diff --git a/src/sliceActions/calculateLayersIntersections.js b/src/sliceActions/calculateLayersIntersections.js
index 5187827..707297e 100644
--- a/src/sliceActions/calculateLayersIntersections.js
+++ b/src/sliceActions/calculateLayersIntersections.js
@@ -13,6 +13,8 @@ export default function calculateLayersIntersections(lines, settings) {
   for (let lineIndex = 0; lineIndex < lines.length; lineIndex ++) {
     const line = lines[lineIndex].line;
 
+    if (line.isFlat) continue;
+
     const min = Math.ceil(Math.min(line.start.y, line.end.y) / layerHeight);
     const max = Math.floor(Math.max(line.start.y, line.end.y) / layerHeight);
 
diff --git a/src/sliceActions/createLines.js b/src/sliceActions/createLines.js
index a45559b..4984ca9 100644
--- a/src/sliceActions/createLines.js
+++ b/src/sliceActions/createLines.js
@@ -1,6 +1,6 @@
 import * as THREE from 'three.js';
 
-function addLine(geometry, lineLookup, lines, a, b) {
+function addLine(geometry, lineLookup, lines, a, b, isFlat) {
   const index = lines.length;
   lineLookup[`${a}_${b}`] = index;
 
@@ -8,13 +8,13 @@ function addLine(geometry, lineLookup, lines, a, b) {
     line: new THREE.Line3(geometry.vertices[a], geometry.vertices[b]),
     connects: [],
     normals: [],
-    open: false
+    isFlat
   });
 
   return index;
 }
 
-export default function createLines(geometry, settings, openClosed) {
+export default function createLines(geometry, settings) {
   console.log('constructing unique lines from geometry');
 
   const lines = [];
@@ -22,34 +22,28 @@ export default function createLines(geometry, settings, openClosed) {
 
   for (let i = 0; i < geometry.faces.length; i ++) {
     const face = geometry.faces[i];
-    const open = openClosed[i];
 
-    if (face.normal.y !== 1 && face.normal.y !== -1) {
-      const normal = new THREE.Vector2(face.normal.z, face.normal.x).normalize();
+    const lookupA = lineLookup[`${face.b}_${face.a}`];
+    const lookupB = lineLookup[`${face.c}_${face.b}`];
+    const lookupC = lineLookup[`${face.a}_${face.c}`];
 
-      const lookupA = lineLookup[`${face.b}_${face.a}`];
-      const lookupB = lineLookup[`${face.c}_${face.b}`];
-      const lookupC = lineLookup[`${face.a}_${face.c}`];
+    const isFlat = face.normal.y !== 1 && face.normal.y !== -1;
 
-      // only add unique lines
-      // returns index of said line
-      const indexA = typeof lookupA !== 'undefined' ? lookupA : addLine(geometry, lineLookup, lines, face.a, face.b);
-      const indexB = typeof lookupB !== 'undefined' ? lookupB : addLine(geometry, lineLookup, lines, face.b, face.c);
-      const indexC = typeof lookupC !== 'undefined' ? lookupC : addLine(geometry, lineLookup, lines, face.c, face.a);
+    // only add unique lines
+    // returns index of said line
+    const lineIndexA = typeof lookupA !== 'undefined' ? lookupA : addLine(geometry, lineLookup, lines, face.a, face.b, isFlat);
+    const lineIndexB = typeof lookupB !== 'undefined' ? lookupB : addLine(geometry, lineLookup, lines, face.b, face.c, isFlat);
+    const lineIndexC = typeof lookupC !== 'undefined' ? lookupC : addLine(geometry, lineLookup, lines, face.c, face.a, isFlat);
 
-      // set connecting lines (based on face)
-      lines[indexA].connects.push(indexB, indexC);
-      lines[indexB].connects.push(indexC, indexA);
-      lines[indexC].connects.push(indexA, indexB);
+    // set connecting lines (based on face)
+    lines[lineIndexA].connects.push(lineIndexB, lineIndexC);
+    lines[lineIndexB].connects.push(lineIndexC, lineIndexA);
+    lines[lineIndexC].connects.push(lineIndexA, lineIndexB);
 
