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RE: lightweight concrete topping on wood

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Light weight or normal weight is used frequently, topping materials like Hacker or Gypcrete are used even more frequently in Multi-Family for sound.  There are many other aspects, and if sound transmission is a true concern, you should pick a rated assembly (STC).  Depending on the type of joist system, the ceiling may require resiliant channel and batt insulation, or double gyp.  Tile areas may require acoustic mat.
Floor toppings will crack, and that is that.  They are not typically reinforced in any manner, which is why some of the self leveling products are more effective, as well as having better sound ratings for thinner applications; most often we see an inch.  With light or normal weight a fiber may be used, and can't hurt.
If the floor is adequate, and carpet and pad or laminate and pad is going over the top, who cares if it cracks?  It isn't structural.  Don't lose sight of the application and make things more complicated than they have to be.
Upgraded carpet pad can be of tremendous help with STC rating.
The glued and screwed plywood will be of greater beneift in stopping wood squeak than the topping.
If your Architect can't give a proper STC assembly, I will send you one if you tell me what joists you have. (offline of course). Contact Hacker Industries or Gypcrete and they will give you rated assemblies using their product.  


From: Andrew Kester, P.E. [mailto:akester(--nospam--at)]
Sent: Tue 1/13/2009 5:05 PM
To: seaint(--nospam--at)
Subject: lightweight concrete topping on wood

For a small two-story apartment building we have a wood framed second floor with wood sheathing. The owner and architect are looking for sound reduction, and also a stiff floor system with reduced "squeaks" due to wood flexing.


Right now we have suggested joists/trusses at 16" oc and ¾" sheathing screwed and glued, with a 2" lightweight concrete topping. I have combed thru the code and found nothing prohibiting using wood to support concrete in this manner, they do mention limiting long-term deflection.


Is there anything in NDS about this or is this prohibited in IBC or other codes? Anyone see any problems with this? We are designing for the extra dead load, and have heard of this being done on other projects quite successfully. Would anyone suggest anything other than poly fibers for micro crack control of the lt wt concrete, maybe some #3s at reentrant corners or don't bother as it is just a sound deadening topping? Should we have them spray the concrete with water sealer, a bond breaker, or anything like that?



Andrew Kester, PE

Orlando, FL