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Unread 09-30-2012, 11:03 PM   #1
NorthbyNorthwest
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Help designing new slate floor

Hello everyone,

I just found this site a couple weeks ago, and it has a wealth information to offer. So I'd like to introduce myself and say thank you in advance for all the questions I have.

My name is Chad and for the last 16 months I have been building a timber frame home. I am not your average DIY'er, I have done all the work from site prep, septic, excavation ect. I have no crew just my wife and a few friends that I call when something gets heavy. About 40% of the wood on the house was milled on-site from trees taken down during the site prep. The rest was locally sourced from friends and neighbors in the logging business.

Here are a few pics of the progress thus far.

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So basically I could use some advice on the next steps to take on getting slate on the floor.
Here is what I have so far; the floor is 3/4 sub with radiant heat in uponor quik trak. I have checked the deflection issue with my engineer and I am fine for the slate we picked.
I have 1000sq/ft of 16x16 Black India from Pental, it's fairly well gauged at about 3/8".
So I need some help deciding on how and what needs to happen to get the slate on the floor.
I appreciate any and all help getting this done right the first time.

Cheers..
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Unread 10-01-2012, 12:27 AM   #2
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Welcome, Chad.

Beautiful photos of your new construction there. Unfortunately, none that might help us at all in evaluating your potential for a successful natural stone floor installation.

You say "the floor is 3/4 sub with radiant heat in uponor quik trak." First I'd wanna know 3/4 what? What material is that first layer of subflooring? And I ask about the first layer because your need two layers for a successful natural stone installation and the Uponor doesn't count.

Second, I'd wanna know about your joist structure. I know you indicate your engineer likes it, but we find frequently they are not sufficiently in tune with the requirements for natural stone as opposed to ceramic tile installations, such as apparently not having advised you to double your subflooring.

Third, I'd be curious to know if the joist structure for this floor (this is the ground floor?) is part of the material milled onsite and whether y'all have given consideration to the inherent instability of that framing material when associated with a natural stone floor covering.

Just out of curiosity, are you planning for your radiant heating systems to be the primary heat in the building?
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Unread 10-01-2012, 02:27 AM   #3
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Hi CX,

Here is an older pano shot that might help evaluation. If not let me know what you are looking for and I can dig thru my photo archive.

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Quote:
First I'd wanna know 3/4 what?
The Sub floor is 3/4" Weyerhaeuser Edge Gold OSB glued and screwed.. Only 1 layer
.woodbywy.com/floors/f_edge_panels.aspx

Quote:
Second, I'd wanna know about your joist structure.
The joist structure consists of Roseburg RFPI 40S 11-7/8" @ 24" on center about 32' with intermediate support at about 15'6" with a 8x12 gluelam and an interior sheer wall. The first 3 joist bays on each side have same RFPI 40S cross blocked every 24".. And cross blocking all the way across the center above the gluelam and at both ends of the building at 12" from the Rim.

This floor is over a daylight basement.
The radiant is the main source of heat both in-slab in the daylight basement and the main floor. We have supplemental heat from a wood stove.

The engineer calculated the floor at dead load of 20psf and 40psf Live.

BTW the slate weight from Pental was 6,128 pounds. That would put it at about 6.2psf.. Or about 11 pounds per 16x16

I hope that helps
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Last edited by NorthbyNorthwest; 10-01-2012 at 02:37 AM. Reason: added info
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Unread 10-01-2012, 09:12 AM   #4
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Quote:
Originally Posted by Chad
The engineer calculated the floor at dead load of 20psf and 40psf Live.
And what did he calculate the deflection to be at those loads, Chad? That's the important question here.

For that joist spacing you'd need an additional layer of plywood even for a ceramic tile installation. The minimum would be nominal 1/2" exterior glue plywood. For your natural stone installation I'd be inclined to increase that to at least nominal 5/8ths" plywood. Then, of course, you'd need a tiling substrate of some kind.

Keep in mind that the subfloor deflection and the joist structure deflection are two separate issues and each must individually meet the required L/720 deflection for a natural stone installation.

