attic walls directly over 1st floor walls, and the the tops of the attic walls will hit the rafters near to where they're currently braced.  My main concern is over the 1st floor living area and the garage which are both 16' span. I'm a little concerned that 2x10 is not enough for those long spans, and/or the extra activity upstairs will cause nail pops in the ceiling below.  Do you recommend 'sistering' 2x12 or engineered trusses to the existing joists? I'm also debating on a couple of dormers with windows versus a 2nd stair/ladder that would exit through the floor onto a back covered patio.  I will have an A/C pro come out to move the inside unit, ductwork etc.  Thanks so much, and I welcome any other considerations I may have missed.

Answer
Dixon,
There are many span tables available, they all require knowing what the lumber truly is as to species and grade.
Your code requirements will be 40#/SF live load, 10#/SF dead load, with L/360 deflection
here is a span table from the southern yellow pine council that is easy to use:
http://www.southernpine.com/spantables.shtml

I prefer using this one from the American Wood Council because you can vary the species and loads:
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

It looks as if your 16' span on  #2 YP is ok:
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp?species=Southern+Pi...


Minimums will always allow some deflection. Note that even L/600 deflection limits would allow a 16' span to deflect .5333" (about a 1/2").  I would not expect you to have many issues with the span as it exists.


Make sure that you maintain cold air movement in the roof structure to avoid frost issues on the roof.  Your insulation work needs to be performed on the walls and ceiling of the new space without blocking the present ventilation.

Sounds like a great project.  Sometimes people look at the square footage and ignore how much space is taken up by stairs both above and below.  I'm glad you are thinking through all the issues.  Your existing HVAC may not be able to handle the additional footage and second floor air dynamics may affect existing rooms.  One solution might be to use doors on the stair well.  It would be great to discuss variables with a good HVAC man before you get too deep in the plans.