I see the advantages of 30% less floor joists with increasing the spacing to 24″ from 16″ but does it outweigh an increased floor deck thickness. I know you can use 3/4″ still but doesnt’ that get a little too much deflection in between with 24″ spacing, plus that added cost?
I’d like to hear some arguments and justifications for each, or a link if this has been discussed before.
Thanks.
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Replies
That's an easy one;
Compare costs of joists at 16" + 3/4" ply to cost of joists at 24" + 7/8" ply.
Pick cheapest.
SamT
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In my house I used 24" OC floor trusses. The floor is NOT spongy between the trusses.
Around here' most guys use 19.2" OC for wood webbed trusses. Don't really kow that there's a reason - It's just done that way, so no one questions it.
I often go to 19.2" for I-joists and 24" for open web trusses.
As to the subfloor deflecting - the wider web truss means there is barely more freespan between joists than with solid wood. unless you have large ceramic floor tiles, it is no concern.
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I am shocked, really shocked, that you would use anything other than 16" for floor joist centering. ;-)
Why is that?
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Maybe I missed something, but it seems that everyone is mixing apples and oranges here. IBC code as I know it won't allow any spacing for #2 SPF beyond 16 on center. I hear you westerners use doug fir which is stronger, so maybe that's a factor in this discussion?But trusses and I-Joists are a different matter, the manufacturer would specify their allowable spread and its pretty standard from I understand that trusses whether floor or roof are made to be spaced at 24" on center. Am I missing something here?
"...its pretty standard from I understand that trusses whether floor or roof are made to be spaced at 24" on center."
Not exactly. What we do is input a truss profile - Like maybe an 18" deep floor truss spanning 24', for instance. The design loads are input, as well as the spacing. You input the web pattern and where you want and splice the truss.
Then you run the truss through an analysis to see if the lumber works or not. Chord and web grades are bumped if needed to make it work. Sometimes on floor trusses webs have to be doubled if there's a lot of force in them.
If the truss doesn't work you go back to the beginning and try something else.
If the lumber is OK, you continue on to the plating routine. A plate size is figured for each joint. Then you're basically done.
I think there's a pretty common misconception that if you oder a floor truss it's automatically designed to be 2' O.C. But that's not the case. Each truss is designed for whatever spacing it's intended to be set on.
If anyone is intreested, I can go over that in more detail. But that should give you an idea of how it's done.
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I see, now I learned something today.
Gravitas Design
the issue is span.. A short span 24 OC is fine longer spans require closer spacing to avoid bouncy floors.. (thickness of subfloor is only marginally affected)..
maximum unsupported 3/4" ply is 19", 24" o.c. would require thicker flooring.
Says who?
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Read it somewhere years ago, still used today by locals. Earlier post said they use 19.2" as centres, Some framing squares have a mark there. Been using that rule still for headering etc. O.S.B. manufacturer told us about 1" and more sheathing, specifically for 24" o.c.
The GP website says their "Sturd-I-Floor" span rated panels are good for 24" O.C.I think the kind I used was rated 48/24.
I called the family doctor this morning and said: "My Wife dislocated her jaw - do you think we could get an appointment in a week or two?"
I'm with you Boss - I never heard anything about 19" unsupported plywood - maybe it's different in Canada where Karp is from.
Free span of plywood using 2x and typical 3-1/2" wide wood web floor trusses [WWFT] - ( I- joists vary from 1-1/2" to 3-1/2" wide - at least from LP):
16" centers - 2x = 14.5"; WWFT = 12.5"
19.2" centers - 2x = 17.7" ; WWFT = 15.7"
24" centers - 2x = 22.5" ; WWFT = 20.5"
In my experience, the greatest cause of bouncy WWFT floors, is the installer ignoring the engineering note which sometimes says " 2x6 stiffback recommended at center location shown....." - translation - since the stiffback is not required, it gets omitted, and the BI can't say much since an LPE approved the installation without the stiffback.
I know that a 2x6 stiffback makes a world of difference in the feel of the floor. I used a lot of the 3-1/2" WWFT in HfH houses - my fellow house lead omitted the stiffback on his house next door - it was no problem to tell which house had it. He never omitted it again.
Jim
Never underestimate the value of a sharp pencil or good light.
"In my experience, the greatest cause of bouncy WWFT floors, is the installer ignoring the engineering note which sometimes says " 2x6 stiffback recommended at center location shown.....""
I'd have to disagree with that. The problem floors I've been involved with have all had bad length/depth ratios.
