diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 86dda80..441e17c 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -248,7 +248,9 @@
 # Time in seconds between build status checks. (integer value)
 #build_interval = 1
 
-# Timeout in seconds to wait for an instance to build. (integer value)
+# Timeout in seconds to wait for an instance to build. Other services
+# that do not define build_timeout will inherit this value, for
+# example the image service. (integer value)
 #build_timeout = 300
 
 # Catalog type of the Compute service. (string value)
diff --git a/tempest/config.py b/tempest/config.py
index cc6d626..dbe9bc2 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -185,7 +185,9 @@
                help="Time in seconds between build status checks."),
     cfg.IntOpt('build_timeout',
                default=300,
-               help="Timeout in seconds to wait for an instance to build."),
+               help="Timeout in seconds to wait for an instance to build. "
+                    "Other services that do not define build_timeout will "
+                    "inherit this value, for example the image service."),
     cfg.BoolOpt('run_ssh',
                 default=False,
                 help="Should the tests ssh to instances?"),