How much should I reduce offer for 15 year old roof?

[Taylor Green]

Hi,

I was wondering how much I should reduce my offer, if at all, for a 15 year old roof that is in good shape.

When I walked thru the property I thought it was roughly 5-8 years old, however the inspector said it was about 15 years old, but still in good shape.

I have never replaced a roof before, but my understanding is that it would be roughly 8k to do. (2000 sq ft)

I figure the roof will last another 10 years, so how should I factor that into my offer price?

Thanks for the help!