diff --git a/lib/Slic3r/Geometry/Clipper.pm b/lib/Slic3r/Geometry/Clipper.pm
index c88500c91..8cb0069fe 100644
--- a/lib/Slic3r/Geometry/Clipper.pm
+++ b/lib/Slic3r/Geometry/Clipper.pm
@@ -5,7 +5,8 @@ use warnings;
 require Exporter;
 our @ISA = qw(Exporter);
 our @EXPORT_OK = qw(explode_expolygon explode_expolygons safety_offset offset
-    diff_ex diff union_ex intersection_ex PFT_EVENODD JT_MITER JT_ROUND);
+    diff_ex diff union_ex intersection_ex PFT_EVENODD JT_MITER JT_ROUND
+    is_counter_clockwise);
 
 use Math::Clipper 1.02 ':all';
 our $clipper = Math::Clipper->new;
diff --git a/lib/Slic3r/Layer.pm b/lib/Slic3r/Layer.pm
index 6c6511986..9fe78fe7d 100644
--- a/lib/Slic3r/Layer.pm
+++ b/lib/Slic3r/Layer.pm
@@ -3,7 +3,7 @@ use Moo;
 
 use Math::Clipper ':all';
 use Slic3r::Geometry qw(scale collinear X Y A B PI);
-use Slic3r::Geometry::Clipper qw(union_ex diff_ex intersection_ex PFT_EVENODD);
+use Slic3r::Geometry::Clipper qw(union_ex diff_ex intersection_ex is_counter_clockwise);
 use XXX;
 
 # a sequential number of layer, starting at 0
@@ -98,7 +98,15 @@ sub make_surfaces {
     my ($loops) = @_;
     
     {
-        my $expolygons = union_ex($loops, PFT_EVENODD);
+        # merge contours
+        my $expolygons = union_ex([ grep is_counter_clockwise($_), @$loops ]);
+        
+        # subtract holes
+        $expolygons = union_ex([
+            (map @$_, @$expolygons),
+            (grep !is_counter_clockwise($_), @$loops)
+        ]);
+        
         Slic3r::debugf "  %d surface(s) having %d holes detected from %d polylines\n",
             scalar(@$expolygons), scalar(map $_->holes, @$expolygons), scalar(@$loops);