I'm not sure if this is the simplist way but could make an array with ten members. The first five would be '1', and the rest 2-5. Then run a randomizer to pick an array at random. Maybe something like:

public Random DiceRoller;
string Dice[] = { "1","1","1","1","1","2","3","4","5","6"}
public string RollDice();
{
string DiceValue = Dice[DiceRoller.Next(Dice.Length)];
retrun DiceValue
}

Not sure if this is what you were looking for or if it helps.