Unless you can pressure treat the logs just brushing something on the outside doesn't do much good. We used to be able to get creosote and fill a 55 gallon drum with 50/50 diesel and creosote and soak our fence posts for about a year. They lasted a long time. Pressure treated timbers are estimated to last 20-30 years, but I've seen some start rotting in less than 8 years.No I don't think so.
We have a mixture of woodland but I'll have to check for sure what I can access at the location.
Will treat the whole length of the logs
We do have yes, but all the big ones were used to make longbows back in the day and after the longbow was superseded by the musket the yew wasn't planted anymore.Do you have any yew trees in your area? They would last a lifetime, even untreated.
Larch is a possibility!What about larch? Those logs don´t get affected by water for a long time.
Dont put your "roof"logs directly on earth and coat them with waste oil or tar. Try to use dried logs. Fresh ones will split and there´s a possibility for puncturing your tarp.