SELECT sum(TotalHoursM) + (TotalHoursT) + (TotalHoursW) + (TotalHoursTH) + (TotalHoursF) AS TOTAL FROM LeaveR
If the column has a 0 value, you are fine, my guess is that you have a problem with a Null value, in that case you would need to use IsNull(Column, 0) to ensure it is always 0 at minimum.
IsNull(Column, 0)