I can't find any span tables for those engineered joists online, but such tables are rarely published for deflection below L/480 anyway. The manufacturer can tell you very quickly what the design deflection would be for your particular installation if given the size, spacing, and span of the joists.

Is the heating system already installed on the floor to be tiled?

My opinion; worth price charged.
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Unread 10-01-2012, 10:56 AM   #5
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I actually have to run by my engineers office and pickup a quick drive gun I loaned out. So I will double check the deflection..

But if I remember right he said the max deflection would be about 3/8"

Here is a link to the RFPI joists
.bit.ly/PGXE2L

Yes the heating system is already down on this floor..

Could you expound of the L/720 deflection or maybe point me to a post that does?
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Unread 10-01-2012, 11:47 AM   #6
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Please do that.

If the 3/8ths" deflection was for the 15.5-foot span it would represent something a little better than L/480 deflection where L/720 is required. A good joist structure for a ceramic tile installation, and about what we'd expect from any engineered structure where natural stone flooring was not specified.

The L/720 deflection is mandated by the Marble Institute of America (MIA) for natural stone floor installations, as is the double-layer subflooring regardless the joist spacing. That's twice as rigid as the usual building code requirement of L/360, which is the same required by ceramic tile industry standards.

The L in the formula represents the unsupported span in inches. The 360 or 480 or 720 are constants used in defining the design deflection depending upon the requirements at hand.

For your span of fifteen and a half feet, or 186 inches, you'd divide by 720 to calculate allowable deflection for a natural stone installation, for example, and get a maximum allowable deflection at the center of your joist span of just over a quarter-inch (0.2583). Your stated deflection of 3/8ths" (0.375) would represent a deflection ratio of about L/496 for that unsupported span.

Any more clear?

You are allowed to post links to other sites now. I can't make that little snippet take me anyplace useful.
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Unread 10-01-2012, 01:00 PM   #7
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http://www.google.com/url?sa=t&rct=j...HIsgVw&cad=rja

Yes that makes perfect sense thank you..
I haven't got a call back yet about the deflection. But according to your calculations and what I have read in the link above 3/8" should be about right for the floor as it is.

So I have to logically ask the next question..
If the calculated weight of the slate is 6.2 psf + the weight of the added substrate material is less then the max dead load. Can I getaway with putting down the slate, regardless of the MIA mandated L /720.

What is the worst case scenario? What is best case scenario?

Keep in mind that I am not hiring a contractor to do this work or dealing with warranties limitations.

Thank you for your help CX..
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Unread 10-01-2012, 01:06 PM   #8
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Quote:
Originally Posted by NorthbyNorthwest
So I have to logically ask the next question..
If the calculated weight of the slate is 6.2 psf + the weight of the added substrate material is less then the max dead load. Can I getaway with putting down the slate, regardless of the MIA mandated L /720.
You can get away with just about anything... whether or not it will stand the test of time is another question. There is a reason the MIA gives the specs they do.

Quote:
What is the worst case scenario?
Epic fail of a tile install and tiles crack, pop off, grout comes away, and the floor in general just plain looks bad and does not last a year.

Quote:
What is best case scenario?
The tile install goes fine and everything holds up. With you not being even close to the L720 I would almost bet there will end up being problems over time. How soon they appear is beyond my level of experience to estimate.
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Unread 10-01-2012, 01:26 PM   #9
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Thanks Kyle,

I am not trying to cause myself problems down the line. Unfortunately between information from the engineer and a family member, who is a tile contractor both telling me I was fine to put down the slate. I am at this place and situation, the slate is on pallets in my front yard and my floor is what it is.

Not at all what I was hoping to hear but at least I know where I stand.

What are my options at this point?
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Unread 10-01-2012, 07:58 PM   #10
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Chad, with the joists being a bit better than L/480, I'd be a lot more worried about your subflooring than the joists, although the lack of joist stiffness is gonna aggravate your situation.

With dimension wood joists it's sometimes possible to stiffen up your subflooring by adding subflooring from below, but with the engineered joists there no good way to do that.

One of the primary reasons for the double layer of subflooring requirement is to eliminate there being any place on any joist top where the subflooring is separated all the way from the joist top to the subfloor surface. The joist tops are where you're most likely to see your first failures in your tile surface. Presuming you experience some failures, of course.

