Roof Spread

So, notionally your rafters are balanced at an angle on the new dwarf walls and cantilever out to the ridge, span down to the walls. If the span from ridge to dwarf wall is shorter than from dwarf wall to house walls then it's likely stable enough with the self weight of the roof but if they sag over time, either side of the dwarf wall you may well see some spread and rotation of the dwarfs. Interesting design but if an SE's signed off on it then no problems!

How did you fix the wall ends of the rafters?
The dwarf walls are Approx a 1/3 of the way down from the ridge. 2/3 distance to the house walls. The rafters are toe nailed to the wall plate with large nails. No birdsmouths etc anywhere. I was thinking of doubling up the rafters from above the stud wall to the external wall for some extra strength? These could be birdsmouthed either end?
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If you've raised these points with your SE and they say it's fine then the sistering would be unnecessary. If it will help you sleep better anyway then go for it; it's your house and you can diminish the insulation of your roof by replacing it with wood if you like.. just hope your BC doesn't take issue with it

Note: It's not just the 2/3 section sagging that would rotate the dwarfs, the same sag applies to the 1/3 section too, but then we're back to the notion that the SE will have considered this when determining the rafters' ability to support the roof and will have ensured any sag will be within tolerance so sistering is unnecessary there too

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