The strongbacks you mentioned will certainly help floor performance. But they're not a complete fix for a bad floor.
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My assumption was that the engineering & truss design was good.
>The problem floors I've been involved with have all had bad length/depth ratios.< I translate this condition as marginal / poor design.
Jim
Never underestimate the value of a sharp pencil or good light.
JTCI,
I have been doing 24" o.c. for years . 2x6 Walls , floors as needed for spans , roofs the same. Only go closer if plans call for it. Inline framing whenever possible. Reduces thermal bridging , costs less, meets codes.(Which is not to imply I don't exceed code when I build, I don't use a sledge hammer when a finish hammer does the job more than adequately either.) Am I such an old guy that only I remember the "Arkansas Framing " energy efficient framing techniques of the early seventies? 2x6 24" o.c. , notches at the bottom of studs for electrical runs, let in headers etc.?
Thanks karp,
Is the thicker flooring a positive trade off for less joists?
Don't thank me yet, Bosshog has taken issue.
As far as making the decision on centres, wider bays make for easier hvac install, maybe less labour. Material costs have to be compared, engineered products are not cheap. 19.2" o.c. is a nice compromise, still use 3/4" ply. I personally would not span any more than that with 3/4". But as you will find out, other posters have other opinions. What ever you decide, have your drawings engineered, including finishes.ie stone, ceramic etc.
I've seen more and more GCs, that sub out everything with the same crews time and time again, have their framing crews line up their 16" o.c. joists directly over the 16" o.c. stud framing. This allows better transition from stud cavity to joist cavity for plumbing, HVAC runs, electrical runs.
Yep, been doing it that way for nearly 30 yrs. although, we did it more to line up the load path. It was important with conventional lumber to keep things in line, not so much with the engineered wood products because the timberstrand rimboard resists the compressive force, not the joists. I still line things up (whenever possible) for mech. access. I also still use 16" centres, so it makes it pretty straight forward, but with 24" o.c. , how do you keep the bays open?
Sounds like you are the type of carpenter I like to hire.
OTOH, not to be rude, but you need to read and learn to understand the grade stamps on sheets goods like OSB and plywood.
All: Dimensional lumber = 16" OC. For engineered products, the spacing is greater 99% of the time excluding special situations like long spans, and even in that case a deeper member is usually a better solution than tighter spacing.
Thanks, do you pay travel expenses?
We must be a little behind the times up here, for custom homes, sub-floor is always 3/4" ply. O.S.B. never caught on. Its used in track homes, but not custom. Joist spacing is still typically 16" o.c. even with the "I" joists. It seems like the mind set , because the architects and engineers are all spec'ing it. I've only seen larger spacing, never actually built one myself. In following this thread, I haven't seen why I would choose one over the other. Still curious, sounds like alot of guys are used to using the larger spacing.
Matt,When you say "Dimensional lumber = 16" OC", are you saying that you can't use 19.2 or 24" spacing with dimensional lumber? The IRC2003 has span tables for 12", 16", 19.2", and 24" for dimensional lumber.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
I'm not saying you can't. I have just never seen it. So, basically, I was talking about common practice. I'll take your word for it so I'm not gonna look it up.
Matt,
I don't know where on earth you ever got the idea that all dimensional lumber = 16" o.c. Guess every building I have built for the last 30 or so years must either be fallen apart or going to soon.
Oh and by the way .. we have had code (UBC , CABO) Since sometime in the late 60's. One can space framing lumber to whatever distance one wants as long as the sheathing/decking can carry the span being asked of it.
O.C. distance has nothing per se to do with a quality job either .. it is th composite structure that matters. Ask a dome builder how far apart his supports need to be .
Like I said 2 posts above, 16" OC for dimensional lumber is what is common practice (here) - insert "in modern times". It's just a rule of thumb. Our code and span tables do allow for greater spacing, it is just rarely, if ever done in new home construction. For that matter, by our code, I think 2x6s are allowed for floor joists too - I've just never seen it on the primary floor system of a house that was built within the last 15 years or so. Sometimes we do 12" OC for long spans, but that's is "iffy".
I have no idea what you all do in Oregon since I've never been to Oregon, and have only been to that corner of the US like for a total of a week. I do know that you all use different lumber than we do. Kinda like when Boss Hog said that most guys do floor trusses at 19.2" OC. I could argue that point, but I don't live where he does.... GirlBuilder said IBC only allows 16" OC. I'd have to wonder about that too but we don't use IBC - we use IRC and before that, we used CABO.
