diff --git a/docs/manual/manual.mk b/docs/manual/manual.mk
index fd9091eb66..d664603a65 100644
--- a/docs/manual/manual.mk
+++ b/docs/manual/manual.mk
@@ -33,8 +33,8 @@ endef
 # The variable <DOCUMENT_NAME>_SOURCES defines the dependencies.
 ################################################################################
 define GENDOC
-$(call GENDOC_INNER,$(1),xhtml,html,html,HTML)
-$(call GENDOC_INNER,$(1),chunked,split-html,chunked,Split HTML)
+$(call GENDOC_INNER,$(1),xhtml,html,html,HTML,--xsltproc-opts "--stringparam toc.section.depth 4")
+$(call GENDOC_INNER,$(1),chunked,split-html,chunked,Split HTML,--xsltproc-opts "--stringparam toc.section.depth 4")
 $(call GENDOC_INNER,$(1),pdf,pdf,pdf,PDF,--dblatex-opts "-P latex.output.revhistory=0")
 $(call GENDOC_INNER,$(1),text,txt,text,Text)
 $(call GENDOC_INNER,$(1),epub,epub,epub,EPUB)