Why should you remove the old tar paper when you put on a new roof?

yes, i am not trying to sound like a smart alek but if you don't know the answer please seek advice from a professional roofer you will be better off in the long run.

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Check with the info on your shingles, some recommend NOT putting down tar paper first others go as far as voiding the warranty if you do install them over tar paper.

Personally, I would not put shingles on my own house without tarpaper, it is a cheap and fast extra layer of protection from water that can cause a lot of damage sometimes before you can see it.