Expert: Roger Zona - 8/26/2010

Question
QUESTION: What is the dead load bearing capacity of gypsum board ceilings and how many inches insulation will it support?

I am a home inspector and I see houses with lots of older homes with added insulation mostly blown in over existing batts but some times blown in over blown in.

How deep can you install it before the ceiling begins to sag or crack or worse cave in?

ANSWER: Hello Ed

Generally, in houses built with wood trusses, the bottom cord or ceiling joist is a 2"x 4" or 2"x 6".  Conventional framing will usually require a larger ceiling joist - 2"x 8".

You can safely install fiberglas batts to a depth of twice the ceiling joist.  Same for blown rock wool.  Don't pack the insulation down and be careful not to step between the joists.  (I'm a bit of a fanatic about safety!)

Older homes with plaster on gypsum lath can support more than just plain drywall.

Hope this is helpful

Roger
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---------- FOLLOW-UP ----------

QUESTION: Roger, thanks for your assistance. I did some additional research into the problem and came up with the following which some folks might find helpful as well.

I've found the weight per r=factor and inches is on most insulation packaging so if you know the product used you can figure out safe loads with the help of the chart below.

Ed

Excerpted from:

United States Gypsum company’s, Gypsum Construction Handbook

Drywall & Veneer Plaster Products

Gypsum Panel Limitations

To prevent objectionable sag in new gypsum panel ceilings, the weight of overlaid unsupported insulation should not exceed:

1.3 psf for 1/2"-thick panels with frame spacing 24" o.c.;

2.4 psf for 1/2" panels on 16" o.c. framing (or 1/2" Sheetrock
brand interior gypsum ceiling board, sag-resistant on 24" o.c.
framing);

2.2 psf for 5/8" panels on 24" o.c. framing.


Answer
Hey Ed -
Thanks for the reply.  Your research is right on target and good to know.  Apparently plaster is used so seldom any more that the product has not been rated.  

Again, thanks for the research and I shall add that web site to my list of "construction product rating".

Roger
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