woodsmith
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Without seeing it it's not possible to say for absolutely certain but I would think you could fill this gap with something more waterproof. A flexible mastic would work but if you have a long, wide and deep gap it will cost you a small fortune in mastic. It may be possible to fill the gap with a foam filler as this would be much cheaper but you may need to use a fire resistant version and it would need some sort of cover as these foams are not usually UV resistant and it would soon degrade in sunlight.
Another option may be to remove the fibre, part fill the gap enough to form a water seal, then put the fibre back.
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