A couple of questions regarding outershell completion

Hi guys, I'm still in the early stages of construction with my new place in south London The live room and drum room are being built into what was previously a tyre garage. The existing area was a blockwork wall construction on a single solid slab cast by the previous owner, with an uninsulated timber frame roof. So far we have channeled down around the edges of the slab to a depth of 12", and I have insulated the ceiling with 4" rigid rockwall and then lined it with acoustic grade plasterboard. Although the plasterboard has already gone up, the cavity in the roof has the capacity to accomodate 2 layers of rockwool, before proceeding any further I am considering taking down the boards and putting up a second layer. Is this a good idea or should the spring system in play here accomodate an air gap as well as the insulation? I am looking to float 2 concrete room constructions on top of the existing slab. These would consist of 4" reinforced slab, blockwork walls and a concrete lid supported by steel joist work. With the fact that the existing slab has been cut, will it make much, if any, difference if I actually float a secong slab? Or would it be equally effective to build from the orginal slab - therefore saving headroom? Lastly, with this internal concrete shell, should I put a second skin of board on the existing ceiling? I would really appreciate any feedback anyone has Andy

here is the plan of the site - this stage of construction is related to the back 'triangualr' section

If you're building isolation rooms on floating concrete slabs, I think the extra insulation in the exterior ceiling will not add much. Use lightweight insulation between the floating rooms and the outer walls. I presume you are floating the rooms based on isolation requirements and the environment noise levels? How was the floating floors engineered?

Hi Glen! I'm very glad to hear that as we've already sealed the ceiling and it would mean taking the boards down to add another layer. In regards to the lightweight insulation, this is in place of what would traditionally have been an air gap between leafs? Yes the rooms are to be floated due to iso and enviro concerns, in fact I am basing this on a design you described back in Jan viewtopic.php?f=2&t=15537 My only reservation with this is still how to calculate the compression the RW3 would undergo and whether it would be suitable for a concrete structure (I've seen a lot of people using this for concrete slab & stud wall/roof structures but not so much for completely concrete shells) As of yet, there are no floating floors. I am concentrating on finishing the outer shell first before doing this. Another option I'm looking at are proprietary spring systems, namely Eurovib, but only if there is no possible way to achieve this through the original method. Lastly, it does bring me back to an earlier question, that as we have already excavated the edge of the slab and cut round it, do we in fact need to cast another slab on top of this or would this be sufficient for isolation?

"I am looking to float 2 concrete room constructions on top of the existing slab." Prove it. You will not float anything just by saying you are doing it...there is much procedure...let us see what you have...

What "proof" are you looking for? I have a shell, with the internal slab cut and I want to know if I'm going to far by floating a new room (with new slab) on top of the one I've already isolated. If it's still a viable option im looking for advice as to whether Rockwool will be a sufficiently pliable spring system or I will need something more substantial.

you might spend some time on the Mason Industries web site - they have some really good guides on floating concrete floors and rooms which can help you calculate the right isolation systems - neoprene, spring etc. you want the natural resonance of the system to be 1/3 (or less) of the lowest frequency you need to address - so if you have to get down to 30hz, you want the floor to be @ 10hz, etc. yes, the air gap between isolation layers is where the lightweight insulation goes.

brookster wrote:

What "proof" are you looking for? I have a shell, with the internal slab cut and I want to know if I'm going to far by floating a new room (with new slab) on top of the one I've already isolated.

