How do i repair rotten truss

Hi, hope someone could give me some advice.
The property is a mid terrace house, the problem is water has been leaking through my roof for a while without me knowing. Result is wall plate is rotten, also where my big problem is the roof truss that sit on the wall plate are also rotten. My question is have the truss got to be replaced,or can i cut back to good timber and some how join together.Or would it be acceptable to bolt another piece of timber next to the orginal.
I hope this make sense.

Depending on how extensive the damage is, you may be able to glue and nail a 12mm plywood gusset to each side of the truss.

Make sure that you treat any rotten timber first

Never cut any timber out of a truss. Fix additional timber to the side if need be

Thanks for the reply.
Wallplate very rotten,ends of all truss that sit on wallplate are rotten extending about 6 inches along truss.

Do you know the original manufacturer/supplier of the roof trusses? You can contact them and they will tell you exactly how to make a good repair. You may be surprised at just how far the repair will extend onto the sound wood.

Our house was built with a faulty roof truss. Despite being told about it repeatedly the builder did nothing. Had to get the NHBC involved The repair extended for 5 feet each side of the ends of the break on both faces of the compromised truss. I asked about redundancy and safety margins in the roof trusses and was surprised to be told that there is no redundancy at all: every truss is required and has to be in good condition or the roof may fail!

Don't remove any material from the truss.

how old is this mid-terraced house?

are we talking truss as in 'A' frame truss:

or modern 'fink' truss: