I have used hardener and then a 2 part builders bog, followed by surface coating with exterior varnish, to patch up dry rot in Oregon timber in a pergola. It has last about 8 years, but it is not load bearing. The patch could fall out if there was downward force.
As your requirement is load bearing (cement sheet) I recommend supporting the rafter with a prop, cutting the rot out, and replacing the rotten part with a new section of rafter. Use a steel sleeve and fasten with nuts and bolts. It will then last as long as the rest of the eave.
best wishes,
Jeffoir.
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