Hi, i am going to be fitting a shower fan and was checking the venting route before i start. I have a run of about 6feet which leads out to the corner of the roof. I have a board running underneath which is asbestos which is held on via nail and is slotted in. I also have a piece of timber that is shown in the pic (ihave removed the end piece) basicaly what i would like to know is if i can cut a section out of the timber to accomodate a 100mm hole which would let me put a grill over and i can cut a 100mm hole in the end piece . Not sure if i am allowed to cut this part of the timber , i could also take the asbestos board out and replace with a grill, could anyone please advice on the best route. Thanks