diff --git a/web/.simplecov b/web/.simplecov
index 1091e3c7a..f7df5462a 100644
--- a/web/.simplecov
+++ b/web/.simplecov
@@ -10,14 +10,23 @@ if ENV['COVERAGE'] == "1"
 
   SimpleCov.formatter = SimpleCov::Formatter::MergedFormatter
 
-  SimpleCov.start do
-    add_filter "/test/"
-    add_filter "/bin/"
-    add_filter "/scripts/"
-    add_filter "/tmp/"
-    add_filter "/vendor/"
-    add_filter "/spec/"
-    add_filter "/features/"
+  SimpleCov.start :rails do
+    # remove the :root_filter so that we can see coverage of external dependencies (i.e., jam_ruby)
+    filters.clear
+
+    # ignore Ruby itself (...not to be confused with jam_ruby)
+    add_filter "/lib/ruby/"
+
+    # ignore Rails subfolders which don't contain app code
+    %w{config coverage db doc features log script spec test tmp}.each do |dir|
+      add_filter "#{dir}/"
+    end
+
+    # ignore all gem code except our jam gems
+    add_filter {|src| src.filename =~ /ruby.*\/gems\// unless src.filename =~ /\/gems\/jam/ }
+
+    # categorize JamRuby in the coverage report:
+    add_group 'Jam Ruby', 'jam_ruby'
   end
 
   all_files = Dir['**/*.rb']