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If you have 3/4" and 5/8" drywall nailed together to make a fire rating, do you have to tape both layers?

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Fire rated wall or ceiling assemblies have fire ratings. Sheetrock, even type X, ("fire code" so called) does not have a rating, outside of being used as a component in a rated wall or ceiling assembly.

If your rated assembly method requires two layers, one of 3/4" and one of 5/8" it will also have details about whether the listed assembly requires taping of joints on both layers, how far the joints must be offset, the required fastener schedule and all the things that bridge the gap from some materials to a 0.5, 1, 2, 3, or 4 hour fire-rated assembly, where walls or ceilings constructed to the method have in fact been fire tested as assemblies, and given a rating.

Randomly slapping some thick drywall together will probably increase fire resistance, but it won't be making a listed, tested assembly of known performance.

With regards to our source of random comments based somewhere other than in fact: https://www.nationalgypsum.com/design-resource-center/faq/faq-ul-fire-resistance-directories States:

“Fire taping” is the name assigned to the process of applying joint compound and tape to joints in fire rated designs. The actual products used are no different than you find in any other part of the gypsum finishing process.

However, the finish of the layer that's going to be covered by the next layer does not need to be especially fine (functional, not pretty is the goal there, when taping the joints on the first layer is required for the assembly method.)

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