# Flooring - 24 OC joists



## aaronled (Jan 25, 2010)

Hello all!

I am brand-spank'n new to this forum...and I have a question for any pros out there -

I bought a house a couple months ago that I would like to completely renovate.  One of the first things "we" would like to do is rip up the nasty flooring and install tile and hardwood.

However - I noticed that the floor joist are 24 OC  
While the home inspector said that this will be fine for any flooring I wish to put down, the skeptic side of me wants to reach out to the masses for some more input here...

The facts -
The house is 9 years old
The joist are TJI pro 150 engineered (11 7/8")
The sub is 7/8" thick (1/8" thicker than the norm - 3/4")
The span (at the longest point) is about 15'
The cross beam supports are steel

The floor certainly doesn't feel "spongy" or anything - but I'm still concerned...  I used to frame houses when I was younger, and I have never installed ANY floor joist wider than 16OC.

The inspector claimed that this floor was more sturdy than a floor comprised of 2x10's 16OC - and I would be fine putting down any floor.

All that being said -
Will I run into any issues with tile and wood floor?  I am looking at a ceramic tile (18x18 or 20x20 - suggestions on thickness?).  The wood floors are still undecided at this point.  

I am concerned that there would be some kind of sag developed over time and cause tile grout (or the tile themselves) to crack, etc.  The wood floor wouldn't be as big of a concern in my mind being that the planks will run perpendicular to the joist - however, there will still be seems in between in spots (I will stagger the joints - but still)

Any advise out there?  Are my concerns warranted - or am I being paranoid!?

Thanks in advance!


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## Wuzzat? (Jan 25, 2010)

"
jjcold
04-15-2004, 04:18 PM
I can't seem to access the TJI info from their website, I can't open the PDF with my available programs. Anyway, could someone post the specs for TJI pro 150 I beam joists, 10 inch depth, set 16" OC, single story home. I'm looking for max span on these joists. My new house has too much floor bounce, and I think they made a mistake to use only one center beam that creates a span of 15'. My house was extended 2 feet on the back end, that made the span 2 feet longer, and I just know the building department wasn't presented with plans that reflect that change. Thanks.
Rich

04-15-2004, 06:29 PM
TJI Pro 150 - 9.5" depth @ 16" o.c. with L/480 deflection criteria
Max Span with 40 Live Load & 10 Dead Load - 15'-7"
jjcold
"



aaronled said:


> *1* reach out to the masses
> *2* I am concerned that there would be some kind of sag developed over time
> *3* would be some kind of sag developed over time
> *4* am I being paranoid!?



*1* http://en.wikipedia.org/wiki/The_Wisdom_of_Crowds

*2* Probably no one publishes service lifetimes or MTBF for their products, but you might get computer projections. 

*3* a sag would compress cracks closed.

*4* http://en.wikipedia.org/wiki/Paranoia
http://en.wikipedia.org/wiki/Paranoid_personality_disorder
If you work the numbers and you are still not convinced and you have other symptoms, then yes.  
But - you are in good company!  The first choice for world leaders is narcissistic, followed by obsessive-compulsive, followed by paranoid [but not in an open society for this last one].


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## aaronled (Jan 25, 2010)

Thanks for the reply...

I have seen the span tables - and I'm within code...  That's not really my worries.  Being "to code" never really tells the whole story.  I am concerned with how heavy tile will fair over time - and I'm reaching out to anyone that may have experience with this situation.



Wuzzat? said:


> "
> *1* The Wisdom of Crowds - Wikipedia, the free encyclopedia
> 
> *2* Probably no one publishes service lifetimes or MTBF for their products, but you might get computer projections.
> ...



1) Thanks for pointing that out - I was worried that using the internet to collect knowledge was just a big 'ol waste of time...now I have confirmation...
2) I'm not an astrologer - but I would think that this would be a hard stat to list being that it would be hard to 'standardize' how people will treat / abuse their floors - weight of furniture / (people  ) - moister - yadda yadda.
3) Could you clarify on this point?  compress cracks closed?  surely if a crack is present in a sag, traffic would deteriorate the tile? no?
4) ...

The reason I am trying to gather some opinions here is b/c I'm trying to determine if it's worth the time / money to add sister joists.  I haven't yet finished the basement and the existing floor will soon be ripped up...so now would be the time if i were to add joists...


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## inspectorD (Jan 25, 2010)

Speaking from experience, the larger the span.. the more you have to worry about...of course.
My concern would be any joints if the plywood was not toungue and groved.These will be weaker areas.
Adding another 3/4 ply on top would help, or go old school with a 1 inch mortar and mesh base on top of the existing ply.

And I would not remove the plywood you have, if it is glued down, you will ruin the TJI flange...just in case you were thinkin of that route.


Just when we need Mr. Cline, he pulls an invisible man.


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## Wuzzat? (Jan 25, 2010)

TJI Floor Joists Span Table)

It's OK for a 40/10 PSF live/dead load @ L/360 and L/480 but not for a 40/20 PSF live/dead load.

As to the strength of 2x10's @ 16 OC this paper gives you the spans
http://www.awc.org/pdf/STJR_2005.pdf

Use 1.9 as E for douglas fir in Table F-2.
I get a 17'-4" span for a 40/10 PSF live/dead load @ L/360, so the 2x10's are stronger.


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## Bud Cline (Jan 27, 2010)

aaronled,

Your structure as it is is fine for the minimum overall deflection required for ceramic tile of 1/360.

Your problem with this system is going to be "deflection between the floor joists". This is ALWAYS the issue with floor joists 24" o.c.

The solution is somewhat simple however. The Tile Council of North America recommends: A double plywood floor is required using exterior grade plywood. The face grain of the plywood should run perpendicular to the floor joists for maximum stiffness. Fastened with screws spaced six inches in the field and four inches around the perimeter of each board.


