"The bridge fell apart"

"The bridge fell apart"
 

I want to express that my friend was hiking one time in the mountains and he came upon an old wooden bridge which he had to cross. As he was walking across it, he broke through (the planks gave way) and he fell into the river below. How do I express that the bridge "broke apart due to the old, rotten wood"?

"El puente se rompió por madera vieja"

I don't know if "romper" is the right word, nor how to say "rotten wood" in Spanish

There are a few verbs that can mean "break"; in your context a couple of them can work equally well, and "romper" is one of them.

A couple of possible translations of "rotten" in your context include "podrido" (= rotten, spoiled, decayed) and "deteriorado" (= deteriorated).

Sometimes (not always) what you can do is figure out the latinate English
word for a common word like when a bridge falls apart. That word is collapse. Often the latinate word translates directly to Spanish. I think the best word is colapsar. El puente de madera se colapsó.

I like the suggestion to use "collapse." That makes a lot of sense.
I talked to my wife about it last night, she said "romper" is not the right word to use in this context, she said she would probably use "partir." Given that, I would now say:

"El puente se partió por madera deteriorada".

"Por la madera" or "porque la madera estaba deteriorada/podrida/vieja..."
(Introducing the cause needs a definite article.)

Thanks Angelica. Is "por la madera" also used in regular speech in this context?

"Por la madera deteriorada", of course. I was only adding the article so the sentence wouldn't sound fragmentary. :)