Any horizontal run of trim, such as head casing and skirt boards, should be protected by a drip-cap and Z-flashing. This is especially important on transition boards, such as the joint between gable siding and the wall below (below).

Often when a drip-cap profile is used, it is assumed to be enough by itself, perhaps just with a coat of paint. But it is vitally important to protect this trim with metal flashing, as well, otherwise the horizontal surface is prone to hold water and can rot out quickly.

To divert water away from horizontal siding when vertical siding is used on gable ends, install a transition board with a drip-cap and Z-flashing.
To divert water away from horizontal siding when vertical siding is used on gable ends, install a transition board with a drip-cap and Z-flashing.

For more on exterior trim details, visit the JLC Field Guide.