depending on the size and nature of the roofing job even more e g four or five these should be skilled workers having routine familiarity in the setting up of guide bars and the laying and smoothing of plaster and concrete layers to form wry surfaces so as to provide a gradient towards an outlet such as floors in wet premises e g slaughter houses the machinery mounted in the trailer comprises a spray coating apparatus 5 housed in a container which has an upper cylindrical portion 6 closed by a top wall 7 and a lower conical portion 8 a central opening 9 in the top wall 7 is connected through a cell feeder 10 to a bottom opening 11 of a cylindrical buffer silo 12 mounted above the spray coating container the cell feeder 10 is similar to the cell sluices normally used in suction blower conveying systems and thus consists of a cylindrical housing in which there is mounted a rotor carrying a number of radial vanes sliding along the cylindrical wall of the housing to form cells serving to sluice material from one environment to another the cell feeder 10 may be constructed as an ordinary cell sluice but since there is no difference of air pressure between its inlet and outlet the requirement of sealing effect is lower the cell feeder 10 is driven by.