How to replace damaged sarking board and roof felt (if needed at all?)

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Established Member
17 May 2012
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South West Wales

I've got a repair to do where an old wood stove flue used to go through the roof, but has since been taken down. Water had been leaking through and caused some rot to the rafter, which I will "sister" with some new timber.

What I don't know is whether it would be a good idea to repair/replace the sarking board, and the felt on top of it? If so, is it possible to do what needs doing from inside the room? Or is the only option to strip back slates and repair from above. Would prefer to do it from inside if at all possible.

Any opinions and suggestions on how best to approach this would be most appreciated.

Many thanks

Unless the perspective in the photo is deceptive, it looks as if the roof pitch is quite low? This would compound the problem. It seems that an inadequate repair was done after the flue was removed. It'll need tackling from above.
Cut away the damp plywood, cut out a piece to match the hole + another one 4" bigger all round. Glue both together

Glue in place with bigger board inside.

Go outside and replace the felt under the tiles.
It may be a royal pain in the bum but you need to do some work both inside and out or you may run the risk of problems with water ingress and damage in the future.