Expert: Nazim Nice - 4/15/2009

Question
QUESTION: i have a existing 1 hour fire rated wall that is 5/8 sheetrock then a 6 inch metal stud with insulation then 5/8 sheetrock. city inspector wants it to be made into 2 hour wall. on the other side there is existing finished space. so how do i add 1 hou rating to the exist 1 hour wall

ANSWER: Hi Randy,

The easiest way is to look for a rated assembly from the Gypsum Association. They publish a book that has a number of tested assemblies.  You may need to provide a tested-rated assembly number to the inspector. You may not find one that has been tested with 6" studs . . . most will be smaller, but your inspector may find the closest match acceptable.  Most will likely have (2) layers of 5/8" GWB each side although some may have 1/2" GWB.  The detail usually specifies a fastening pattern.  

You can purchase the GA Manual from online sources.

If you have a close working relationship with your inspector, you may just try asking him if (2) layers of 5/8" GWB each side with 24" min. staggered joints will be sufficient (this is GA File WP 1522).  

If you need to make it 2 hours without work on one side - that may be tough.  I suspect you'd likely have to ignore the 1 hour fire rating of the existing wall and construct a new 2 hour wall 'shaft wall' right behind it.  You'd end up with a pretty thick wall, and you'd likely be using proprietary shaft-wall products, not something you'd pick up at the local lumber yard.  

Hope this helps.  Good luck!

-Nazim

---------- FOLLOW-UP ----------

QUESTION: thank you . the biggest problem is I can only work from one side. and the inspector said to build a 3-5/8 metal stud up aginst the existing wall and put 5/8 drywall on each side. this makes me loose about 5 inches of floor space the full 100 feet of the space. i feel that is a lot of space to loose.

Answer
Hi Randy,

It looks like GA WP 1520 may work for you. It is proprietary, meaning you have to buy certain products, but it looks close to what you are asking for.

Here is a link to the wall construction.

http://www.bpbna.com/us/english/gypsum_wallboard/cad_files/GA-FILE-NO-WP-1520.pd

It most likely was tested with the resilient channels to get a sound transmission rating of 55-59.  I'd see if the inspector would let you eliminate the resilient channels if sound transmission is not an issue.

Hope this helps!

-Nazim