First off, I would recommend not using a "truss" family for this, as it's just a beam with the ends chamfered off.
You are just over complicating your life.

Just use a beam. Place a reference plane at either end that follows the angle of your end-cut. Now just cut the geometry of the beam to the reference planes.

If you make this a group (beam + reference plane at each end) then you can copy that beam around your roof as required and edit only one and the entire set of beams will update.

If you wish to place it on the Skillion slope of the roof, the easiest way is to create a reference plane at the underside of your roof line in a section. Name that reference plane (in the properties box once you select it) and name it something logical like "Skillion Roof Bldg 1 Top of Rafter" or whatever suits.

Go to a plan view
Now under the structure tab on the far right go to the "define workplane" and choose the "Skillion Roof Bldg 1 Top of Rafter" name and NOW in your plan view place your wooden beam family (justified at top of beam) and then go through the process of cutting the geometry to the other end cut reference planes.

The truss family is used (like in the image) for a component which is comprised of a Top and Bottom chord, and web pieces. What you are creating is just a simple beam, no need to complicate your life for no benefit.

Oh and also the family you provided is pretty much empty.