There are addons for GRASS 6.x ([addons][1]) and there are others for GRASS 7 ([addons][2])

Since the future version 7 (still being developed) marks a major break with the 6.x versions (new vector API, etc.) it is doubtful that all GRASS 6.x addons works without problems (there is no r.roughness in [GRASS 7 raster addons][3])

So, if you want to use r.roughness without bugs or problems, use the 6.4.x version


  [1]: http://grasswiki.osgeo.org/wiki/AddOns/GRASS_6
  [2]: http://grasswiki.osgeo.org/wiki/AddOns/GRASS_7
  [3]: http://grasswiki.osgeo.org/wiki/AddOns/GRASS7/raster