Isolated Slab vs. Floated Floors vs. Both??? Please Help Me

Ok, I'm trying to get some of this stuff figured out so I don't sound like an idiot talking to my architect/contractor. I understand the differences between the different floor systems, but what would be ideal for new constuction, costs aside. Just figure in isolation and ease of running lines and cables. I was talking to a buddy of mine and he said that they would probably pour the whole slab and then soft cut it to isolate the rooms. The only problem I see with this would be the in floor heating and running conduit in the floor. Isolation wise it seems like the isolated slab would be best, but for ease of running lines and cables floating floor seems like it'd be easier. Should I just have the slab cut and float floors over the isolated slab and run everything in the floated floor? Any insight would be great...if this stuff has been covered please point me in the right direction. Thanks, Jordan

Welcome to the Construction forum! 8) I looked at your design forum thread also, to put your questions in perspective. The "to float or not to float" question was a significant one for me and my little project. First, I highly recommend buying Rod's book because he covers this in good detail. Note that your understanding of how slabs are made isolated is incorrect. It is not accomplished by pouring a large slab and then cutting it into pieces. Rather, they are separate pours. This is an important distinction. You cannot place heavy walls supporting heavy ceilings on the edge of a cut slab. You need structural footings around the perimeter of slabs, whether it's one big slab or multiple slabs in close proximity to one another. Otherwise, there's the concept of floating floors. Actually, it should be descrbed as floating rooms, because ideally you are floating a floor for the room, then attaching its walls to the floating floor, and then attaching a ceiling to those walls -- thus, then whole room floats. (The same procedure applies to isolated slabs, by the way.) There are several problems I always see with floating floor designs. First of all, the floated floor needs to be extremely heavy in order to avoid nasty resonance problems which would make isolation worse, not better. They have to be floated on elastomers that are "Goldilocks" loaded (not too much load and not too little load). Getting all the calculations correct requires a lot of patience and expertise. Another problem I routinely see is that most floating floor designs I see don't comply with building codes, especially in areas that are prone to seismic activity. After extensive research for my project, I determined that it might be possible to use Mason Industries' seismic snubbers in conjunction with their jack-up floated concrete slab system. However, after consulting with a local acoustics engineer, it was determined that an isolated slab would be fine in my case -- not to mention a lot cheaper. Another thing worth mentioning about high-end floated floor systems -- even its manufacturers indicate that they're actually intended for noise isolation between upstairs and downstairs. If you're building slab-on-grade, you don't have downstairs occupants to worry about. Yet in retrofit situations, floated slabs are often impossible from an engineering perspective -- the floor upon which it would sit is typically not built to withstand the tons extreme weight of a floated concrete slab, however thin. In a nutshell, most floated floors are (a) unnecessary, (b) unsafe/impossible/illegal, or (c) improperly calculated, or (d) any combination of the above. There are exceptions, but I think they are rare. When I joined the forum here almost two years ago, it seemed like it was all about floated floors. Since then, we've learned a lot about them -- mostly how extraordinarily difficult it is to do them right, how often they're not necessary, and the worst combination of all, how often they can be done improperly and make things worse instead of better. If you decide to do isolated slabs, you can certainly run PVC conduits from room to room underground before the slabs are poured. I hope that helps. I'm sure Rod's book will help much more. 8) --Keith :mrgreen:

Cool thanks :D I'll definatley pick up that book. How far apart are the slabs from one another? I believe somewhere I read that you can use Celotex (sp?) to use as a spacer and you can leave it in when you're done. I was just worried if there's only a small gap in between and they shift just a little bit over time the conduit and water lines could be sheared. Not the end of the world if the conduit breaks, but I'd be in some trouble if a water line breaks. Thanks. Here's the new revised floor plan. I'm meeting with an architect later this month or early next month, so I'm trying to make sure I know what I'm talking about.

jschulze, Listen to what Sharward (Kieth) says. He's done the research. Make sure your architech knows what you are planning to put on the floor in terms of walls/ceilings. It might be possible for him/her to design the footings in for the individual rooms, and then after it's cured, you could cut out the space between the footings instead of having separate pours. This is all assuming that you even need to isolate the slabs at all. I believe somewhere in Rod's book he mentions just how good your walls and ceilings have to be before flanking noise (from a NON-isolated slabs) becomes a problem. As far as your conduit goes, have you considered using rubber or flexible tubing where the slabs separate? I think this is a requirement for gas lines in California because of earthquakes. It allows a lot of flex without breaking. len

jschulze wrote:

How far apart are the slabs from one another? I believe somewhere I read that you can use Celotex (sp?) to use as a spacer and you can leave it in when you're done.

