Do I Split Cost of Termite Control?

We have an apartment which we rent out that comprises the entire upper floor of a building in our back yard.

 

The lower part of the building is storage.

 

Last year we had to have the entire building fumigated because we had termites. 

 

Do I put the whole fumigation cost down as a rental expense, or just half of it because only half the building is actually used as a rental? Or none at all for some reason?

 

Thank you!