Been up on my garage roof - maintenance work bit of bitumen more chippings etc. All not too bad for twenty odd years. Good gradient so rain flows of OK ... just puddles at end near gutter. The roof board has sagged ( Chipboard ) due to increased space here between last joist ! I am going to try and brace it back a bit from inside but want really to level dip with something before over felting etc. Any suggestions what to use ? Thinking about some sort of levelling compound which is waterproof ....... bitumen is too tick & would take quite a bit to do ? Any help would be most appreciated Cheers