diff --git a/deps.nix b/deps.nix
index bb3431c9..494136ab 100644
--- a/deps.nix
+++ b/deps.nix
@@ -8,8 +8,8 @@ let
     name = "Nix-0.15";
     src = fetchsvn {
       url = https://svn.nixos.org/repos/nix/nix-perl/trunk;
-      rev = 24765;
-      sha256 = "12ah8c8p9bx55hd17lhcfc74bd4r1677dxy0id3008pww1aklir7";
+      rev = 24774;
+      sha256 = "1akj695gpnbrjlnwd1gdnnnk7ppvpp1qsinjn04az7q6hjqzbm6p";
     };
     NIX_PREFIX = nixSqlite;
     doCheck = false; # tests currently don't work
diff --git a/src/lib/Hydra/Helper/Nix.pm b/src/lib/Hydra/Helper/Nix.pm
index 79d39dfe..8cf970cf 100644
--- a/src/lib/Hydra/Helper/Nix.pm
+++ b/src/lib/Hydra/Helper/Nix.pm
@@ -9,7 +9,7 @@ use Hydra::Helper::CatalystUtils;
 
 our @ISA = qw(Exporter);
 our @EXPORT = qw(
-    isValidPath queryPathInfo
+    isValidPath
     getHydraPath getHydraDBPath openHydraDB txn_do
     registerRoot getGCRootsDir gcRootFor
     getPrimaryBuildsForView 
@@ -23,17 +23,6 @@ sub isValidPath {
 }
 
 
-sub queryPathInfo {
-    my $path = shift;
-
-    my $hash = Nix::queryPathHash($path);
-    my $deriver = Nix::queryDeriver($path);
-    my @refs = Nix::queryReferences($path);
-
-    return ($hash, $deriver, \@refs);
-}
-
-
 sub getHydraPath {
     my $dir = $ENV{"HYDRA_DATA"};
     die "The HYDRA_DATA environment variable is not set!\n" unless defined $dir;
diff --git a/src/lib/Hydra/View/NixManifest.pm b/src/lib/Hydra/View/NixManifest.pm
index 927cc548..8a672084 100644
--- a/src/lib/Hydra/View/NixManifest.pm
+++ b/src/lib/Hydra/View/NixManifest.pm
@@ -21,7 +21,7 @@ sub process {
         "}\n";
     
     foreach my $path (@paths) {
-        my ($hash, $deriver, $refs) = queryPathInfo $path;
+        my ($deriver, $hash, $time, $narSize, $refs) = Nix::queryPathInfo $path;
 
         # Escape the characters that are allowed to appear in a Nix
         # path name but have special meaning in a URI.
@@ -40,6 +40,7 @@ sub process {
             (defined $deriver ? "  Deriver: $deriver\n" : "") .
             "  NarURL: $url\n" .
             "  NarHash: $hash\n" .
+            ($narSize != 0 ? "  NarSize: $narSize\n" : "") .
             "}\n";
     }
 
diff --git a/src/script/hydra_build.pl.in b/src/script/hydra_build.pl.in
index 9a6f5a66..77448c1b 100755
--- a/src/script/hydra_build.pl.in
+++ b/src/script/hydra_build.pl.in
@@ -199,10 +199,6 @@ sub sendEmailNotification {
     sendmail($email);
 }
 
-sub getSize {
-    my $size = `du -bcs @_ 2> /dev/null | tail -1 | cut -f 1 `;
-    return int($size);
-}
 
 sub doBuild {
     my ($build) = @_;
@@ -233,7 +229,7 @@ sub doBuild {
         # Run Nix to perform the build, and monitor the stderr output
         # to get notifications about specific build steps, the
         # associated log files, etc.
-        my $cmd = "nix-store --realise $drvPath " .
+        my $cmd = "nix-store -j1 --no-build-hook --realise $drvPath " .
             "--max-silent-time $maxsilent --keep-going --fallback " .
             "--no-build-output --log-type flat --print-build-trace " .
             "--add-root " . gcRootFor $outPath . " 2>&1";
@@ -371,13 +367,20 @@ sub doBuild {
   done:
     my $logfile = getBuildLog($drvPath); 
 
-    my $logsize = defined $logfile ? getSize($logfile) : 0;
-    my $size = isValidPath($outPath) ? getSize($outPath) : 0;
+    my $logsize = defined $logfile ? stat($logfile)->size : 0;
 
+    my $size = 0;
     my $closuresize = 0;
+
     if (isValidPath($outPath)) {
-        (my $hash, my $deriver, my $refs) = queryPathInfo($outPath) ;
-        $closuresize = getSize(@{$refs});
+        my ($deriver, $hash, $time, $narSize, $refs) = Nix::queryPathInfo($outPath);
+        $size = $narSize;
+
+        my @closure = Nix::computeFSClosure(0, 0, $outPath);
+        foreach my $path (@closure) {
+	    my ($deriver, $hash, $time, $narSize, $refs) = Nix::queryPathInfo($path);
+ 	    $closuresize += $narSize;    
+        }
     }
 
     txn_do($db, sub {