When and how did the tradition of rebuilding the Shinto shrine of Ise every 20 yrs or

When and how did the tradition of rebuilding the Shinto shrine of Ise every 20 yrs originate?

I have no idea. My guess would be that at first, every twenty years or so, it got run down, and maybe building techniques had improved, so they rebuilt it, then, after a few score years, it just became...well...a tradition.

Edit: Well, not that Wiki is an unimpeachable source, but apparently I wasn't too far off:

The shrine buildings at Naik? and Gek?, as well as the Uji Bridge, are rebuilt every 20 years as a part of the Shinto belief of the death and renewal of nature and the impermanence of all things (Gabi-sabi) as a way of passing building techniques from one generation to the next. The next scheduled rebuilding of Ise Shrine is due in 2013.

In the lead-up to the rebuilding of the shrines, a number of festivals are held to mark special events. The Okihiki Festival is held in the spring over two consecutive years and involves people from surrounding towns dragging huge wooden logs through the streets of Ise to Naik? and Gek?. In the lead-up to the 2013 rebuilding, the Okihiki festival was held in 2006 and 2007. A year after the completion of the Okihiki festival, carpenters begin preparing the wood for its eventual use in the Shrine.

Say yes to good and no to bad.

I don't know that there is an official answer to this question, my hypothesis is at the end.

Two of the main principles of Shinto are purification and renewal. Shinto ceremonies always start with purification, as do visits to the jink (shrine). Ema (tablets that wishes are written on) and Ofuda (blessed items to which the Kami can descend during household prayers) are burned in a ceremony yearly. Jinja and homes are cleaned just before new years in a specific order to remove the spiritual "dirt" (as well as any physical dirt) so that the year will start fresh.

The main Kami that descends to Ise Jingu (properly called Jingu, without the Ise) is Amaterasu from whom it's said that the Japanese Emperors and Empresses descended. Because of it's importance, an annual cleaning might not be enough to keep away the spiritual dirt that gathers over time.

The first rebuild occurred in 690, three years after Empress Jito took the throne (It would have been several hundred years old at the time). The second rebuild in 710 took place during the reign of Empress Gemmei in the year when the official residence was moved to Nara. Prior to Empress Gemmei the official residence was moved at the start of each reign. The 730 rebuild took place during the reign of Emperor Sh?mu, who was a devout Buddhist.

So in lieu of a better hypothesis, the first rebuild was likely due to deterioration of the original building, the second due to the move of the Imperial residence to Nara, and after that it took on a life of it's own. A side effect of the twenty year rebuilding is that building techniques which might have been lost have been kept alive.