# Alternating row colours in tabulary?

I managed to get alternating row colours working when using tabular but when I converted them to tabulary to make the tables match \textwidth the row colouring became messed up.

Now it completely ignore the defined starting row and also changes the colour of the rows depending on the number of rows in the table. For example if there are an odd number of rows the top cell is grey, but when I add another row and make the number of rows even, the top cell becomes white. My code for such a table is below:

\documentclass{article}
\usepackage{tabulary}
\usepackage[table]{xcolor}
\definecolor{light-grey}{gray}{0.9}
%
\begin{document}
%
\begin{table}[h]
\rowcolors{2}{light-grey}{white}
\begin{tabulary}{\textwidth}{|l|L|}
\hline
\textbf{ID} & \textbf{Requirement} \\ \hline
REQ1      & Requirement 1 \\
REQ2      & Requirement 2 \\
\hline
\end{tabulary}
\caption{Functional Requirements}
\label{table:Functional Requirements}
\end{table}
%
\end{document}


This gives:

write into the preamble after loading tabulary:

\usepackage{etoolbox}
\preto\tabular{\global\rownum=0\relax}


This gives:

\let\Tabular\tabular