A roof system including roof support and roof, preferably a flat roof. The roof support includes (a) a plurality of first joists and (b) a plurality of second joists. Each first joist acts as a roof girder. Each second joist extends between a pair of adjacent first joists. Each first joist and each second joist includes a top chord, a bottom chord and web members connecting the chords. Each top chord is hollow, rectangular cross section and sealed; each bottom chord and web member is preferably a hollow sealed pipe. As a result the roof support can be made of joists with a minimum of flat surfaces and nooks and crannies where dust and particles can collect, so that the roof support is easier to wash and regularly sanitize.