diff --git a/guix/build/gnu-build-system.scm b/guix/build/gnu-build-system.scm
index d0f7413268..d84411c090 100644
--- a/guix/build/gnu-build-system.scm
+++ b/guix/build/gnu-build-system.scm
@@ -598,6 +598,8 @@ (define (process-directory directory)
                                         (string-suffix? ".tgz" file))
                                     (gzip-file? file)))
                              #:stat lstat)))
+      ;; Ensure the files are writable.
+      (for-each make-file-writable files)
       (for-each reset-gzip-timestamp files)))
 
   (match outputs