In most cases pressure treated timber is used to create the substructure / support frame (ie purlins and joists).  It is important that this timber has been treated properly to Hazard Class ‘4’, ground contact specification.  It is most important that the timber was kiln dried / air dried prior to pressure treating, otherwise the treatment will not penetrate all the sapwood.  This can be the difference between the timber lasting 10 years or lasting 20 years plus.