Fireblocking does not actually stop fire; it cuts air flow (draft) in concealed building cavities.  Air Flow through concealed cavities could allow a fire to spread more quickly, and fireblocking therefore prevents buildings from being quickly engulfed in fire.  Fireblocking gives the occupants more time to escape and gives firefighters more protection by helping to prevent catastrophic building collapse.  In multi-family dwellings, larger open spaces are divided by draftstops to prevent the spread of flame.  Building Areas with different uses, such as a garage and house, require fire-resistive separation, as do buildings with different occupancies, such as two family dwelling or a dwelling above a commercial building.
Typical Areas that need to be Fireblocked:
Block concealed wall spaces every 10ft horizontally and vertically
Block any ceiling/floor levels
Block vertical intersections such as soffits drops & cove ceilings
Concealed spaces under stairs at top and bottom of stringers.
Seal gaps around ducts and pipes at the floor and ceiling penetretation
Material to be used:
2x lumber
or better gypsum board
Compressed fiber
Separation and protection from garages:
Use a min uf gypsum board on garage side of walls common to house
Use a min gypsum board on walls that support ceilings to common house
No ducts openings in garage
Ducts through wall or ceiling common to house need to be 26 g steel.
Use non combustible floors with slope to vehicle door or a drain
Note:  Please check with your building officials and or local building authorities before you begin a new building.  Local building codes change all the time.  This article is only intended to provide general information on this matter.
Fire Block