Your inspector is dead-wrong and obviously has no knowledge of what is required for a tile floor.  You were very wise to question his claims.


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## aaronled (Jan 27, 2010)

Thanks for the input!

I have been back and forth over this for a while now.  

I'm truly contemplating adding in joist to be 12OC...  I know this is a big job - BUT, the basement is not finished and I will be ripping up the existing floor anyway - SO if I were to ever do this, now would be the time.

If I do this, I'm not so much worried about re-running the electrical (and all the plumbing perpendicular to the joist are ran underneath the joist instead of through - the duct-work doesn't appear that threatening either) - however, I would like to add them in without ripping up the sub floor...

I don't know if this is wise (to beat them into place - and somehow glue them) - but like inspectorD pointed out, I will be contending with a glued flange on the existing joist, and I would prefer to leave them be.  You guys think this is a dumb approach?

One more Q (for Bud Cline - if he stops back by ) - If I were to leave as-is, would this "deflection between the floor joists" still be a concern being that the sub floor is 7/8 tongue and groove?

Again, guys, much appreciated!


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## Wuzzat? (Jan 27, 2010)

aaronled said:


> would this "deflection between the floor joists" still be a concern being that the sub floor is 7/8 tongue and groove?


So the "beam span" is 2' and the beam "height" is 7/8"; what type of wood is it?

A beam in bending has the top surface in compression, so cracks above a curved beam would be compressed close.  The bottom is in tension so this surface is stretched and lengthened.


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## aaronled (Jan 27, 2010)

Sorry, Wuzzat - I'm not understanding your question...

The joists are 24OC
The span (at the widest point of the house) is 15'
the beams are steel (don't know the height dimensions - but they certainly are beefy enough for the cross run)
7/8" is the thickness of the sub-floor (tongue and groove...plywood)

Not sure where the "curved beam" is coming into play here...  

From my experience with tile - a crack in a sagging point will only get worse with time.

Sorry if I'm not understanding you right...little lost


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## Wuzzat? (Jan 28, 2010)

aaronled said:


> Sorry, Wuzzat - I'm not understanding your question...
> 
> The joists are 24OC
> The span (at the widest point of the house) is 15'
> ...


Sorry: 'beam' should have been 'joist'.

With a load on it, the subfloor will sag between the 24" joists, in addition to the 15' joist sagging over its span.
To calc. the small deflection you will have over 24" you need to know the modulus of elasticity, E or MOE, of the wood.  For Select Structural, SS, larch it's ~1.9 million PSI.

According to table F-1 in this link
http://www.awc.org/pdf/STJR_2005.pdf
E ranges from 0.8 to 2.4 million PSI for lumber used in structures.

Here's a microscopic look at beams/joists in bending
http://en.wikipedia.org/wiki/Neutral_axis


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## aaronled (Feb 19, 2010)

Sorry for the delayed reply - busy month 

Thanks, Wuzzat, for all the info!

After thinking about this for the past month, I would really like to spearhead adding joists...

As I said, I do have construction experience; however, I have only installed floor joist on new builds / additions.

I said in a previous post that I would LIKE to add the joist without ripping up the subfloor (and damaging the existing flanges).  I would like to position each joist laying on its side where it's going to go (in the centers - not sister joist), add glue on the flange, then beat the joist upright into place, then someone on top will screw down via chalk lines.

This may be a long shot, but is there anyone out there that has done this?  CAN it be done? 

Furthermore, Would this help to make the floor feel more solid?  Would it be worth the money / effort?  The floor is not 'bouncy' per se - but it would be nice to stiffen the floor (as well as close the 24" gap before installing the new tile / hardwoods)

Thanks again.


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## Wuzzat? (Feb 19, 2010)

aaronled said:


> I would like to position each joist laying on its side where it's going to go (in the centers - not sister joist)


If your current deflection is L/360, what deflection would you like to have?  L/480?

From the deflection design goal, the joist size [this gives I, the moment] and material [this gives E, the modulus of elasticity] can be figured.

But since I = b(h^3)/12, increasing h [height] by 10% gives a 33% increase in moment [(1.1)^3 = 1.33], but increasing b [width] by 10% gives only a 10% increase in moment.  
That's why joists are on edge.  It's the best use of the material for the money spent.

In your case you might do better running small steel I beams down the middle between the joists.  I have a list of moments and E values for various types of steel I beams.

Or, if you can find a way to fasten a 2x under each existing joist to increase its height and therefore its I, you can dial up however much stiffness you want, at a loss of some headroom.


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## aaronled (Feb 22, 2010)

I think what I wrote may have been taken out of context.  Yes, I would like to lay the joist down on their sides in between the existing joist - BUT THEN, they would be stood on edge like the existing ones (the laying down on the side bit was just to convey the gluing and installing method.)  The end result would be just like the existing joist - only now they would be 12oc instead of 24oc.  I was trying to paint a picture of how I would go about it - sorry if it is confusing or not coming out right!  

Having said that - your suggestion about the small steel beams sounds interesting.  This makes me very curious - as it SEEMS it would be an easier way to install (could be visualizing it wrong though!)  This might be worth looking into seeing if it's a viable option...codes, price for materials, etc...

The BIGGEST goal here is to provide a nice flat (without dipping) sub for new floors - however, if I can increase the L value, then that would be great too!  As another poster pointed out, i could just beef up the sub-ply by adding more sheets...however, I'm not so keen on this idea being that it would seem to add a lot of extra weight on an already questionable deflection rating...  Again, the basement is not finished yet (but soon will be) - and I think now would be the time to move on this (if I do)!


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