diff --git a/lib/carto/functions.js b/lib/carto/functions.js
index 47129ef..f569589 100644
--- a/lib/carto/functions.js
+++ b/lib/carto/functions.js
@@ -94,6 +94,13 @@ tree.functions = {
 
         return hsla(hsl);
     },
+    replace: function (entity, a, b) {
+        if (entity.is === 'field') {
+            return entity.toString + '.replace(' + a.toString() + ', ' + b.toString() + ')';
+        } else {
+            return entity.replace(a, b);
+        }
+    },
     //
     // Copyright (c) 2006-2009 Hampton Catlin, Nathan Weizenbaum, and Chris Eppstein
     // http://sass-lang.com
diff --git a/lib/carto/tree/dimension.js b/lib/carto/tree/dimension.js
index dfa7de2..471e439 100644
--- a/lib/carto/tree/dimension.js
+++ b/lib/carto/tree/dimension.js
@@ -35,8 +35,12 @@ tree.Dimension.prototype = {
     // conversions such that `100cm + 10mm` would yield
     // `101cm`.
     operate: function(op, other) {
-        return new tree.Dimension(tree.operate(op, this.value, other.value),
-                  this.unit || other.unit);
+        if (other.is === 'field') {
+            return new tree.Quoted('', this.value + op + other.toString());
+        } else {
+            return new tree.Dimension(tree.operate(op, this.value, other.value),
+                      this.unit || other.unit);
+        }
     }
 };
 
diff --git a/lib/carto/tree/field.js b/lib/carto/tree/field.js
index f6ebe8d..e18eb39 100644
--- a/lib/carto/tree/field.js
+++ b/lib/carto/tree/field.js
@@ -11,6 +11,9 @@ tree.Field.prototype = {
     },
     'eval': function() {
         return this;
+    },
+    operate: function(op, other) {
+        return new tree.Quoted('', this.toString() + op +  other.toString());
     }
 };
 
diff --git a/lib/carto/tree/quoted.js b/lib/carto/tree/quoted.js
index 885e4c5..67d0add 100644
--- a/lib/carto/tree/quoted.js
+++ b/lib/carto/tree/quoted.js
@@ -16,8 +16,13 @@ tree.Quoted.prototype = {
     },
 
     operate: function(op, other) {
-        return new tree.Quoted(this.quote,
-            tree.operate(op, this.toString(), other.toString()));
+        if (other.is !== 'string') {
+            return new tree.Quoted(this.quote,
+                this.toString() + op + other.toString());
+        } else {
+            return new tree.Quoted(this.quote,
+                tree.operate(op, this.toString(), other.toString()));
+        }
     }
 };
 
diff --git a/test/rendering/building_height.mml b/test/rendering/building_height.mml
new file mode 100644
index 0000000..79ba517
--- /dev/null
+++ b/test/rendering/building_height.mml
@@ -0,0 +1,23 @@
+{
+    "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
+    "Stylesheet": [
+        "building_height.mss"
+    ],
+    "Layer": [{
+        "name": "world",
+        "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
+        "Datasource": {
+            "file": "http://tilemill-data.s3.amazonaws.com/test_data/shape_demo.zip",
+            "type": "shape"
+        }
+    },
+    {
+        "class": "new",
+        "name": "countries",
+        "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
+        "Datasource": {
+            "file": "http://tilemill-data.s3.amazonaws.com/test_data/shape_demo.zip",
+            "type": "shape"
+        }
+    }]
+}
diff --git a/test/rendering/building_height.mss b/test/rendering/building_height.mss
new file mode 100644
index 0000000..6122c84
--- /dev/null
+++ b/test/rendering/building_height.mss
@@ -0,0 +1,4 @@
+@n: 4;
+#world {
+    building-height: 2 * 3 * [HEIGHT] + 2 + [NOTHEIGHT] + (@n * 2);
+}