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RE: lightweight concrete topping on wood

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Andrew -


Whatever you do, don't use 2". If you use anything, use 1-1/2" but you still need to add a second sill plate so that the GWB has something to attach to. I've worked on projects where the architect specified 3/4" gypcrete to avoid the double sill plates. The weight is less, but this material has been problematic in areas that get wet. 1-1/2" lightweight is a better solution. Also, you might consider double stud walls (on separate plates) instead of staggered studs on a single plate. GWB ceilings on resilient channels is a good idea. The gypsum association has some good wall and floor assemblies for STC ratings.



T. William (Bill) Allen, S.E.


Consulting Structural Engineers
V (949) 248-8588 F(949) 209-2509

-----Original Message-----
From: Andrew Kester, P.E. [mailto:akester(--nospam--at)]
Sent: Tuesday, January 13, 2009 5:06 PM
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