diff --git a/roots.h b/roots.h
index 6e3b243..a14b7d9 100644
--- a/roots.h
+++ b/roots.h
@@ -41,6 +41,12 @@
 // it is mounted.
 int format_volume(const char* volume);
 
+// Reformat the given volume (must be the mount point only, eg
+// "/cache"), no paths permitted.  Attempts to unmount the volume if
+// it is mounted.
+// Copies 'directory' to root of the newly formatted volume
+int format_volume(const char* volume, const char* directory);
+
 // Ensure that all and only the volumes that packages expect to find
 // mounted (/tmp and /cache) are mounted.  Returns 0 on success.
 int setup_install_mounts();