This is a structural design issue - it cannot be "guessed at" because the contractor thinks it can be done. It sounds like your contractor is in over their head. Here's what I see and read:

- First, research collar ties. They are often mistaken as to what they are utilized for. They are strictly to prevent the rafters from pulling apart at the ridge from uplift/bending. So yes, when they are used properly, metal straps can substitute them. They are used in stick built ridge board framing systems in combination with rafter ties. However . . .

- It looks like an old manufactured truss system with mending plates. At least the mending plates would say so, but they seem a bit light and can't tell what is attaching the web chords or the bottom chords. So, the entire gist of trusses is a specific design (meaning exact chord length and placement with mending plate size) for your loads that allow you to use a smaller size of member than if you went with a traditional stick built roof, and being able to span longer distances unsupported. You cannot just convert an existing truss system to a stick built, or vice versa.

- If you decided to basically build a new cathedral roof inside your old roof frame, that can be done, and at great difficulty. It is not just the nails that should be sticking through the roof sheathing for asphalt shingles, but the heel space at the top plate.

- A handbuilt truss is not going to be adequate, unless an engineer spec'd design is being field checked for compliance with dimensions and fasteners (the typical "glue and screw plywood gussets" is a hillbilly thing best left for sheds, not houses). But, it would be impossible to insert a manufactured scissor truss into the same span, because you only have 2' of spacing to negotiate it to get it above the top plate. And you still have the shingle nails, so the existing top chord needs attached to the new roof diaphragm, so the top chord of the new truss may be approaching a rafter size.

- So then you lean towards stick build, where the rafters will be much larger, and whether that can fit into the heel space without being notched or chamfered beyond limits. But without a ridge beam, you need rafter ties, which negates your cathedral ceiling. So then you have to install a ridge beam, where spanning 31' would want a column or two and foundation work to accompany.

BTW a flitch plate has nothing to do with collar ties. Apples to steak.

thank you for that very detailed reply. we did have a structural engineer provide the above option so i’m hoping he knows what he is doing. The other option was detailed as “We can remove the existing trusses and raise the exterior wall heights in that area 12". This would allow for a larger feeling room, then install scissor trusses. The new roof would sit proud of the existing trusses alleviating any differences in the existing/new trusses. This would require replacement of all or some of the existing shingles/fascia/gutters/soffit. This would give an interior lift of about 1.5" - 2" per ft or around 18" - 24" of lift in the center peak plus an additional 12" of gain from the exterior wall height increase.”

He suspected this would be signficantly more expensive

Buy a different house. That one will never be what you want it to be, even though you spend an enormous sum on trying to remake it. With a different engineer and contractor that won't tear the house down unintentionally. Intentionally is another story. You might want to investigate doing that teardown and rebuild.

Even if someone comes up with a proper scissors truss scenario, the "vault" will be very slight and you might wonder why you spent all that money for something so minor. And it might not be very attractive.

Altering the existing roof structure will give you limited result. Replacing the roof and roof's structure will give you almost unlimited results.

When your bang is only a pop, it's not worth pulling the trigger.

If a structural engineer was telling you that adding collar ties to a truss roof system will allow a steeper ceiling pitch, it's best to find another engineer as that one is a quack.

If you listen to that “engineer”, the house will be a whole lot of nothing. As in, a rubbish pile.