Rafter?

I'm guessing most people don't use the word "rafter" much in their everyday conversation, unless they are into construction. Basically, a rafter is a thick piece of wood that supports the roof, and is similar to a thick wooden tent stake. In Korean, it's called "서까래" and in Japanese it's called 垂木. 
There are many materials that are used for karate breaking demonstration, such as roof tiles, bricks, ice, wooden boards, stones, etc. Wooden board is the easiest and most commonly used. It's probably the easiest because you are hitting along the grain of the wood so the wood naturally splits. On the other hand, a rafter is quite difficult to break because you would be attempting to break the wood against the grain. To give you a perspective, one blogger barely managed to break a rafter by stomping on it repeatedly, wearing heavy hiking boots. Even a sledge hammer couldn't break the rafter in one strike. The thing that is so amazing about Nakamura Sensei's breaking demonstration of a rafter is that it's not so much a "break" but rather a "slice", as with a sharp knife. 

Notice the smoothness of the rafter broken by Nakamura Sensei. It's like a sliced block of cheese. It isn't jagged. I don't know much about material science, but I think under most circumstances a wood like that would break from a stress or a strain placed on it, and its own leverage would play a big part. The rafter looks to be around 60 cm (12 inches, 1 foot). That means Nakamura Sensei's breaking is a result of a superhuman speed, and not the leverage of the material being broken.

Ancient masters have said in karate, one's hands and feet must be considered as swords. Nakamura sensei's karate is truly at the level of a lethal weapon.