I have installed a 4" oatley vent flashing and drilled holes in roof and walls with a 4-1/8 bit. Unfortunately the radon coupler out of the concrete is only 3". This was a plumbing mistake and long past. I am pretty sure I will just adapt from 3" to 4" so the roof flashing seals properly.

There is 3" pipe already ran to the attic from the basement. Should I rerun 4 inch pipe all the way from the concrete 3" coupler, or just use the adapter in the attic before I go out the roof?

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