I’m new to the forum but thought it would be a good way to find some info I’m needing. I’ve been in construction for about 5 years now and am building a 28 by 26 edition for a couple in Kentucky.
They want to space the joists at 32″ centers. They are trying to achieve an older cabin/cottage or post and beam type look. Does anybody know where I can find information on joist sizing and floor thicknesses for this type application without consulting an engineer?
Thanks!
Replies
Yeah. Use eight by twenty eight oak joists with four by eight decking.
Jonathan, try these two books on timber framing. Depending on code enforcement in your area the span tables listed may or may not be acceptable, but regardless these are two bibles of timber framing.
http://www.amazon.com/exec/obidos/ASIN/1561581291/qid=1112832952/sr=2-2/ref=pd_bbs_b_2_2/103-1195175-2566223
http://www.amazon.com/exec/obidos/ASIN/0882663658/qid=1112832952/sr=2-3/ref=pd_bbs_b_2_3/103-1195175-2566223
Mike
1 1/8" plywood subfloor is designed to span 48" centers, though my personal opinion is that that's a stretch.
But for 32" centers it should be fine, discounting any extra beef needed for bearing walls, masonry structures, book storage, etc.
The deflection of a joist varies directly with the load. At 32" spacing, each joist is carrying 1/3 more load than it would at 24", so the joist stiffness must be increased by one third to maintain the same deflection. Stiffness of a joist varies directly as the width, so increasing the width by 1/3, say from 1-1/2" to 2", will increase the stiffness by one third. Alternatively, joist stiffness varies as the cube of the depth, so increasing the depth by 10% (actually 10.0642416%) will increase the stiffness by 1/3. (1.1 cubed = 1.331)
The deflection of the subfloor is a little trickier because you have more load and greater span between the joists. With the greater load and the greater span, the original subfloor - whatever you would have specified for 24" spacing - will deflect 3.16 times as much as it would have at 24". So the subfloor has to be 3.16 times as stiff to achieve the same deflection. Stiffness varies as the cube of thickness, so increasing the thickness of the subfloor by 47% will give you the stiffness you need. (1.47 cubed = 3.18) If you would have used 3/4" ply at 24" spacing, 1-1/8" will work at 32" spacing.
"Stiffness of a joist varies directly as the width..."
Stiffness is more a function of the length/depth ratio.
Width adds strength. Depth adds both strength AND stiffness.
Our biggest problem is that nobody wants to take responsibility for anything—but don't quote me on that.
the only thing that will look correct is to use what they used to call car siding.. it's an 1 3/4 thick tongue and grooved six inch wide boards.. the timbers below will need to be at least three inches thick and probably thicker for the proper look..
If you size everything to meet the minimium structural requirements things won't look right.. that plus any timber will sag because it's installed green to minimise that effect you'll need to oversize the timbers!
The joists shouldn't be any big deal. Just find a span chart for joists 16" O.C., but with double the load.
For instance - Joists 16" O.C. at 100 PSF would have the same PLF load as joists 32" O.C. at 50 PSF.
For the plywood, you're probably better off checking with a manufacturer of something like the 1 1/8" thickness mentioned.
In the interest of CYA, I'd have an engineer spec the joists...and anything else funny your client wants you to do. I'm not suggesting you can't calculate the spans, but having an engineer sign off removes any worries. I have a structural engineer review anything that's not obvious (and some things that are). Rather than making me look unsure, bringing in the engineer adds professionalism.
Make the client pay for the engineer, by the way. Or build it into the job.
Thanks for all your replies, that gives me several leads!
Jonathan