In accordance with Acceptable Solution C/AS1 4.1.1 each household unit shall be fire separated from other household units. Although this does not specifically mention eaves, it typically requires the vertical fire separation to be extended out into the eaves, terminating at the fascia.  

With GIB® Intertenancy Barrier Systems eaves fire separation can be resolved by extending the GIB Barrierline® into the eave as shown in Figure 1. This is suitable for eaves up to and including 1200mm wide. The GIB® Intertenancy Barrier Systems literature shows how a cantilever can be formed. 

For a double frame IT wall system eaves separation may be resolved by applying a suitable two-way FRR wall system within the eave space as shown in Figure 2. 

Figure 2 – Double Frame IT Wall System eave separation

If similar two-way fire protection was to be applied to a GIB® Intertenancy Barrierline® system, then mineral wool or equivalent would be needed to pack the gap at system transition. See Figure 3. 

Figure 3 – GIB Intertenancy Barrierline® System with eaves separation 

For smaller spaces blocked with a minimum 90mm solid timber (or 45mm rafters each side), no plasterboard lining would be required to maintain a 30-minute fire separation. This is shown in Figure 4.