Multi line header does not play well with geometry headsep parameter

Question

I would like to use the geometry package alongside fancyhdr. To control the distance between header and main text in my article documentclass, this question recommends use of the headsep parameter:

\documentclass[11pt, a4paper]{article}
\usepackage[includehead, headsep=5pt]{geometry}
\usepackage{fontspec}
\usepackage{fancyhdr}


\pagestyle{fancy}
\fancyhf{}
\fancyhead[C]{A header}%\\a new line in the header}

\renewcommand{\headrulewidth}{1.5pt}

\begin{document}
  \section*{Education}

\end{document}

This seems to work fine, and modifying the headsep indeed modifies the distance between the header and the section title. However, it so happens that my header is multi-line. as soon as I uncomment line 9 as follows:

\fancyhead[C]{A header\\a new line in the header}

The section title clashes with the header, which I do not want. How can I get headsep to remain aware that my header is now multi line?

Answer 1

Starting with the OP's code, I just augmented the \headheight and decremented the \topmargin by equal amounts. Then, the header behaves as I presume the OP would desire.

MWEs below show single-line and double-line header.

\documentclass[11pt, a4paper]{article}
\usepackage[includehead, headsep=5pt]{geometry}
\headheight=\dimexpr\headheight+15pt 
\topmargin=\dimexpr\topmargin-15pt 
\usepackage{fontspec}
\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhf{}
\fancyhead[C]{A header a new line in the header}

\renewcommand{\headrulewidth}{1.5pt}

\begin{document}
  \section*{Education}

\end{document}

\documentclass[11pt, a4paper]{article}
\usepackage[includehead, headsep=5pt]{geometry}
\headheight=\dimexpr\headheight+15pt 
\topmargin=\dimexpr\topmargin-15pt 
\usepackage{fontspec}
\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhf{}
\fancyhead[C]{A header\\ a new line in the header}

\renewcommand{\headrulewidth}{1.5pt}

\begin{document}
  \section*{Education}

\end{document}

Answer 2

To get a nice layout of a page

use the layout package to understand the various dimensions involved in designing a page and their relationships.

To have two lines in the header space you need to set headheight=25pt (5) or more, if plan to add more lines or a figure. Then to separate the header from the text area set headsep=20pt (6) or similar. Finally to allocate enough space from the top of the physical page to the top of the text area set the top= 1in or more.

.

The headheight, the headsep, and others are fixed spaces reserved for your use. They are not aware of overflows nor they collapse in the absence of content.

There’s more than one way to skin a cat. If you overspecify geometry will complain.

The log file is also instructive.

*geometry* driver: auto-detecting
*geometry* detected driver: luatex
*geometry* verbose mode - [ preamble ] result:
* driver: luatex
* paper: a4paper
* layout: <same size as paper>
* layoutoffset:(h,v)=(0.0pt,0.0pt)
* modes: includehead 
* h-part:(L,W,R)=(89.62709pt, 418.25368pt, 89.6271pt)
* v-part:(T,H,B)=(60.0pt, 683.6402pt, 101.40665pt)
* \paperwidth=597.50787pt
* \paperheight=845.04684pt
* \textwidth=418.25368pt
* \textheight=633.6402pt
* \oddsidemargin=17.3571pt
* \evensidemargin=17.3571pt
* \topmargin=-12.26999pt
* \headheight=25.0pt
* \headsep=25.0pt
* \topskip=11.0pt
* \footskip=30.0pt
* \marginparwidth=50.0pt
* \marginparsep=10.0pt
* \columnsep=10.0pt
* \skip\footins=10.0pt plus 4.0pt minus 2.0pt
* \hoffset=0.0pt
* \voffset=0.0pt
* \mag=1000
* \@twocolumnfalse
* \@twosidefalse
* \@mparswitchfalse
* \@reversemarginfalse
* (1in=72.27pt=25.4mm, 1cm=28.453pt)

This is a MWE

\documentclass[11pt, a4paper]{article}
\usepackage[ headheight=25pt, headsep=25pt, top = 60pt, includehead]{geometry}
\usepackage{fontspec}
\usepackage{fancyhdr}

\usepackage{layout}% added<<<<<

\pagestyle{fancy}
\fancyhf{}
\fancyhead[C]{A header \\ a new line in the header}

\renewcommand{\headrulewidth}{1.5pt}

\begin{document}
    \layout% added<<<<<
    \section*{Education}    
\end{document}