Hello everyone, this is my first build. Rumble is an excellent off-tank that can deal huge amounts of damage if you know how to play him right. This build keeps him tanky while still having alot of damage. This is just a quick build and is obviously still in progress. Please read the whole build before voting and constructive criticism is appreciated.

Pros
-Good off tank
-Excellent tower diver(scrap shield gives who movement as well as a shield)
-Can pursue effectively
-Has two slows
-His heat is alot better than mana(My opinion)
-He is also a total beast in lane. His ULT will help you greatly against squishy tower-huggers

Cons
-Cannot jungle
-Is hard to farm with
-Once overloaded is hard to flee

Greater Mark of Magic Penetration
The magic penetration runes are pretty self explanatory. They are useful in almost any situation and really count early game.
Greater Seal of Armor
I try to build him pretty tanky in the runes. Really, armor runes are pretty useful throughout the whole game.
Greater Glyph of Cooldown Reduction
I get the cooldown runes because I was torn between mercury treads and boots of lucidity. The cooldown runes mean that rumble still can execute abilities quickly, while still have the magic resistance from merc treads.
Greater Quintessence of Movement Speed
Greater Quintessence of Health
I go with two movement speed quintessences because you need him pretty quick. Both to get in and out of fights. The health quintessence is also needed for survivability early and mid game. Constantly I get to safety with only 20 - 30 health and I owe it to this little guy.

For masteries i went with the classic 9-21-0. I do this to make him quite tanky throughout the game. I took ghost and ignite as summoner spells though I mastered neither. I go with ghost because it allows for good pursuit/getaway along with his slows. Ignite is also useful for finishing off enemies who are running towards their turret and against enemy healers.

I start off with Doran's shield, to give you additional health and armor, and then rush mercury's treads and rylai's Crystal sceptre. Then I get sunfire cape and force of nature for some durability. And then I finish it off by getting rabodan's and swapping doran's shield for void staff.

I try to max flamespitter first, and scrap shield second. With one point on Electro-Harpoon at level 3. With flamespitter on level 5 at 12, as well as a solid scrap shield output, you can successfully chase with W while having Q on, panning out massive damage.

I find that rumble isn't a successful jungler. This is because of his lack of healing abilities and that his flamespitter does 50% less to minions. Farming does not seem to be a problem for me, especially after i get sunfire cape. Also, because his ULT has a low base cooldown, you can use that to kill off enemy creeps that are (for example) advancing when you are attempting to take a turret.

A good cs (creep score) by the 10minute mark is around 60 (duo lane)
A good cs by the 10minute mark is around 120 (solo lane)

I build him tanky for a reason. He is no use sitting on the edge of a fight. He is the most effective by spamming his Q and E. Do not use your ULT to steal a kill. You will waste your most valuable weapon for a minute and annoy your teammates. It is good to use your ULT to slow an escaping enemy that your teammates are chasing though.
After using one Electro Harpoon, try to use the other one. Just fling it into a random bush and see if it stops short. The second ones are free and it may help your teammates to know if their is an enemy in the bushes.

I hope you find this build useful and, once again, constructive criticism is appreciated.