You should be able to save yourself the cost of an engineer unless the structure is unusually large or complex. It would help if you told us the span and slope of the roof and if there are any intermediate bearing walls, hips, dormers and the snow load.
A roof truss relies on rafter chords, a bottom chord and intermediate chords arranged in triangular patterns so that all chords are either in tension or compression. Because wood is stronger in compression & tension than in bending the size of the chords can be reduced which saves money. Of course, the strength of the connections become critical but that is solved by building the truss in a factory under strict quality control.
The removal or relocation of even one chord and/or the addition of almost any unanticipated load on the bottom chord will induce BENDING moments in the other chords that they are not large enough to take.
An engineer would have to look at the roof structure not as a truss but as a simple-span rafter system which would involve adding new rafters and floor joists sized for the loads. Such a structure does not not require an engineer because the framing sizes and configurations are clearly and simply shown in any building code. Any licensed builder can design it and get it accepted by the local building department.
If the spans are larger than shown in the building code you will need to use LVL's or I-Joists which would require an engineer. You could hire one or use one provided by a manufacturer through a local lumberyard.
The only issue not addressed by the building code is bouncyness of floors. If you don;t know how to avoid this problem just use the next larger joist size if you are close to the span limit in the code table.
Q
