If your group is all about the "game" then you handled the situation just fine using abstract dice rolls to determine the outcome. The two things you could have done to improve the situation were to make the players aware of how it would be resolved ahead of time and then not allow them to argue after the fact.
If your group is based on role-playing characters through a story then utilizing player knowledge for the character's benefit is a failure of role-playing and they should not be rewarded for engineering a complicated scientific or mathematical plan with characters that lack that kind of knowledge.
Explosives? Somehow securely attached to a boulder well enough that they aren't detonated or destroyed before the avalanche reaches a precise destination? And this is all calculated precisely and timed perfectly? And the target is predictable and stationary?
Suppose all of those questions can be over-looked, the players should not know the results of their plan just by watching it happen. They should see no more than the destruction of the roof, the explosion, and a cloud of dust. They should have to work to figure out the results.
Also . . . CONSEQUENCES! There should ALWAYS be consequences for every action. The bigger the action the bigger the consequences. What is the logical fall-out of their actions?