Morning All - i am getting quotes for a flat roof replacement and two companies have said they would lay felt up to and over the existing ridge tiles rather then remove the tiles and lay the felt underneath.

this is the second roof since we've been here - both have lasted well into their 15 years and both have been laid under the tiles.
is over the tiles as good as under? 

I have had two flat roof replacements, and one new one. Of a similar design. Each time under the tiles.

But, I'm almost sure the answer to your question is that the quality of the work is the key. Over - under probably doesn't matter. Attention to detail does. 

 

One question: how - in the '...over ...' case is the felt secured at the edge - how will the edge deal with wind and rain?

Think it depends on the relative heights. Google found this drawing. The upstand formed by the "ridge board" is tall enough to stop water getting in. If it was too low I think you would have to go over the ridge tile and down othe side. 

 

Thanks for the quick response. the above diagram could have been taken from my roof. Both roofers suggested a 2 inch overlap but how the edge deals with the weather is my concern and of course how good the existing ridge mortar is for the next 15 years. but in all honesty we've never had any tiling issues on the flat roof.

I thought the first quote was a "cheap" option, also reflected by the second roofer who wanted to go "under" and surprised when the same solution was offered by the third roofer. 

I will get one more quote - which will probably make it 50/50. Maybe a fifth !! Maybe i'm just waiting for confirmation of what i think I want to hear as a layman. 

I think “over” would look crap and open to wind damage etc. All things being equal I would go under IMO.

Over is a rough as toast and a bodge. Should be under, new board and fillet at the base of the board and then run down over the roof membrane and under the first row of tiles. 

Re-fixing the ridge could be done with a dry ridge system too. 
 

Are they quoting to use felt or rubber ..??

Under the ridge tiles, unless you are very short of upstand height. Over will look like a right bodge.