What is Existing Fire Resistance Rating of Wall?

11/12/2019

We have 2x4 wood studs which are spaced 16-inches on center. There's a layer of 3/8" gypsum board on each side, plus on one side there's 1 layer of 3/8" plywood (douglas fir).

How do I determine the fire resistance rating of this partition assembly?

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4 Comments

Brian M.

11/12/2019 12:20:57 pm

Reference: 5th Edition SFPE Handbook of Fire Protection Engineering pg 1981.

3/8" gyp = 10 min
16" OC wood wall = 20 min
3/8" gyp = 10 min
3/8" douglas fir plywood = 5 min

Total fire resistance rating on the side WITH the plywood = 45 min.
Total fire resistance rating on the side WITHOUT the plywood = 40 min.

yunlong LIU link

11/12/2019 01:02:45 pm

Why each side has different FRR? why the side with plywood got higher FRR?

Doug link

11/12/2019 05:28:28 pm

In non-legal jargon, a fire starting in the room with the plywood has to burn through the 5 minute plywood before getting to the 40 minute wall. The fire takes 45 minutes to burn through before hitting flammable material.

The plywood, being flammable, is not part of the fire protection system when a fire starts on the non-plywood side. The fire would be slowed down for only 40 minutes before hitting flammable plywood material.

Jon Nisja link

11/12/2019 07:52:26 pm

Back in the early 1980s the Dept. of Housing and Urban Development (HUD) published their Fire Ratings for Archaic Materials Guide: https://www.huduser.gov/Publications/PDF/fire.pdf.

3/8" gypsum on each side of wood studs = 25 minutes or 1/3 hourly rating (based on page A-55 of the guide). The plywood on one side gives you another 5 minutes (per SFPE Handbook).

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