I’m in the planning stage on a joist floor over crawl space (my own house, not a pro carpenter). There will be stone tile in some areas and some heavy live load items I plan to double the joists under. My original plan called for #2 douglas fir 2X10’s 24″ OC, 10 foot span between the stem wall and a central girder (2 2X10’s). An engineer who was reviewing the plans for other reasons suggested changing the spacing to 16″ OC and making the girder from 4 2X10’s. Now, I thought this a bit excessive, but figured it couldn’t hurt to play it safe as he’d presumably done some calculating, whereas I was just following an L/360 table and over estimating for good measure.
Now the question comes out. While playing with an online span calculator, I find that I can use the same grade of 2X8’s and still span 10′ at 16″ OC, with the maximum loads the calculator uses (40 live, 20 dead), and still be at L/720. Does this sound right? Any thoughts as to who’s further out in left field, me or the engineer? And I am assuming that the L/720 deflection will be ok for just about anything I want to cover the floor with.
TIA for any thoughts on the subject,
John
Replies
I have read your post twice and cannot tell anything about the area the beam is supporting, so I cannot comment on that.
But as for floor joists, it is rare for joists to be placed on 24-inch centers, and your engineer is just recommending you move back to what is generally considered acceptable for good floor performance.
You'll want L/720 floors for your stone finish, and need to do whatever sizing is appropriate for that deflection, using loading of up to 50 live and 20 dead.
Whether brick, slate, granite, or heavy tiles, you'll be spending a lot on that floor finish, and it is unwise to try and shave down the floor structure underneath to save money. Framing lumber is cheap compared to what you are are doing atop it.
If you go 24" OC on the joists you had better use some serious subfloor.
Thanks! Thats just what I wanted to know. I put 50 in for the live load and span shrunk dramatically.So it sounds like 2x10's are the way to go, unless I want to order structural select 2x8's or put them on 12" centers. I'm leaning towards the 2X10's, as the live load can get almost up to 90 before the span gets too small.Does make me wonder why floor joist span tables include a 24" spacing. I guess thats for Motel 6's where you can feel someone walking in the hall 200 yards away.John
Your engineer is right
As to the 16" oc layout. there are two deflections you are dealing with, especially for a stone/tile floor. one is th edeflection the joujsts allow. but the other is the deflection of the subfloor between the joists. That is the basis of another comment here.
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Not much to add to the above but I'm just gonna tell you what is standard practice where I live (NC): Most all primary floor systems are framed with 2x10, 16" OC assuming that dimensional lumber is going to be used. A center girder would normally be min three 2x10s, but that also depends on how far apart the piers are that support the girder.