Fire rated drywall under sheathing?

I inspected an attic yesterday and noticed that on the underside of the sheathing in a span of two of the truss areas that there was gypsum board/drywall. Is this for fire rating?

I would suspect this is a townhome and your answer is yes.


Super common in my area in townhouses to see rock/gypsum tacked to the underside of the roof within 4’ of the shared wall (often times it’s all falling down).

I believe it is a requirement in many, if not most, places.

I haven’t seen it before in my area. So now I know for sure. I was assuming it was a fire rating issue.
Thank you all for the replies. I appreciate your input. Learning every day.

Look up “1 hour rated assemblies”. You’ll see this gypsum layer on exterior walls, partition walls between units, and under roofs (particularly flat ones). DensDeck® is a leading brand name of enhanced drywall for this purpose.

Look for proper detailing of water barriers to protect the gypsum.

Look for certification stamps. In most cases regular drywall won’t do, it must be Type X and the right thickness. The 1/2" thick drywall you pictured would NOT meet the 1-hour fire requirement today. As with any system, look for failure (is it sagging, removed, no longer meeting the design intent?)

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Yeah, it’s a good thing to kind of keep in the back of your mind as you’re inspecting attached housing. The first few feet on either side of the wall can all under some different rules. I once had a client ask why the outlets on one wall (turned out to be the shared wall) were on the floor. I was thankfully having a good mental day and realized it was due to fire separation. I’ve also seen 5/8" X drywall in a tall crawl space on the shared wall that I assumed was for fire rating. I think some of the stuff varies by jurisdiction but it’s just a good thing to beware of.

In a related story I recently did an inspection for a firefighter and we had a good talk about newer v. older building and fire containment. He said all the newer requirements (now many coming up on 40-50 yrs.) made a HUGE difference in fires spreading. As firefighters they are definitely aware of a building’s age as they go in some as it helps them plan and know what to expect. I thought that was a pretty cool link between our industries.

And, weird rules. For example in my area if a building is 90 degrees to the property line, only the property line wall need a 1 hour rating in the first 5 feet of setback. However, if the building is rotated an angle the 5 foot projection onto the wall needs the rating (even if that projection is only 1 or 2 feet).

Rules are rules.

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