Fire-Foam roof torching?

I have a slate roof with 3 elevations. Two are underfelted with new-ish slates, but the other is old and somewhat nailsick and not underfelted. In the current unfavourable market, I opted to just have the roof patched up for now and had slipped tiles fixed and the ridge tiles rebedded. The house is in central England, so weather not too severe though house is fairly high up.
The sick roof slope had a powdery torching that was filling the loft (and my stuff) with fine dust. This was in extremely poor condition with much (about 1/4) having already fallen off so I decided to remove the rest before the roofers came. As poor as it was, I'm now considering that it may have been helping to keep at least parts of the roof together in bad weather!.
I was going to replace the torching with browning but can now see this would take an age, and I'd wonder how waterproof it would be... so I'm now thinking of applying a bead of foam filler in the angle between the top of the battens and slates. Screwfix do a firefoam that may or may not be suitable...
Is this a bad idea?
Paul.
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Traps damp /wetness. Foam on the slates to bond them together is one thing, but foam stuck to wood is a recipe for trouble, especially on an old slate roof, which are anything but watertight. It also makes repair problematic, and slate does need ongoing repairs.
FWIW unfelted roofs can be reslated from inside the loft.
I would guess in theory one could fix expanded metal lath to the rafters and bond it to the slates with foam, keeping the foam off the wood. But, never tried it, so upto you to think about it.
Regards, NT
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