And the fact that your total dead load may be less than the load used to calculate your joist deflection is of no concequence at all in regard to the between joist deflection, which is your more potent enemy.

'Bout all I can suggest at this point, presuming you are not willing to remove the heating system and add subflooring, is that you get your "engineer and family member" to jointly issue you an insurance policy against failure that would be sufficient to cover the replacement cost of the tile installation and the necessary improvement to the subflooring under the new installation. I'm sure they'll have no problem doing that.

My opinion; worth price charged.
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Unread 10-01-2012, 09:11 PM   #11
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Quote:
One of the primary reasons for the double layer of subflooring requirement is to eliminate there being any place on any joist top where the subflooring is separated all the way from the joist top to the subfloor surface. The joist tops are where you're most likely to see your first failures in your tile surface. Presuming you experience some failures, of course.
I'm not sure I follow your explanation of this point of failure, could you elaborate?

Would the the quick traks and 1/4" or 1/2" hardibacker or anything else help with this point of failure?

Pulling up the heating at this point would be more plausible then after the fact.
But adding 3/4 more sub-floor causes issues with finished floor heights for door jams and cabinets.. I guess there is no real easy answer for this beside returning the slate and going with something else. ugh!

I don't have anyone to blame for problems but myself, which is nice because I cant sue myself.
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Unread 10-01-2012, 09:25 PM   #12
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Chad, when you add the second layer of subflooring you stagger the panels such than all the joints in the first layer are covered by the second layer.

I don't know if those Quik Trak thingees provide anything at all in the way of structure to your subflooring. Never have installed any and have not researched them at all. But I would assign them a zero value unless the manufacturer can demonstrate otherwise.

No CBU adds anything of structural value to the floor, even according to their manufacturers, but you're gonna need something over the heating strips before you tile.

You could switch to ceramic tile, but you really need more subflooring even for that. There are manufacturers of CBUs that allow their product to be installed over single layer plywood over 24" centers for ceramic tile installations, but you couldn't make me do it on a bet.

You don't need another layer of 3/4" plywood, by the way. A second layer of 1/2", properly installed, would suffice.

Or you can install over what you've got and toss the dice. And never move heavy furniture over that floor.

I hate to be the bad cop on this, but those 24" joist spacings are just bad news for the tile industry.

My opinion; worth price charged.
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Unread 10-01-2012, 09:56 PM   #13
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I am guessing the deflection of the sub-floor between the joists would cause the abutting edges over the joist to rise. Which would cause the failure if the slate above? So the addition of the 1/2'' would stop this process to some extent? I wouldn't think the addition of 1/2'' over the 3/4 would bring the deflection up to anywhere near L/720 between the joist.

It would be interesting to do the heating calculations to see how much efficiency I would lose if I put the 1/2 over the quick traks. Would this be acceptable compromise?

Sorry to beat a dead horse, I just need to explore any and all options before I take action.

Thanks CX
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Unread 10-01-2012, 10:06 PM   #14
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The between-joist requirements are decided more by material testing than design deflection, Chad, and 1/2" plywood properly installed over 3/4" plywood is a pretty sound subfloor even at that spacing.

No way to get the consistent test results with natural stone that we can get with ceramic tile, but the MIA is mostly happy with 1/2" over 3/4" subflooring even at that spacing.

Adding 1/2" plywood over the heating system isn't gonna get you the same floor as adding it directly over the subflooring. I can't say it won't help, and I can't even guess how much it might help, but I know it's not gonna be the same with that "spacer" in there.

Maybe the stars will align for you and remain that way, eh?
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Unread 10-01-2012, 10:54 PM   #15
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Well we have come this far from not an option, to plausible if I add the 1/2 over the 3/4. I will put some thought into our conversation, and move forward informed..

Quote:
Maybe the stars will align for you and remain that way, eh?
Haha Star alignment didn't get the house this far, good friends and good advice did, except for the 24'' OC joists

Speaking of which, how would you properly installed the 1/2'' over the 3/4'' glued and screwed? I understand what you are telling me as far as overlapping the existing 3/4.
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