diff --git a/heatcalc/heatcalc.css b/heatcalc/heatcalc.css
index dfa7614..2321ff0 100644
--- a/heatcalc/heatcalc.css
+++ b/heatcalc/heatcalc.css
@@ -78,9 +78,11 @@ td.hover, tr.hover td{
 }
 #heatcalc tbody th{
     z-index: 100;
+    position: relative;
 }
 #heatcalc thead th:nth-child(1){
     z-index: 200;
+    position: relative;    
 }
 
 #heatcalc td, #heatcalc th{