diff --git a/TestGrid.md b/TestGrid.md
index 60aea61..3abe8c8 100644
--- a/TestGrid.md
+++ b/TestGrid.md
@@ -32,6 +32,16 @@ Usage statistics are available for the public test grid. (Note: ticket #13 menti
  * [other time scales](http://allmydata.org/tahoe-munin/tahoecs2.allmydata.com-tahoe_introstats.html)
 ![](http://allmydata.org/tahoe-munin/tahoecs2.allmydata.com-tahoe_introstats-week.png)
 
+### Space used by the shared public directory
+
+This counts the total size of all immutable files reachable through the
+shared public directory listed above.
+
+ * [other time scales](http://allmydata.org/tahoe-munin/tahoecs2.allmydata.com-tahoe-rootdir-space.html)
+![](http://allmydata.org/tahoe-munin/tahoecs2.allmydata.com-tahoe-rootdir-space-week.png)
+
+
+
 ### Load Average
 
 This measures how long a 1 second callback is overdue, on each machine that