diff --git a/src/morevna.py b/src/morevna.py
--- a/src/morevna.py
+++ b/src/morevna.py
@@ -19,6 +19,12 @@ def _checkFile(f):
 
 def buildTree(args):
 	_checkFile(args.datafile)
+	if os.path.isfile(args.treefile):
+		treeMod=os.stat(args.treefile).st_mtime
+		dataMod=os.stat(args.datafile).st_mtime
+		if dataMod<treeMod and not args.force:
+			print("tree file is up to date")
+			return
 
 	tree=HashTree.fromFile(args.datafile)
 	tree.save(args.treefile)
@@ -71,6 +77,7 @@ parser=ArgumentParser()
 subparsers=parser.add_subparsers()
 
 pBuild=subparsers.add_parser("build")
+pBuild.add_argument("-f","--force",action="store_true",help="force tree rebuild")
 pBuild.add_argument("treefile", help="stored hash tree location")
 pBuild.add_argument("datafile")
 pBuild.set_defaults(func=buildTree)