Mitek does not include the self-weight in the analysis. It does provide you the weight of the truss leaving the designer responsible for including enough DL for self weight. I have tackled this problem because we design and manufacture a number of long span agriculture trusses. We want to account for enough self-weight, but no overload the truss either. Here is how I did it:
Write a spreadsheet to calculate how much TC DL and BC DL is required given a particular truss spacing, my spreadsheet looks something like this:
--Maximum TC Pitch:
--Weight of truss: (you get this from Mitek, etc)
(Truss weight / (Spacing)(Span) / 2 gives you PSF DL due to truss alone equally distributed to top and bottom chords
Additional TC DL:
(These boxes have pull down automated menus with cooresponding PSF values. These values are summed and multiplied by the pitch adjustment and added to the portion of DL from the truss self-weight)
Additional BC DL:
(These boxes have pull down automated menus with cooresponding PSF values. These values are summed added to the portion of DL from the truss self-weight. If you have scissor trusses you may use a DL pitch adjustment on BC)
For your example (4 PLY GIRDER 36', (5:12), 24" O.C. Trib, 1400# Self weight, Assuming 15# felt, medium asphalt shingles, 7/16" OSB or Com-Ply for top chord loading...1/2" gypsum on bottom chord) The required DL TC = 14.68 PSF and DL BC = 11.92 PSF, again given that spacing.
You can download a dead load calculator from WTCA called LoadGuide, but, I caution you that the purlin weight at a given spacing is incorrect. They are too heavy. I called them and they agreed. The spreadsheet should take you about 2 hours to put together but it will save you a lot of time.