with felt support trays regularly come across so called roofing experts who fit felt support trays incorrectly fitting them on top of the under felt rather than below directing any water getting under the tiles into the property rather than the gutter & an other great one they do is have the trays laying back the way rather than angled down the roof towards the gutter allowing water to drip of the end of the trays into the property creating damp, its very soul destroying at times to inform someone who has spent thousands on there roof done by a so called expert usually recommended by a friend that what they have done is the reason for there current dampness issues
on a tiled roof whether that be clay or concrete retro fitting them is straight forward, on a slate roof retro fitting then creates a weakness in the roof as you cannot nail your top slate into place & can only use a tie this will lead to a row of un nailed slates across your new roof creating a weakness in the roof covering