Yup, you spelled Celotex correctly. There are a lot of products under that name though. Perhaps you're referring to SoundStop. You may be able to use that as a "perimeter isolation board." However, closed cell foam insulation may be better because it won't decompose. I bought a 50 foot x 6 inch roll of closed cell foam that is made specifically for concrete cold joints. Perhaps using two rows of that would be better... :roll: As far as your specific question is concerned -- distance between isolated slabs -- I don't think it matters much. I have about 2 to 4 inches around my new slab. If I were to do it over again, I might have actually left no gap at all and used closed cell foam around the entire perimeter. I should hasten to point out that nobody here recommends SoundStop (a.k.a. "sound board") for walls. That product is more expensive than drywall and performs worse than drywall. We like spending less money for better results around here. ;-) --Keith :mrgreen:

You need structural footings around the perimeter of slabs, whether it's one big slab or multiple slabs in close proximity to one another.

Hello Kieth. Say, do you know how engineers reconcile your statement with jacked up floating floors. They are usually a thin slab that must support the weight of the entire room on the edge. Just curious. fitZ

That's a great point, Fitz! You've stumped me! Maybe Rod can comment...? :roll:

cadesignr wrote:

You need structural footings around the perimeter of slabs, whether it's one big slab or multiple slabs in close proximity to one another.

Hello Kieth. Say, do you know how engineers reconcile your statement with jacked up floating floors. They are usually a thin slab that must support the weight of the entire room on the edge. Just curious. fitZ

FitZ, I am confused by the question - perhaps you could rephrase it so I could respond. It appears the quote above relates to slab on grade (where you do (in fact) need haunches to carry load bearing walls) but then your question relates to floating slabs (where haunches are not a part of the equation for the elevated slab - but might well be required for the slab below carrying the point loads of the floor above). I am soooo confused.......... :? :? :? Sincerely, Rod

I am soooo confused.......... Hello Rod. Sorry I didn't get back sooner. This bbs server doesn't always let me know of a new post on a thread I've responded to. Anyway, let me clarify my question for you. Here is Kieths statement...

You need structural footings around the perimeter of slabs, whether it's one big slab or multiple slabs in close proximity to one another.

OK Rod, let me put it this way. WHAT is the difference? IF.."You need structural footings around the perimeter of slabs".. , then what EXEMPTS a floating SLAB from this requirement? Exactly WHAT makes a FLOATING SLAB any different than a slab on grade? They BOTH carry the load ON EDGE. And if you say the "floating floor" has a slab with footing undernieth of it, I'd say I've seen pictures of floating slabs poured in the MIDDLE of an "on grade" slab. That tells me there are TWO thin slabs supporting the load with no footings whatsoever. Does that clarify my question Rod? The point being, I don't understand what gives the edge of a floating slab any more strength than a thin slab on grade with no footing.? Hell, a slab on grade even has the ground under it to help support it. I UNDERSTAND there is steel mesh/rebar in a floating slab. And if it is this steel mesh/rebar that EXEMPTS a floating floor from a "footing" requirement, then why couldn't the same thing be done in a standard on grade slab, thereby eliminating the need for a footing...no difference, right? Maybe a pic or two would help. Whats the difference between these two conditions, and what requires the on grade slab to have a footing, REGARDLESS if its carrying a roof load or not( room within rooms do not have a roof load but from what Kieths statement implies, even a room within a room ISOLATED slab requires a footing, no?) Thanks Rod. fitZ

BUMP!!

cadesignr wrote:

OK Rod, let me put it this way. WHAT is the difference? IF.."You need structural footings around the perimeter of slabs".. , then what EXEMPTS a floating SLAB from this requirement? Exactly WHAT makes a FLOATING SLAB any different than a slab on grade? They BOTH carry the load ON EDGE.

fitZ, Now I understand...... OK......... first off - an elevated slab is a structural slab..... and is designed to "bridge" the span from load point to load point. In other words - the slab itself becomes a series of beams interposed with the slab - even though those "beams" don't have to vary in thickness when compared to the floor.

