diff --git a/sfc/slicer.py b/sfc/slicer.py
index 4b21d27..8a48223 100755
--- a/sfc/slicer.py
+++ b/sfc/slicer.py
@@ -213,13 +213,13 @@ if height is not None and height < 0 and z_levels[-1] > height:
 	
 for next_z in z_levels:
 	wires = shape.slice(Base.Vector(0, 0, 1), next_z + epsilon)
-	if z_step is None or last_z is None or last_z + z_step >= next_z:
+	if z_step is None or last_z is None or last_z - z_step <= next_z:
 		dump_level(wires, next_z + z_off)
 	else:
-		d = next_z - last_z
-		n = (d // z_step) + 1
+		d = last_z - next_z
+		n = int(d // z_step) + 1
 		for i in range(0, n):
-			dump_level(wires, last_z + (i + 1) * (d / n) + z_off)
+			dump_level(wires, last_z - (i + 1) * (d / n) + z_off)
 	last_z = next_z
 	
 #