Housing in Japan

Was browsing through Danny Choo's Living in Japan Guide articles when this passage caught my eye in the Tokyo Property Purchase article:

• The average life span of a house in Japan is 25 years which is just one of the reasons why folks dont make these sort of purchases as an investment. Generally speaking, when people buy property in Japan, they usually plan to live there until their time is up.

I could not understand that when I read it and it still throws me for a loop. How the heck do you build a multistory house that only lasts 20 years?

check on Wikipedia got me this:

The taxable value of a house is controlled by its building material. Wooden houses are considered to have a lifespan of twenty years, and concrete ones to have a lifespan of thirty years

Color me confused. How do you build a huse out of wood that is so deliberately shoddy that it falls apart after 20 years? Build load bearing walls out of 1x2 studs deliberately 22 inches on center? Use brads in stead of nails? I want to know more ...

found this in a Tokyo Real Estate blog:

------I've always been taught that the average life span of a wooden framed house is about 30 years. Very short compared to houses in the US. The building materials used and the severe humidity has a factor in this. If you have the budget I recommend building an RC house. You can expect the cost to be twice that of building a wooden framed house but they last over twice the life span, are much safer (especially in an earthquake) and the fire insurance is a fraction of what a wooden house would be!!

oh ok, humidity rotting wood and other materials out. It gets to the point of being cheaper to replace it than try to maintain and repair it.

addendum: this also makes sense from a cost perspective, since the land is by far the most expensive thing, once your cost to repair or replace goes past a certain point then it would be easier and cheaper to replace the whole house than be nickel and dimed to death.