And if you say the "floating floor" has a slab with footing undernieth of it, I'd say I've seen pictures of floating slabs poured in the MIDDLE of an "on grade" slab. That tells me there are TWO thin slabs supporting the load with no footings whatsoever.

Well I have to tell you that you would never see that on my projects - nor on any engineered project with any engineer who ever wanted to work with me again. In the case of elevated slabs - the slab (or floor if the slab is above grade level) has to be designed to carry the entire load - room plus slab - AND it has to be designed to carry the concentrated point loads that the floor imparts. SO in the case of an elevated slab over a slab on grade - the slab on grade doesn't really require continuous haunches - but at the very least it does require either a thickend and reinforced concrete slab over properly compacted structural gravel - OR it requires a series of isolated footings below the slab - with a simple slab over them to carry the point loads. You say: I'd say I've seen pictures of floating slabs poured in the MIDDLE of an "on grade" slab. That tells me there are TWO thin slabs supporting the load with no footings whatsoever. But first I have to ask - is this a case of you know this to be a fact - or you just assume that the bottom slab is not specially designed to carry the load? If you know this for a fact - then they got it wrong.

The point being, I don't understand what gives the edge of a floating slab any more strength than a thin slab on grade with no footing.? Hell, a slab on grade even has the ground under it to help support it.

And - in the case of an existing slab - not specifically designed to carry the load - what a lot of assumption on anyone's part that the slab is poured over earth that was compacted properly...... and if your loads happen to coincide with an area where the earth wasn't - what then?

I UNDERSTAND there is steel mesh/rebar in a floating slab. And if it is this steel mesh/rebar that EXEMPTS a floating floor from a "footing" requirement, then why couldn't the same thing be done in a standard on grade slab, thereby eliminating the need for a footing...no difference, right?

I suppose it could be - although I wouldn't consider it on any of my projects. It is a heck of a lot cheaper to construct a structural haunch with the remaining slab a simple slab in comparison than to pour a slab that is mean to be completely structural in it's nature. So from a cost performance point of view - my process is cheaper - MUCH cheaper.

Maybe a pic or two would help. Whats the difference between these two conditions, and what requires the on grade slab to have a footing, REGARDLESS if its carrying a roof load or not( room within rooms do not have a roof load but from what Kieths statement implies, even a room within a room ISOLATED slab requires a footing, no?)

Sorry - I would never even consider placing an elevated slab directly over an unreinforced slab on grade. Which is what I see in your first picture...... but then again - I also always reinforce the haunch. BTY - not a footing actually - rather a haunched slab - so the pour is monolithic - while a footing is seperate......... But the answer is "yes it does" - OR it requires a specially designed structural slab - calculated to carry the load.......... As I said - the haunch is much MUCH cheaper. And even in the case of a thickend structural (reinforced) slab - I would only allow it to be that way if I personally was on the project - or (in the case of an existing building) if the project had been formally inspected by an independent testing lab to verify concrete strength - placement of rebar - full compaction testing - curing proceedures, etc., etc. Just making the assumption that any old slab is ok to carry these kinds of loads is an invitation for disaster. Sincerely, Rod

NOW I understand....too. :lol: Thanks for your insight Rod. fitZ

NOW I understand....too.

:roll: Uh....wait a minute Rod :? I prematurely said that. No, I don't understand. At least the part:

OK......... first off - an elevated slab is a structural slab..... and is designed to "bridge" the span from load point to load point.

Understood, but because of lack of exposure to real world applications, I don't see how the jacks(isolators) can be set under an edge, if intuition serves me. It would seem, in the event of future settling, the access to the jacks would not be placed where they would be covered..ie. a wall, which for all intents and purposes would logically be placed at the edge of the "structurall slab", no? If that is correct, then that implies a portion of the "beams" must be cantilevered from a perimeter load point to the edge of the slab, is that correct? If so, then the rebar that is tied to the "isolators(FS)" must form the structural backbone of the beams, and as such must also cantelever to the edge of the slab, correct?? If so, then the edge must be reinforced via rebar as well, which must be the backbone for an ..."edge beam", or some other concept. Is this right or wrong Rod. Just trying to understand the conccpts. I've read everything I can on the Mason jack up system, and just needed a little more input to understand the "realities". Thanks Rod. fitZ