I’m looking at finishing the attic in a house built in 1939. Existing attic floor joists are very dense older southern pine 2×6’s (1-3/4″ x 5-3/4″) at 16″ oc. the worst span condition is 12′-2″ (about a foot more than the 11′-2″ span that most joist tables recommend for a dense southern pine 2×6″) i’ll probably be adding a knee wall and the ceiling on the first floor is plaster, so i’m worried about deflection cracking the plaster.
Can anyone suggest options other than adding deeper joists in this area?
Replies
loose weight. :)
Seriously I really can't think of anything other than bigger joists. I know you don't want to sacrifice any head room, but even old 2x6' (which in my old house really are 2"x 6") arn't enough for a 12' foot span, and certainly not for plaster. Maybe someone else here has some magic in them.
Good luck.
Justus Koshiol
Running Pug Construction
Edited 12/4/2002 10:16:27 AM ET by Justus
Detail the knee walls like a floating wall in truss construction so that you aren't transferring any rafter loads to the joists. (I can almost always tell whether the knee walls are in Cape Cods by the first floor ceiling crack locations.)
Of course, I would never advocate overspanning a joist, but I'd be willing to bet that a 63 year old joist is a heck of a lot stronger and can span more than a new one.
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"I may have said the same thing before... But my explanation, I am sure, will always be different." Oscar Wilde
There are several way to accomplish what you want to do without changing the depth of your joists.
One would be to install a glulam beam, or a few LVL's perpendicular to, and directly above the joists, so that the inside face of the beam is in the same plane as the kneewall. Then use the appropriate framing hardware (most lumber yards have Simpson catalogs to pass out) to attach the joists to the beam. Now you not only are taking the weight of the roof off the joists, you are shortening the span of the joist as well.
Another way of accomplishing the same thing, but harder to get structurally approved without an engineer, is to turn the kneewall into a box beam by inserting diagonal framing between studs and sheething both sides of the wall, staggering plywood joints. These box beams are great for retrofits, I've used them several times over the years, but like I said, it's harder to be certain just how much weaight they can actually take. DO NOT use drywall screws as fasteners, nails are much stronger.
A third way would be to add 4x6's, or 6x6's as supplemental joists, maybe 12"o.c., you could figure the span/spacing requirements from a common span table for whatever species framing lumber you have available.
You might be able to have special floor trusses built. Call around to local floor truss companies, it might be cheaper than you think.
The strength of a beam increases with the square of the depth, and linearly with width. Doubling the width by sistering new 2x6's would roughly double the strength of your floor without increasing the depth. This is out of the realm of standard tables and will require specific calculations. Your building department will probably want an engineer's wet stamp before giving you a permit.
The following is a very rough calculation, not to be relied upon for construction. Liability is limited to what you paid for this advice. Consider a uniformly loaded simple beam, maximum bending moment is given by:
M = (wL^2)/8
You're OK as is with L = 11'-2", and want to change it to 12'-2" Doing the arithmetic, 146" squared over 134" squared is about 1.19. You can double M by sistering, and you only need to multiply it by 1.19. Therefore it is likely that when your real engineer does the real calculations, it will turn out that sistering your joists will work. If you measure and draw everything well enough, your engineer may be able to do the job without the expense of a site visit.
-- J.S.
I had a real engineer do something similar for a 10'6" span and an 11'8" span. In both cases, we used doubled 2x6 and the floors are sound still.
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Not a problem....get steel flitch plates the size of your joists and sandwhich it between the existing joist and anothr 2x6 bolted every 12" or better. Kind of expensive but I bet it works. Dbl check with an engineer,
Have fun
Namaste
Andy
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