diff --git a/lib/Slic3r/Print/Object.pm b/lib/Slic3r/Print/Object.pm
index aa24eedf3..537817f52 100644
--- a/lib/Slic3r/Print/Object.pm
+++ b/lib/Slic3r/Print/Object.pm
@@ -59,9 +59,14 @@ sub slice {
             $print_z += $first_layer_height;
             $print_z += $self->config->layer_height * ($self->config->raft_layers - 1);
         
-            # at this stage we don't know which nozzles are actually used for the first layer
-            # so we compute the average of all of them
-            my $nozzle_diameter = sum(@{$self->print->config->nozzle_diameter})/@{$self->print->config->nozzle_diameter};
+            # compute the average of all nozzles used for printing the object
+            my $nozzle_diameter;
+            {
+                my @nozzle_diameters = (
+                    map $self->print->config->get_at('nozzle_diameter', $_), @{$self->print->object_extruders}
+                );
+                $nozzle_diameter = sum(@nozzle_diameters)/@nozzle_diameters;
+            }
             my $distance = $self->_support_material->contact_distance($first_layer_height, $nozzle_diameter);
         
             # force first layer print_z according to the contact distance
diff --git a/t/support.t b/t/support.t
index 2091b4432..5967d5c69 100644
--- a/t/support.t
+++ b/t/support.t
@@ -1,4 +1,4 @@
-use Test::More tests => 25;
+use Test::More tests => 26;
 use strict;
 use warnings;
 
@@ -221,12 +221,37 @@ use Slic3r::Test;
 
 {
     my $config = Slic3r::Config->new_from_defaults;
+    $config->set('skirts', 0);
+    $config->set('start_gcode', '');
     $config->set('raft_layers', 3);
     $config->set('nozzle_diameter', [0.4, 1]);
+    $config->set('layer_height', 0.3);
     $config->set('first_layer_height', 0.8);
     $config->set('support_material_extruder', 2);
+    $config->set('support_material_interface_extruder', 2);
+    $config->set('support_material_contact_distance', 0);
     my $print = Slic3r::Test::init_print('20mm_cube', config => $config);
-    ok Slic3r::Test::gcode($print), 'first_layer_height is validated with support material extruder nozzle diameter when using raft layers';
+    ok my $gcode = Slic3r::Test::gcode($print), 'first_layer_height is validated with support material extruder nozzle diameter when using raft layers';
+    
+    my $tool = undef;
+    my @z = (0);
+    my %layer_heights_by_tool = ();  # tool => [ lh, lh... ]
+    Slic3r::GCode::Reader->new->parse($gcode, sub {
+        my ($self, $cmd, $args, $info) = @_;
+    
+        if ($cmd =~ /^T(\d+)/) {
+            $tool = $1;
+        } elsif ($cmd eq 'G1' && exists $args->{Z} && $args->{Z} != $self->Z) {
+            push @z, $args->{Z};
+        } elsif ($info->{extruding} && $info->{dist_XY} > 0) {
+            $layer_heights_by_tool{$tool} ||= [];
+            push @{ $layer_heights_by_tool{$tool} }, $z[-1] - $z[-2] if $tool == 0;
+        }
+    });
+    
+    ok !defined(first { $_ > $config->nozzle_diameter->[0] + epsilon }
+        @{ $layer_heights_by_tool{$config->perimeter_extruder-1} }),
+        'no object layer is thicker than nozzle diameter';
 }
 
 __END__
diff --git a/xs/src/libslic3r/Print.cpp b/xs/src/libslic3r/Print.cpp
index d3bb43b3c..0dab8e70b 100644
--- a/xs/src/libslic3r/Print.cpp
+++ b/xs/src/libslic3r/Print.cpp
@@ -300,7 +300,7 @@ Print::step_done(PrintObjectStep step) const
 
 // returns 0-based indices of used extruders
 std::set<size_t>
-Print::extruders() const
+Print::object_extruders() const
 {
     std::set<size_t> extruders;
     
@@ -316,6 +316,17 @@ Print::extruders() const
         if ((*region)->config.top_solid_layers.value > 0 || (*region)->config.bottom_solid_layers.value > 0)
             extruders.insert((*region)->config.solid_infill_extruder - 1);
     }
+    
+    return extruders;
+}
+
+// returns 0-based indices of used extruders
+std::set<size_t>
+Print::extruders() const
+{
+    std::set<size_t> extruders = this->object_extruders();
+    
+    // add support material extruder(s)
     FOREACH_OBJECT(this, object) {
         if ((*object)->has_support_material()) {
             extruders.insert((*object)->config.support_material_extruder - 1);
diff --git a/xs/src/libslic3r/Print.hpp b/xs/src/libslic3r/Print.hpp
index 53c28833b..489eebc08 100644
--- a/xs/src/libslic3r/Print.hpp
+++ b/xs/src/libslic3r/Print.hpp
@@ -198,6 +198,7 @@ class Print
     Flow brim_flow() const;
     Flow skirt_flow() const;
     
+    std::set<size_t> object_extruders() const;
     std::set<size_t> extruders() const;
     void _simplify_slices(double distance);
     double max_allowed_layer_height() const;
diff --git a/xs/xsp/Print.xsp b/xs/xsp/Print.xsp
index e1329c449..1a6635c30 100644
--- a/xs/xsp/Print.xsp
+++ b/xs/xsp/Print.xsp
@@ -161,6 +161,14 @@ _constant()
     void set_step_started(PrintStep step)
         %code%{ THIS->state.set_started(step); %};
     
+    std::vector<int> object_extruders()
+        %code%{
+            std::set<size_t> extruders = THIS->object_extruders();
+            RETVAL.reserve(extruders.size());
+            for (std::set<size_t>::const_iterator e = extruders.begin(); e != extruders.end(); ++e) {
+                RETVAL.push_back(*e);
+            }
+        %};
     std::vector<int> extruders()
         %code%{
             std::set<size_t> extruders = THIS->extruders();