flat roof leaking 'tar stains' when raining

[middy1]

replaced wooden fascia board on 10 year old garage flat felt roof with UPVC boarding. This was done by cutting existing board below felt overlap, fitting new boards and applying a sealant between.
Since fitted, when it has rained, there have been streaks of 'tar like' substance coming from numerous places (probably 20/25)on and running down the fascias. Whilst there may have been an occassional nick of the felt the leaks are more numerous. Various sealants have been tried and failed. A sealant has also been applied over the felt overlap and has not worked. Any suggestions on cause and remedy?

 

[Mw Roofline]

You don't need to apply a sealant in between the fascia and under the welt as it will always be dry up there. If it has been sealed with a bithuminous (can't spell it) sealant like roof and gutter sealant it will run down the fascia as it goes a bit funny.

 

[middy1]

thanks for response but the sealant is to fill the gap between the new fascia and the remainder of the old wooden one that was cut at the felt level and is just a propriety sealant.It may be bitumin that is leaking/leeching out but why?