diff --git a/tools/building.py b/tools/building.py
index d4f6b42129..624245c202 100644
--- a/tools/building.py
+++ b/tools/building.py
@@ -123,7 +123,7 @@ def PrepareBuilding(env, root_directory, has_libcpu=False, remove_components = [
     global Rtt_Root
 
     Env = env
-    Rtt_Root = root_directory
+    Rtt_Root = os.path.abspath(root_directory)
 
     # add compability with Keil MDK 4.6 which changes the directory of armcc.exe
     if rtconfig.PLATFORM == 'armcc':