When to Retreat for Termites

[Matt Miller]

I have a couple rentals, roughly 10 years old that have not had any kind of re-treatment for termites since they were built.    At what point should I look at preventative treatment again?  Any other suggestions you have are appreciated.

Some additional details:  

Both homes are on slab foundation.

I will be contacting exterminators in the near future, but want to know what I may or may not need upfront, so I don't get taken for a bunch of unneeded services or extra cost.  

From what I understand treatments used in older homes last decades, but the sprays used in new homes only last about 7 years.  Again, this may all be incorrect, but is what I have heard from a couple sources now.