Rotten top plate - how to address?

[d000hg]

Our garage is actually a Victorian coach-house and is a lovely structure with original slate roof. We've talked about converting it into a studio or something BUT on replacing a rotten fascia board have discovered the wall plate (I think this is the term, it runs along the wall and the trusses sit atop it) on one side is extremely rotten. Like you can grab handfuls of it as dust, there are evident strands of mould trailing around.

The trusses do not appear as badly infected and prior to this there was no evidence the roof had issues - no sagging or dipping, the slates are all neatly in place, etc.

Is the only way to fix this to remove the entire roof to put in a new length - on a garage this is an annoying expense but I guess not too hard, but if it was a house that would be an extortionate cost to replace one bit of wood! Maybe there are clever techniques to do this from underneath I can research if I knew the name or something?

 

[d000hg]

I can probably get photos though it appears a very typical roof, let me know any specifics to focus on if so?

 

[foxhole]

Roofing forum?

 

[d000hg]

Roofing forum?

Oh, we have one here? Sorry I missed it and just saw woodwork. Can anyone move it?

 

[TicTac]

You'll need to treat the complete area with a a suitable spray, and the end of the rafters, and the joists as well. If the joist are solid enough, you may be able to put a scaffold board on them to spread the load, and then use a car jack to take the pressure off of the rafter, and then slide a new piece of wall plate underneath. Just make sure someones keeping an eye on the ceiling for stress cracks as you do it.

 

[bobasd]

post photo of the outside at the eaves/fascias showing the generl setup including the wall plate detail.and photos of the inside showing the rafters and joists or the trusses ,which do you have, where they land on the wall plates.
have a look at all fascias around the garage.
find out why water might have got in at the eaves?ie the cause?

you take the weight off by setting up a 2" x 4" frame, like a partition, below the joists and slightly wedge the rafters up.you then replace whole lengths of wall plate and strap them into the face brickwork below. youll need a reciprocating saw for nail cuttin.

if the wall plates are as bad as you say then the tails sitting on the plates must be pretty bad as well, photo them. i'm guesin that theres no plastered ceiling?

dont go cutting out 16" of wall plate at a time by propping one joist at a time.

 

[TicTac]

you take the weight off by setting up a 2" x 4" frame, like a partition, below the joists and slightly wedge the rafters up

Thanks for that Bobasd, brain fade on my part forgetting that, I'd go for acrows and scaffold boards though. And yes, if the facias come off, then the wall plate can go in from the outside. Sorry for the balls up.