It doesn't have to be a tie, simply neither getting to 270.
By far, the most reasonable way this could happen, is if Johnson was to win New Mexico. He was governor of NM, but he's still polling 3rd, at about 20%.
I don't think Trump could win this state, without winning plenty of other states, so I thought it might be nice, if Trump could throw his support in NM to Johnson.
If Trump wins his must wins, OH,FL,NC,IA, and he wins NV, but loses the blue battle grounds, and CO, and NH. Then, Hillary would eak out a win with NM.
BUT, if Trump voters switch over to Johnson, then in the above situation, neither Hillary, or Trump have 270, so it goes to the house.
Remember, when it goes to the house, it's not every member has a vote - each state has a delegation, and each state has 1 vote. The Republicans would win easily.
Map, congress by party
Remember, when it goes to the house, it’s not every member has a vote - each state has a delegation, and each state has 1 vote.
And if the state’s delegation is tied, the state gets no votes.