Redraw truss as attic truss

[GeneDavis]

I've a simple one floor house and have truss-framed the roof, and now want a six-truss run, 10 feet in length, to all be attic trusses so we can have a platform deck on which to mount HVAC equipment.

 

The depth of the bottom chord for the rebuild  is unknown but it's certainly not 3.5".

 

Do I just draw my "room" with invisible attic walls and guess the floor structure at, say, 7.25"?  And the rebuild the affected trusses?