Originally Posted by pyper
There is only one person qualified to tell you if you can modify your truss in any way, and that is the engineer at the truss manufacturer.
Look them up online and give a call -- the answer is probably a simple "no."
To do what you want, you probably need to tear off your roof and re-frame it.
Engineered trusses are just that: "engineered." They are built so that they use the least amount of wood but are still strong enough to hold up the roof and transmit all the load to the exterior walls (typically). Plus they hold the tops of the walls in place. They do this by breaking up the framing into a series of triangles, which transmit loads in different directions. When they are connected these triangles form a unitary structure. Cutting any truss without a full recalculation of the loads and stresses involved is asking for truss failure.
Given that you want to build a loft, with associated live and dead loads, you may have to reinforce the walls as well, to take the extra loads.
Your best bet is to hire a structural engineer. But be prepared for him to tell you that it's going to be very expensive to do what you want to accomplish. As pyper said, you may have to remove the roof to reframe the whole thing.