Timber Framed Campsite Shower Block - Treated or Untreated?

[dunny1234]

Hi, I'm about to start building a timber frame toilet/shower block on my campsite. All the groundworks have been completed and the base is 200mm above ground level.

The inside of the building will be well ventilated, but I'm unsure whether to use treated or untreated 2 by 4 inch timber.

If I use treated, I'm worried about the plaster cracking when the timbers shrink. But, will untreated timber last a long time?

So, there's my question, timber framed building - treated or untreated???

Thanks in advance

[welsh brickie]

always treated