The first thing i recommend is a correct identification of the fungi. Dry rot does not usually infect external timbers as it prefers damp, humid, dark, enclosed areas poorly ventilated areas. Many wet rot fungi are mistaken for dry rot because of the dry brittle appearence. the terms dry rot and wet rot can be quite misleading as both fungi require food (timber) and moisture.
Should the problem be of wet rot, its easily solved. remove the food(timber) and thats that. Any new timber should be treated.
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