The proof is in the pudding, as they say. The concern with simply cutting a slab and decoupling it from the other side of the slab is a start...but not an end. When you think about the weight, typically tons of weight, you run into the ability of the simple slab to support this weight. Usually this is not an issue as the slab will be setup on the perimeter to support this weight. I venture a guess that yours is not like this. It is a slab that has been cut and now you are thinking of piling on tons of weight on this very perimeter thinking it will support it. Most likely it will not. A typical perimeter will have a footer somewhere's in the area of 16-24 inches wide by 16-24 inches deep. This is done in expectation of the the added weight ( dead load/ live load), the potential seismic loads or environmental impact, the actual existing ground compaction e.g. the ability of the ground to support the slab and the additional weight. So whatever you do after this point of cutting the slab will change the way the concrete exists in the existing environment. Basically you have a few "plates" on the ground waiting for the right amount of weight to be placed in the correct spot to make it "move". If you go to Mason Industries website and read what ever they offer, there may or may not be information that addresses this short-coming on your part. The short coming is that you do not have the additional support @ the perimeters, the footers have been effectively removed once you cut the slab. What does that mean to you? It means that you have a fragile slab that is no longer capable of supporting the previously prescribed weight that was designated to the actual slab before it was cut. You have weakened this area and it needs to be re-supported since the weight you are about to install could break any part of the slab. You also run an additional risk in that you have no idea what kind of shape the ground is in below the existing slab. You add weight and the slab can tilt if enough of a void exist below it...at best the un-supported slab will crack causing your structure to shift in the direction of gravity... AND...there is always the existing structure to consider and how this modification of eliminating the contiguous concrete slab from one footer to another will effect the remaining footer/slabs ability to actually continue to support the existing structure.

Hi guys Thanks for the replies. I have looked through Mason thoroughly, and we even had a quote for a system from their UK branch to be installed for this floor a few years back but it was prohibitively expensive. Brien, that's exactly the kind of insight I'm looking for - thank you. We have cut down into the slab but not into the footings of the surrounding walls. The slab is cast on a chalk/soil base and as such does not have any footing. The footing is placed further down than the slab goes and supports only the external walls. At present I have an approximate 100mm channel cut round the whole slab. Loading aside, I can't find info as to whether this would suffice as a valid 'spring' in the system. I'm working on the basis, as I have previously read, that the earth itself is an ideal decoupler, being that something of such mass provides such a low resonant frequency. I'm at a stage now, whereby, I have a three options Build a 9" wall (concrete block/side down) around the perimeter - risk of too much loading & whether the channel provides enough isolation Float a new slab on the existing one (we are looking into proprietary systems - even the possibility of car tires but am unsure about the issue with how they perish. Also this means losing more ceiling height) To excavate the whole existing slab and cast a new one in the same place using something other than the earth to decouple the slab. I just can't find the answer to what wold be the best way forward and anyone I've spoke to has opinions but can't give a confident answer. Thank you both for your input on this - if you could give any more guidance I would be most appreciative! Andy

i think the concern Brien raised is the structural integrity of the slab with the new load - it might reside on earth but without some additional support on the edges where you're loading is concentrated, you could end up with a serious issue. it may be that your best option is to dig it out and replace it with a slab which can properly support the expected load. and during that processing consider some options for decoupling the slabs from each other and perhaps a combination of a footing for the inner isolation walls and floating slabs inside that.

I totally agree with Glen and Brien: there are structural issues with the way you are going now, and you probably should get a qualified structural engineer involved before you go any further. Nobody here can tell if your plan is safe or not: you need an engineer to go there, take a look, do whatever testing he needs to do, figure the numbers, and give you a written report. Floating a floor is a big deal: very hard to do right, and very expensive. Cutting the slab is not the same as floating the floor, and produces the issues that you have now: you don't know if the remaining slab without any footing can support the many thousands of kilograms that you need to put on top of it. Most likely it can't, but only a structural engineer can tell you for sure. Here is a thread about floating floors, and the right way to do it. I'd suggest that you read through that carefully, follow the links, and try to get a better understanding of what is involved. viewtopic.php?f=2&t=8173 http://www.nrc-cnrc.gc.ca/obj/irc/doc/p ... /ir802.pdf I'd also suggest that you get Rod's book, as it has a section that deals with what Glenn describes: pouring a new slab with a footing, after you demolish the one you have right now. - Stuart -

"Nobody here can tell if your plan is safe or not: " I thought I just said it was a bad idea?