While uncommon, there are some situations that require top-down fire resistance, for example, an office mezzanine built over a warehouse below. The New Zealand Building Code documents require floors to be rated from the underside, based on the FRR of the fire-cell below. Although often ignored, potential top-down exposure is sometimes a consequence of building design and layout.
An FRR is by definition the result against a standard furnace test, and furnaces are not designed for top-down exposure. A standard test furnace is placed vertically for wall testing or rotated horizontally for a floor or ceiling specimen. Turning the furnace upside-down to expose an element from above is not practical. Turning the specimen upside-down does not fairly characterise gravity effects and would not be representative of actual construction.
The performance of top-down fire resistance can be assessed by considering wall and floor/ceiling test results. For a given FRR, the performance specification for ceiling linings is generally higher than for walls. Compare for example wall specification GBTL60 (13mm GIB Fyreline each side of timber studs), with specification GBFC60 (16mm GIB Fyreline on timber joists).
Because gravity acts more severely on ceiling linings, a higher performance lining is usually required. However, when installed on top, gravity assists by holding linings on a floor or floor framing.
In the updated GIB Fire Rated Systems Manual, there are two details outlined, one version that is non-loadbearing, and the other which is loadbearing, like that office mezzanine example.
A top-down or two-way FRR can be created by placing the corresponding wall lining on top of the framing and the relevant ceiling lining below. Figure 1 shows the solution using a 60 minute example.
When it is desirable to provide a floor with loadbearing capability, such as for storage or light foot traffic, the solution is to first install structural sheet flooring which in-turn needs to be protected against top-down exposure. The protective GIB Fyreline lining is placed on top of the flooring and then protected with a thin wear layer such as mdf, carpet or flooring vinyl (depending on intended use). This is illustrated in Figure 2.
Download the new GIB Fire Rated Systems Manual from the GIB website or call the GIB Helpline on 0800 100 442 if you have questions.