How to fix sagging timber roof trusses?
TL;DR
You will probably compress your roof and make things structurally worse using &quot;permanent&quot; ratchet straps.

With the ratchet straps you are only hiding a symptom which can be seen from inside your home. It would make things structurally worse in the long run.
Look at your roof line from the outside, is the ridge starting to do something like this? Ratchet straps will exacerbate the issue.

I'm sorry but there is no easy fix. Either the roof was engineered wrong, built poorly, or someone took out a load-bearing wall.
You could try filing a claim with your homeowner's insurance. Hopefully they're willing to get this fixed before it becomes exponentially expensive and/or deadly. If you involve a structural engineer before calling your homeowner's insurance then you would have verifiable proof of the issue and the insurance would not be able to easily cancel your policy to avoid honoring your claim. Do note that this would very likely raise your premium.

One final thought:
Talk to a real estate lawyer. A 15 year old house should not be having this issue. You might be able to sue the builder or previous owner for this issue.