diff --git a/lib/spack/spack/test/environment.py b/lib/spack/spack/test/environment.py
index 2b499869c5d48a7a201f4fa094441adcb7d11976..e9f0a5182fd120d32ff424f2ac60537e14adffe6 100644
--- a/lib/spack/spack/test/environment.py
+++ b/lib/spack/spack/test/environment.py
@@ -64,11 +64,14 @@ def test_filter_system_paths(self):
         filtered = filter_system_paths([
             '/usr/local/Cellar/gcc/5.3.0/lib',
             '/usr/local/lib',
+            '/usr/local',
             '/usr/local/include',
             '/usr/local/lib64',
             '/usr/local/opt/some-package/lib',
             '/usr/opt/lib',
             '/lib',
+            '/',
+            '/usr',
             '/lib64',
             '/include',
             '/opt/some-package/include',
diff --git a/lib/spack/spack/util/environment.py b/lib/spack/spack/util/environment.py
index 15eb52d8e21ba184d8f22fca28ba3defab316f6c..420cce8245899c5fc6072e29df891556eec74a32 100644
--- a/lib/spack/spack/util/environment.py
+++ b/lib/spack/spack/util/environment.py
@@ -24,9 +24,10 @@
 ##############################################################################
 import os
 
-system_paths = ['/', '/usr/', '/usr/local']
+system_paths = ['/', '/usr', '/usr/local']
 suffixes = ['lib', 'lib64', 'include']
-system_dirs = [os.path.join(p, s) for s in suffixes for p in system_paths]
+system_dirs = [os.path.join(p, s) for s in suffixes for p in system_paths] + \
+    system_paths
 system_bins = [os.path.join(p, 'bin') for p in system_paths]