diff --git a/src/sliceActions/calculateLayersIntersections.js b/src/sliceActions/calculateLayersIntersections.js
index 76b0e9f..114bc2b 100644
--- a/src/sliceActions/calculateLayersIntersections.js
+++ b/src/sliceActions/calculateLayersIntersections.js
@@ -9,9 +9,9 @@ export default function calculateLayersIntersections(lines, settings) {
   const layerIntersectionPoints = Array.from(Array(numLayers)).map(() => []);
 
   for (let lineIndex = 0; lineIndex < lines.length; lineIndex ++) {
-    const line = lines[lineIndex].line;
+    const { line, isFlat } = lines[lineIndex];
 
-    if (line.isFlat) continue;
+    if (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 188f1c3..5a6e3fc 100644
--- a/src/sliceActions/createLines.js
+++ b/src/sliceActions/createLines.js
@@ -25,7 +25,7 @@ export default function createLines(geometry, settings) {
     const lookupB = lineLookup[`${face.c}_${face.b}`];
     const lookupC = lineLookup[`${face.a}_${face.c}`];
 
-    const isFlat = face.normal.y !== 1 && face.normal.y !== -1;
+    const isFlat = face.normal.y > 0.999 || face.normal.y < -0.999;
 
     // only add unique lines
     // returns index of said line