[Cura thinks file is not watertight]

Hi Guys,

I have no idea why cura keeps telling me the file is not watertight. 

When I go to slice the file it creates a top and bottom where there is none and infills it, this happens no matter what orientation the part is and depending on orientation is different.

 

If anyone has any ideas where Im going wrong that would be greatly appreciated

[Measure Tool plugin]

Hey guys, seems this tool has gone from the market place, any chance it will come back?