In Perth, timber sagging is usually caused by the age of your timber supports, climate stress, leaky roofs, or a combination of warping and gravity over time that affects older homes. In modern homes, timber sagging is usually caused by poor quality materials being used from the beginning.
In the event of timber sagging, you may need to call in the professionals to repair or replace your roof timber supports. In extreme cases, re-roofing or a complete roof replacement may be better options.