Cross Gable Roof Extension Help! 5

[sithlord382]

Hi I have been looking at a house (circa 1960s) to purchase that has an approved planning application attached to it, for a side extension that will result in a cross gable roof structure. I went to view the property today and went into loft to find some sort of truss system instead of the open space rafter configuration which I am normally used to seeing.
I am interested in purchasing the property, carrying out the proposed works and re-selling but I need to roughly figure out how much the proposed works would cost before I go ahead, to essentially see if it is worth it.
My question is how would the new gable roof (which is perpendicular to the existing gable roof) be constructed? The existing roof has a W shaped truss system and surely you can't just snip away at these trusses to make way for the new roof structure as it will effect the structural integrity. Would the new roof construction need to be a truss system (therefore do i need to get a truss specialist involved in determining a quote) or can it be open plan with the standard ridge beam, roof rafters etc.

Thanks in advance

 

[Nicolas P.]

Any opinion on whether or not the continuous sheathing of the roof and the valley boards are keeping outward thrust from the rafters in check?

 

[KootK]

I suspect that most of our gang would consider that ridge beam condition which, conveniently, has no thrust. The plywood can certainly handle some thrust but then you'd have to answer some difficult questions about the fasteners and jointing in the plywood sheets etc.

 

[Nicolas P.]

You have a very good point...fasteners through sheathing and rafters beneath it would now have to be analyzed, as well as lateral bracing of those same rafters.

It's just that I've seen so many overframe (or california or blind valley whatever the name is in your part of the country) that are supported on one end by a girder truss (which makes sense) but on the other end, it's only supported by the valley board over the sheathing and rafters. I haven't seen anyone do the calculation to see if the rafter or truss where the end of the valley is supported can actually be used as a "post" or "column" to transfer the load all the way to the foundation footings. contractors and architects seem to just assume it will...