A description of trusses...

What is a truss?
A truss is an individually designed component, engineered to provide a structural frame for the support of a floor or roof.  Prefabricated from high quality, stress graded lumber and joined together with steel nail-plate fasteners.  It is a flexible, practical and fully-engineered solution to all floor and roof requirements.  They are economical, as there can be up to 40% less lumber used in a traditionally formed floor or roof and competitively priced.  
They can reduce labor cost on site due to the amount of prefabrication.

Overhang

Most rooflines extend past the exterior walls providing a protective overhang for windows, doors and the house itself. If you want to keep the same overhang for your addition, measure your existing overhang on a horizontal plane. Because you will more than likely cover the outside edge of your overhang with a board or two (fascia), you will need to remove a section of existing fascia to view the existing overhang. This is generally done in an area where the addition meets the existing house. With the fascia removed, the overhang can be measured accurately and the material needed to duplicate the existing fascia can be determined.

Heel Height

You now have the ability to check your slope and overhang. One more variable is required for a perfect match between your addition and existing house - heel height. The overall heel height is the vertical distance of rafter material (trusses or common framed) above the outside edge of your top plate.

Cantilevered Trusses

Previously, we discussed truss overhangs. Typically, the overhang of a truss is merely an extension of the top chord as illustrated in the drawing to the right labeled 'overhang.' There are two other alternatives for overhangs. One extends the bottom chord out over the bearing, commonly referred to as a cantilevered overhang. Another overhang option is provided by the an energy truss.

One of the advantages of utilizing a cantilevered overhang is the resulting rise of facia height. If a project requires circle tops over windows or a transom above a door, an overhang extending from the top chord reduces the amount of wall area that can be viewed "from the street." If you have 9 feet high walls, a 6 on 12 roof pitch, and an overhang of two feet, the overhang will drop 12" below the plate line. This will allow only eight feet to place window and door treatments that can be viewed without overhang obstruction. Using the same specifications as above, a cantilevered overhang allows all nine feet of wall to be seen.

Energy Trusses

An energy truss offers the same advantages as the cantilevered truss (raising the facia); however, the soffit is not returned. By the way, a soffit return is the lumber filler placed horizontally from the end of the top chord to the wall. The energy truss allows the project designer an option of raising the overhang several feet above the plate line. This provides the illusion of higher walls and the additional space for brick or stone work over tall windows and doors.

Along with esthetic reasons, the cantilevered and energy trusses prove to be beneficial with respect to installation of attic insulation. By raising the roof line considerably higher than the outside walls, insulation can be applied in a more effective manner to the plate line.