Do two layers of 1/2" drywall provide a fire rating equivalent to one layer of 5/8" drywall?

Question

On an existing 1/2" drywall partition, does adding another layer of 1/2" drywall over the existing bring the partition wall up to an equivalent fire rating as 5/8" drywall ??

Answer 1

Just to shed common sense on this. Almost in all cases is it cheaper and less of a hassle to demo the 1/2" and install 5/8" than to install 1/2" over 1/2". So to answer the question, just install the 5/8" so you aren't having a philosophical debate on fire rating with a local inspector.

Answer 2

You can stack up many layers of firecode drywall (Type X and C) but that does not make a fire resistant assembly. Local inspecting authorities generally require that fire rated construction be based on a tested assembly by a trusted testing authority, such as Underwriter's Laboratories or the Gypsum Association. If you can't find a tested assembly that matches what you're doing, then you won't have a rated assembly.

Answer 3

There are many factors to look at, like wall type. Fire testing was done on many type wall construction example Shaft Walls, Separation walls, Wood Studs, Metal studs, with insulation and without. Also the type of gypsum panel used Type X board or Type C board. Fire code C has a better rating than Type X.

To include the installation of the board joints Two layers of 1/2 inch require the first layer to be horizontal stagger joints face layer vertically staggered joints.

The type of finish could effect the rating like adding Veneer plaster could increase the rating.

Just adding a layer of drywall may have very little effect on the rating.

Answer 4

A second layer will increase the fire rating. The drywall seams are staggered. Some inspectors require the second sheet to be horizontal. I always thought the seams staggered vertically was better but maybe it is easier to verify the offsets with the second sheet horizontal.