diff --git a/src/plugins/server/server_doc.ml b/src/plugins/server/server_doc.ml
index f38f08705f929dc2d66c7b02a8acd4678acee133..e1ba1ae684f463109cdb645408104f229264227c 100644
--- a/src/plugins/server/server_doc.ml
+++ b/src/plugins/server/server_doc.ml
@@ -307,16 +307,17 @@ let metadata page : json =
 (* -------------------------------------------------------------------------- *)
 
 let pp_one_page ~root ~page ~title body =
-  let full_path = Filepath.normalize (root ^ "/" ^ page) in
-  let dir = Filename.dirname full_path in
+  let full_path = Filepath.Normalized.concat root page in
+  let dir = Filename.dirname (full_path:>string) in
   if not (Sys.file_exists dir) then Extlib.mkdir ~parents:true dir 0o755;
   try
-    let chan = open_out full_path in
+    let chan = open_out (full_path:>string) in
     let fmt = Format.formatter_of_out_channel chan in
     let title = Md.plain title in
     Markdown.(pp_pandoc ~page fmt (pandoc ~title body))
   with Sys_error e ->
-    Senv.fatal "Could not open file %s for writing: %s" full_path e
+    Senv.fatal "Could not open file %a for writing: %s"
+      Filepath.Normalized.pretty full_path e
 
 (* Build section contents in reverse order *)
 let build d s = List.fold_left (fun d s -> s() :: d) d s
@@ -338,13 +339,13 @@ let dump ~root ?(meta=true) () =
          let body = Markdown.subsections page.descr (build [] page.sections) in
          pp_one_page ~root ~page:path ~title (intro @ body) ;
          if meta then
-           let path = Printf.sprintf "%s/%s.json" root path in
-           Yojson.Basic.to_file path (metadata page) ;
+           let path = Filepath.Normalized.concat root (path ^ ".json") in
+           Yojson.Basic.to_file (path:>string) (metadata page) ;
       ) !pages ;
     Senv.feedback "[doc] Page: 'readme.md'" ;
     if meta then
-      let path = Printf.sprintf "%s/readme.md.json" root in
-      Yojson.Basic.to_file path maindata ;
+      let path = Filepath.Normalized.concat root "readme.md.json" in
+      Yojson.Basic.to_file (path:>string) maindata ;
       let body =
         [ Md.H1 (Md.plain "Presentation", None);
           Md.Block (Md.text (Md.format "Version %s" Fc_config.version))]
@@ -363,15 +364,16 @@ let () =
   Db.Main.extend begin
     fun () ->
       let root = Senv.Doc.get () in
-      if root <> "" then
-        if Sys.file_exists root && Sys.is_directory root then
+      if not (Filepath.Normalized.is_unknown root) then
+        if Sys.is_directory (root:>string) then
           begin
-            Senv.feedback "[doc] Root: '%s'" root ;
+            Senv.feedback "[doc] Root: '%a'" Filepath.Normalized.pretty root ;
             Package.iter package ;
             dump ~root () ;
           end
         else
-          Senv.error "[doc] File '%s' is not a directory" root
+          Senv.error "[doc] File '%a' is not a directory"
+            Filepath.Normalized.pretty root
   end
 
 (* -------------------------------------------------------------------------- *)
diff --git a/src/plugins/server/server_doc.mli b/src/plugins/server/server_doc.mli
index 6bb0462554ea7348d1133a53bd925ed4f62098ab..d86aaa97d5be52ac58a8cbb44ce264eb18c52ae8 100644
--- a/src/plugins/server/server_doc.mli
+++ b/src/plugins/server/server_doc.mli
@@ -72,6 +72,6 @@ val package : Package.packageInfo -> unit
 
 (** Dumps all published pages of documentations. Unless [~meta:false], also
     generates METADATA for each page in [<filename>.json] for each page. *)
-val dump : root:string -> ?meta:bool -> unit -> unit
+val dump : root:Filepath.Normalized.t -> ?meta:bool -> unit -> unit
 
 (* -------------------------------------------------------------------------- *)
diff --git a/src/plugins/server/server_parameters.ml b/src/plugins/server/server_parameters.ml
index 3eedff1c7280142bea5b0bdd536b36eb50a384d6..c7086902c978644aa8f1d127c3bc115d0d378654 100644
--- a/src/plugins/server/server_parameters.ml
+++ b/src/plugins/server/server_parameters.ml
@@ -60,11 +60,12 @@ let server_doc = add_group "Server Doc Generation"
 let () = Parameter_customize.set_group server_doc
 let () = Parameter_customize.do_not_save ()
 
-module Doc = P.String
+module Doc = P.Filepath
     (struct
       let option_name = "-server-doc"
       let arg_name = "dir"
-      let default = ""
+      let file_kind = "Directory"
+      let existence = Fc_Filepath.Must_exist
       let help = "Output a markdown documentation of the server in <dir>"
     end)
 
diff --git a/src/plugins/server/server_parameters.mli b/src/plugins/server/server_parameters.mli
index 4186a397421ee6135487e34a06ad7b1bc90b3f13..74d29a8d0c1a1575ff25bbb0906f97b80905516d 100644
--- a/src/plugins/server/server_parameters.mli
+++ b/src/plugins/server/server_parameters.mli
@@ -24,7 +24,7 @@
 
 include Plugin.General_services
 
-module Doc : Parameter_sig.String (** Generate documentation *)
+module Doc : Parameter_sig.Filepath (** Generate documentation *)
 module Polling : Parameter_sig.Int (** Idle waiting time (in ms) *)
 module AutoLog : Parameter_sig.Bool (** Monitor logs *)