I always tell people that I understand that you have to trust the information you receive from a tradesman (unless you know better) but that is really to prevent myself from miscalling their poor decisions.  However……  A few years after having the front of my house harled I noticed a damp patch at a high corner.  Not having high enough ladders I asked a roofer, whom I trust(ed), to have a look and he said that all was well.  The damp patch continued and I asked another roofer to look for me and he declared all was well but suggested another downpipe as the gutter may be overflowing.  This was installed and it continued.  Only when I scaffolded and got up there myself did I see the issues – from a building surveyors perspective, one who understands how materials interlink and the pathways for water – although it wouldn’t have taken a brain of Britain to see the problems that existed.