A plastering machine for applying a surface coating layer to a building surface, such as to a building wall, comprising: - a coating material supply connected to a mouthpiece via a conduit, a flexible hose defining a first portion of said conduit, - said mouthpiece having a top portion and a bottom portion and including: - - a rear wall, - - a first side wall and - - a second side wall (31") opposite said first side wall, - - said rear wall being connected to or integral with said first side wall and said second side wall (31") to define a coating material recess for containing coating material supplied to said mouthpiece through said conduit, said recess preferably tapering in a direction from said top portion to said bottom portion, - a lower edge of said rear wall at said bottom portion being configured to delimit, preferably together with said building surface, a coating material exit, such as a gap, for discharge of said coating material contained in said recess in use of said machine, characterised in - said first side wall including opposite said rear wall a first flange and - said second side wall (31") including opposite said rear wall a second flange (34") having a plurality of faces configured for being placed against said building surface in use of said machine - said first and second flanges (34', 34") being oriented to extend away from said recess, - a third spacer device extending in said recess, optionally from said top portion, said third spacer device having a face configured for being placed against said building surface in said use of said machine, - said face of said third spacer device being in a position closer to said first flange than to said second flange (34"), - said face of said third spacer device together with said plurality of faces being configured for keeping said lower edge spaced from said building surface in said use of said machine, by said faces abutting against said building surface. - a coating material supply connected to a mouthpiece via a conduit, a flexible hose defining a first portion of said conduit, - said mouthpiece having a top portion and a bottom portion and including: - - a rear wall, - - a first side wall and - - a second side wall (31") opposite said first side wall, - - said rear wall being connected to or integral with said first side wall and said second side wall (31") to define a coating material recess for containing coating material supplied to said mouthpiece through said conduit, said recess preferably tapering in a direction from said top portion to said bottom portion, - - a rear wall, - - a first side wall and - - a second side wall (31") opposite said first side wall, - - said rear wall being connected to or integral with said first side wall and said second side wall (31") to define a coating material recess for containing coating material supplied to said mouthpiece through said conduit, said recess preferably tapering in a direction from said top portion to said bottom portion, - a lower edge of said rear wall at said bottom portion being configured to delimit, preferably together with said building surface, a coating material exit, such as a gap, for discharge of said coating material contained in said recess in use of said machine, characterised in - said first side wall including opposite said rear wall a first flange and - said second side wall (31") including opposite said rear wall a second flange (34") having a plurality of faces configured for being placed against said building surface in use of said machine - said first and second flanges (34', 34") being oriented to extend away from said recess, - a third spacer device extending in said recess, optionally from said top portion, said third spacer device having a face configured for being placed against said building surface in said use of said machine, - said face of said third spacer device being in a position closer to said first flange than to said second flange (34"), - said face of said third spacer device together with said plurality of faces being configured for keeping said lower edge spaced from said building surface in said use of said machine, by said faces abutting against said building surface.