My headline collides with my text / chapters.
\documentclass[a4paper, 12pt, oneside, numbers=noenddot, ngerman]{scrartcl}
\usepackage[ngerman]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{amsmath, amssymb, amsthm}
\usepackage{fullpage}
\usepackage{setspace}
\usepackage{booktabs}
\usepackage{multirow}
\usepackage{float}
\usepackage[automark]{scrpage2}
\begin{document}
\pagestyle{scrheadings}
\clearscrheadings
\clearscrplain
\clearscrheadfoot
\ihead{text1}
\ohead{text2}
\chead{text3}
\setheadsepline{0.4pt}
\setlength{\parindent}{0pt}
\pagenumbering{roman}
\singlespacing
\tableofcontents
\newpage
\pagenumbering{arabic}
\setstretch{1.4}
\section{Section1}
\end{document}
If I use
\setlength{\headsep}{0.45in}
The problem ist fixed however my the top margin before any content, in this case the header is too much for my taste.

%\usepackage{fullpage}– Fran Apr 22 '15 at 20:37\setlength{\headsep}{0.45in}command. @cfr The 0.4pt are for the thickness of the separation line beneath the header. The 0.45in were for the top margin, the values and units were meant that way. – idkfa Apr 22 '15 at 20:42