Tar Roof Lifespan

James from Pennsylvania asked the following: 

I am interested in a purchasing a home that has a metal roof (in Ohio). However, the metal roof is fully covered with tar. The owner states the tar needs redone every 4-5 years. Is that a normal or common roofing system? I'm concerned that there are underlying issues and they're simply covering everything up with tar. Or that the weight of the tar will be an issue over time. Please let me know if you have any thoughts or concerns. Thanks!

Below is what our roofing expert, John Kenney from Cotney Consulting Group, had to say to your question: 

Roofing expert, Shawn Snodgrass from Kalkreuth Roofing and Sheet Metal, also responded: 

Anytime a coating has been used on a residential roof, the original roof likely has already run out it’s life span. Coatings are not recommended by any of the metal roof manufacturer’s and voids any warranty that may have been in place at that time. Furthermore, I would not recommend coating a metal roof with any asphaltic product as they have a tendency to run when temperatures rise, filling the gutter with tar. I would also not recommend coating a metal roof multiple times as there in no way to determine the total weight as it relates to how the home was constructed.
I hope this helps.