diff --git a/src/thirdparty/build_ui.sh b/src/thirdparty/build_ui.sh
index 17d4372d7..0e4ad2ccf 100755
--- a/src/thirdparty/build_ui.sh
+++ b/src/thirdparty/build_ui.sh
@@ -37,7 +37,10 @@ if [ ! -d $CATAPULT_HOME ]; then
   pushd $CATAPULT_HOME
     git checkout $CATAPULT_COMMIT
   popd
+fi
 
+# If the autogenerated catapult files aren't present, then generate them.
+if [ ! -f $CATAPULT_FILES/index.html ]; then
   python2 $VULCANIZE_BIN --config chrome --output $CATAPULT_FILES/trace_viewer_full.html
   cp $CATAPULT_HOME/tracing/bin/index.html $CATAPULT_FILES/index.html
 fi