I am encountering the following warning message while modelling AirflowNetwork in EnergyPlus:

** Warning ** GetAirflowNetworkInput: SurfaceAverageCalculation is entered for field = Wind Pressure Coefficient Type, but no roof surface is defined using an AirflowNetwork:MultiZone:Surface object.
** ~~~ ** Reconsider if this is your modeling intent. Simulation continues. 
** Warning ** GetAirflowNetworkInput: SurfaceAverageCalculation is entered for field = Wind Pressure Coefficient Type.
** ~~~ ** The AirflowNetwork model provides wind pressure coefficients for 4 vertical exterior orientations and 1 horizontal roof.
** ~~~ ** There are only 0 exterior surface orientations defined in this input file using AirflowNetwork:MultiZone:Surface objects. 
** ~~~ ** Reconsider if this is your modeling intent. Simulation continues.

I have clearly defined all the exterior surfaces (Walls and Roof). I can however, see the wind pressure coefficients that are generated in the output. Could you please shed some light on it.