Id recommend using that EPDM (im no builder either) my mums had a garage built similar size and they used the 18mm chipboard then a massive roll of this rubber membrane roof lining, its guaranteed for like 30 years, doesnt chip, crack, split, element proof, wont shrink etc, he was that chuffed with it hes using it on the extension theyre building, looks better and lasts longer then felt, and i think it was about £150 for a large roll. google it