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40 psf is a high value for snow load that could result in an uneconomical truss design.You could check for snow load reduction provision (UBC97 Eqn 14-1) if not applied. Moreover structural beams that will be used to support truss will necessarily required to be cater for torsional design as they will be subjected to high torsional moments in two ways, 1, Axial forces in bott & top chord of truss will create a couple acting about the longitudinal axis of beam i.e creating torsion equivalent to axial force times depth of truss. 2, While providing slotted holes (roller supp) to eliminate thermal stresses you have to embed a bracket plate into beam that will be supporting truss and correspondingly will be subjected to vertically directed pressure (from truss end shear) acting at an eccentricity equals to 0.5xbeam width + distance of beam edge to centroid of loaded plate area i.e creating additional torsion moment in beams equals to truss's end shear times eccen.

What is your wind load? Wind load would be more critical for your truss design than snow load provided you don't assume a very high snow load. Umair has mentioned good points here. Do note that the beam end supporting pinned end of truss would have torsion in it . The end with slotted hole will create no torsion in the supporting beam.