Repair to old slate roof

Hello, Photos showing this at http://www.flickr.com/photos/89810542@N02/ Please could I ask for advice from anyone who has experience of repairing old slate roofs? Two adjacent rafters on the same side of the roof have slipped down where they rest against the ridge pole, and are now partly below the ridge pole. If they go another inch or so they will be below the ridge pole so will have no support at the top.
I can't see why they have moved so I assume they have simply bent a little between the ridge pole and the purlin. The ridge pole appears to be still straight, and the corresponding rafters on the other side of the roof are ok.
Is it better to leave the slates and timbers where they are, and try to reinforce the timbers from the inside, and if so any suggestions on how to do the repair?
Alternatively, should I remove some ridge tiles and slates, and try to add new sections of rafter from the outside?
Or some other method? Any practical hints gratefully received. It's a barn with stone walls, roof span 9.3m (30ft). Three purlins on each side. Rafters 50mm x 50mm. Quite old, but I don't know how old. Cheers, James
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It might have slipped. Check to see if it's slid on the wall plate at the other end, or if the wall plate has gone rotten, or if the wall is bowing out at that point.
I presume it just relies on the rigidity of the walls and the purlins to stop the roof spreading? So it's strange it's done it at one end where the spreading support is best.
Also take care - if enough of the structure is seriously compromised, it might suddenly collapse.

I think you have to start by working out what's gone wrong.

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Andrew Gabriel
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