Are you saying that a lot of what you have built with dimensional lumber is > 16" OC?
Matt, What I am trying to say is that 16" o.c. is one spacing that works under certain circumstances. 24" o.c. is far more common here I believe.
While it may be common or the standard practice in some or even all areas it is not a requirement anywhere I know of. I have built with o.c. for floors for instance at anywhere from 8" up to 8'(using beams as joists) Each was specified because of design consideration on the part of the engineer/architect or designer.
New construction joist size and spacing is always specified by the eng/archi for primary floors here in NC. For decks for example, one has to rely on code and expierence. My code book is in my truck and tattered. :-)
It's interesting that you see a lot of 24" oc spacing for dimentional lumber framed floors there on the west coast. So, I learned another thing today...
Thanks for the info. :-)
>> One can space framing lumber to whatever distance one wants as long as the sheathing/decking can carry the span being asked of it. <<
Well.... That's debatable... So, you are saying that if the grade stamp on a piece of plywood says 32/16 you can space your 2x4 rafters at 32" ? :-)
Never mind... I know what you meant... :-)
Check out the flooring section at this Weyerhaeuser site:
http://www.ilevel.com
I think it does a pretty good job at describing floor "quality" and people's perceptions of floors.
ps: If i screw this up, please forgive me: it's my first post!
Glad to see you've quit lurking and started posting.Keep it up.
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Thank guys. I am using open web trusses w/ 2 1/2" chords. The trusses will be designed to take out the bounce, no problem. I guess I should have differentiated bounce from, lets call it "squish", or the plywood or OSB deflection between joists. I dont' want to have a L/560 or greater joist system and then have deflection between the joists w/ the decking.
Good answers though, I'll use you guys to settle all of our in house debates from now on.
"I'll use you guys to settle all of our in house debates from now on."Sure thing - you'll always be right then!LOL
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Are you sure you're using open web trusses, and not I-joists, with 2-1/2" chords? I'm not assuming that you don't know the difference, rather I've never (to the best of my recollection) seen floor or roof trusses with anything smaller than a 2x4 member.Maybe I just need to get out more.And for what it's worth, we just used open web floor trusses at 24" O.C. spanning 24' (16" deep truss) and the sheathing was 3/4" Advantech. I was wondering if we were going to have a rigid floor and if the subfloor would have too much deflection. Surprisingly, neither one seems to be a problem.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
I'm not 100% certain that he means 2X3 WW floor trusses, but it's possible. A few years back the company I worked for then was approached about the possibility of building 2X3 WW floor trusses in order to be more competitive with I-joists. The theory is that there's generally only ONE span in a house that's maxed out and needs the 2X4 lumber. The rest could be made out of 2X3 in order to keep the cost of the floor trusses down.The company I worked for didn't go for it. I haven't heard much about it for 5+ years, so I'm guessing it was never a wide spread practice. But it is possible...
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Yes Jon, it is an open web truss w/ 2 1/2" chords. It is an all steel top and bottom chord from Premium Steel. 2 1/2" at the top dim. then it rolls down into a "T" shape to grab the web members.
Good point guys about the direct bearing 16" joists on 16" studs. I have drawn it this way often and even tried to eliminate doubling up the top plate to save time and resources but the engineers and or the builders keep putting it back in out of habit.
Got any pictures? I've never seen any floor trusses quite like that.
If it wasn't for plumbers, you'd have no place to go.
If you are building a subdivision it would be worthwhile to go to 24 OC and save a few bucks a house.
However if you are building one then count. How many more floor joists are we talking about here. You might be talking about 100$ total. More than likely you'll spend that on the miscolored paint in the structure.
In the big picture 100 bucks spent making the floor stiffer will help you when you put down ceramic tile, and it will go a long way to killing the bounce.
I'd concentrate on making sure the plumbing flanges are clear of the joist locations. You don't want to be cutting up your supports to set a toilet.
As a footnote I did put Floor Joists on 24 inch centers glued-nailed on 12 inch I-joists w 1-1/4 inch T&G OSB subfloor. It is solid but it would have cost 52 $ to go to 16 inch OC.
Your real issue is attaching qualitative decisions to quantitative specs. The Warehauser (sp) specs on the Tru-Joist are what you need to figure. It is simply span length, the centering, and the topping and possibly the bottom (if you are drywalling the bottom of the floorjoists say in a basement rec room.)
Now walls and rafters, that is a different story. With insulation the less web conductivity the better. With flooring see if the specs and the $ speak to you.
Edited 4/5/2007 11:54 pm by booch