-      lines[indexA].normals.push(normal);
-      lines[indexB].normals.push(normal);
-      lines[indexC].normals.push(normal);
-
-      lines[indexA].open = open;
-      lines[indexB].open = open;
-      lines[indexC].open = open;
-    }
+    const normal = new THREE.Vector2(face.normal.z, face.normal.x).normalize();
+    lines[lineIndexA].normals.push(normal);
+    lines[lineIndexB].normals.push(normal);
+    lines[lineIndexC].normals.push(normal);
   }
 
   return lines;
diff --git a/src/sliceActions/detectOpenClosed.js b/src/sliceActions/detectOpenClosed.js
index bcfbad9..228280e 100644
--- a/src/sliceActions/detectOpenClosed.js
+++ b/src/sliceActions/detectOpenClosed.js
@@ -1,42 +1,45 @@
-export default function detectOpenClosed(geometry) {
+export default function detectOpenClosed(lines) {
   console.log('detecting open and closed lines');
 
-  const pools = getPools(geometry);
-  const openVertices = getOpenVertices(geometry);
+  const pools = getPools(lines);
+  const openLines = lines.map(line => line.connects.length === 2);
 
-  const openFaces = [];
   for (let i = 0; i < pools.length; i ++) {
     const pool = pools[i];
 
-    const isOpen = pool.some(face => openVertices[face.a] || openVertices[face.b] || openVertices[face.c]);
+    const isOpen = pool.some(lineIndex => openLines[lineIndex]);
 
-    if (isOpen) openFaces.splice(openFaces.length, 0, ...pool);
+    for (let j = 0; j < pool.length; j ++) {
+      const lineIndex = pool[j];
+      const line = lines[lineIndex];
+      line.open = isOpen;
+    }
   }
-
-  return geometry.faces.map(face => openFaces.includes(face));
 }
 
-function findPool(pools, faceA) {
+function findPool(pools, lines, lineIndex) {
+  const { connects } = lines[lineIndex];
   for (let i = 0; i < pools.length; i ++) {
     const pool = pools[i];
 
-    if (pool.find(faceB => faceA.a === faceB.a || faceA.a === faceB.b || faceA.a === faceB.c)) {
+    if (pool.find(lineIndex => connects.includes(lineIndex))) {
       return pool;
     }
   }
 
+  // no pool found
+  // create new pool
   const pool = [];
   pools.push(pool);
   return pool;
 }
 
-function getPools(geometry) {
+function getPools(lines) {
   const pools = [];
 
-  for (let i = 0; i < geometry.faces.length; i ++) {
-    const face = geometry.faces[i];
-    const pool = findPool(pools, face);
-    pool.push(face);
+  for (let lineIndex = 0; lineIndex < lines.length; lineIndex ++) {
+    const pool = findPool(pools, lines, lineIndex);
+    pool.push(lineIndex);
   }
 
   for (let i = 0; i < pools.length; i ++) {
@@ -46,8 +49,9 @@ function getPools(geometry) {
       const poolB = pools[j];
 
       for (let k = 0; k < poolA.length; k ++) {
-        const faceA = poolA[k];
-        if (poolB.find(faceB => faceA.a === faceB.a || faceA.a === faceB.b || faceA.a === faceB.c)) {
+        const { connects } = lines[poolA[k]];
+
+        if (poolB.find(lineIndex => connects.includes(lineIndex))) {
           poolA.splice(poolA.length, 0, ...poolB);
           poolB.splice(0, poolB.length);
         }
@@ -57,15 +61,3 @@ function getPools(geometry) {
 
   return pools.filter(pool => pool.length > 0);
 }
-
-function getOpenVertices(geometry) {
-  const vertices = Array(geometry.vertices.length).fill(0);
-  for (let i = 0; i < geometry.faces.length; i ++) {
-    const face = geometry.faces[i];
-    vertices[face.a] ++;
-    vertices[face.b] ++;
-    vertices[face.c] ++;
-  }
-
-  return vertices.map(numFaces => numFaces < 4);
-}
diff --git a/src/sliceActions/slice.js b/src/sliceActions/slice.js
index 2b3f47f..6fdbdfa 100644
--- a/src/sliceActions/slice.js
+++ b/src/sliceActions/slice.js
@@ -13,12 +13,11 @@ import applyPrecision from './applyPrecision.js';
 import removePrecision from './removePrecision.js';
 
 export default function(geometry, settings) {
-  geometry.mergeVertices();
   geometry.computeFaceNormals();
 
   // get unique lines from geometry;
-  const openClosed = detectOpenClosed(geometry);
-  const lines = createLines(geometry, settings, openClosed);
+  const lines = createLines(geometry, settings);
+  const openClosed = detectOpenClosed(lines);
 
   const {
     layerIntersectionIndexes,