Shaft Enclosure at Top - 707.12

IBC 2006
Type V-A, R2

At the top of elevator shafts/stairs we have been terminating the 2HR assembly into a 2HR roof/assembly, however, I question whether this horizontal assembly should be more like a shaft assembly installed horizontally to provide protection from both directions since there is only a 1HR assembly outside the shaft limits. The roof is typically a wood-famed truss so you have a "plenum" of sorts that is only 1HR and only provides protection from the one direction. If the 1HR assembly fails say at 61 minutes the adjacent shaft "cap" assembly is then exposed.

The commentary is perfectly clear when a shaft terminates at a floor (Figure 707.12) but not at a roof. We could extend the shaft walls to the underside of the sheathing as noted. I have attached our detail. Thanks.

Glen

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