table with vertical spacing

\documentclass[letterpaper]{article}
\usepackage{helvet}
\renewcommand{\familydefault}{\sfdefault}

\newcommand\T{\rule{0pt}{2.6ex}}       % Top strut
\newcommand\B{\rule[-1.2ex]{0pt}{0pt}} % Bottom strut

\begin{document}
\begin{tabular}{ p{8em}  r  r  l }
\hline
          & Total     & Average   & Unit \T\B  \\    \hline
Area 1    & 419773    &   9.15    &  m\textsuperscript{2} \T \\
Volume 1  & 0         &      0    &  m\textsuperscript{3} \B \\    \hline
\end{tabular}

\end{document}