I have a two bedroomed terrace house and a local person has contacted me suggesting he clean the roof of the moss and seal it - not sure what seal it means. Can anyone please advise, is it necessary to have the roof tiles cleaned of moss and I also notice some bits of grass growing there, looking at the roof front and back there does seem to be a fair bit of it. He has also suggested £850 which would include cleaning out the guttering afterwards and I have no idea if this is reasonable for all the work. This is the first time I have met him. Can anyone please advise if this work is really necessary? Many thanks for any help.