Simply add
\fancyfoot[C]{\Roman{chapter}\,--\,\thepage}
to your current settings. Since, for consistency's sake this should also apply to the first page of each chapter (in ehich the plain style is used), a redefinition of plain will also be needed:
\fancypagestyle{plain}{%
\fancyhf{}
\fancyfoot[C]{\Roman{chapter}\,--\,\thepage}
\renewcommand{\headrulewidth}{0pt}
}
A complete example:
\documentclass[a4paper]{book}
\usepackage{fancyhdr}
\usepackage{lipsum}
\pagestyle{fancy}
\fancyhf{}
\fancyhead[LE,RO]{}
\fancyhead[LO,RE]{\slshape \leftmark}
\fancyfoot[C]{\Roman{chapter}\,--\,\thepage}
\renewcommand{\headrulewidth}{0pt}
\fancypagestyle{plain}{%
\fancyhf{}
\fancyfoot[C]{\Roman{chapter}\,--\,\thepage}
\renewcommand{\headrulewidth}{0pt}
}
\begin{document}
\chapter{Test chapter one}
\lipsum[1-40]
\chapter{Test chapter two}
\lipsum[1-40]
\chapter{Test chapter three}
\lipsum[1-40]
\end{document}
Some images of some of the footers in different chapters:


Of course, activate these settings from the point in which chaters will be numbered on. Since this applies particularly for the redefinition of plain, you could conditionally make the redefinition of the footer depending on wheter you are on the \mainmatter or not:
\makeatletter
\fancypagestyle{plain}{%
\fancyhf{}
\if@mainmatter
\fancyfoot[C]{\Roman{chapter}\,--\,\thepage}
\else
\fancyfoot[C]{\thepage}
\fi
\renewcommand{\headrulewidth}{0pt}
}
\makeatother
The code:
\documentclass[a4paper]{book}
\usepackage{fancyhdr}
\usepackage{lipsum}
\pagestyle{fancy}
\fancyhf{}
\fancyhead[LE,RO]{}
\fancyhead[LO,RE]{\slshape \leftmark}
\fancyfoot[C]{\Roman{chapter}\,--\,\thepage}
\renewcommand{\headrulewidth}{0pt}
\makeatletter
\fancypagestyle{plain}{%
\fancyhf{}
\if@mainmatter
\fancyfoot[C]{\Roman{chapter}\,--\,\thepage}
\else
\fancyfoot[C]{\thepage}
\fi
\renewcommand{\headrulewidth}{0pt}
}
\makeatother
\begin{document}
\frontmatter
\tableofcontents
\mainmatter
\chapter{Test chapter one}
\lipsum[1-40]
\chapter{Test chapter two}
\lipsum[1-40]
\chapter{Test chapter three}
\lipsum[1-40]
\end{document}
\cfoot{\Roman{\thechapter} -- \thepage}? – cfr Mar 03 '14 at 22:32plainwhich was not needed there, but it has to be done here. – Gonzalo Medina Mar 03 '14 at 23:55