What? Buildings last forever, unless damaged. The rain will take them to 50% health if you have no roof, but other than that there's really no need to repair.

1) Is it completely roofed? Unroofed elements take damage under rain.
2) Is it built by the shore, at sea level? Elements take damage while partly in water, or built on muddy terrain.
3) Is it attacked by mobs?
4) Is it correctly built? How are colored elements when you hover over them in repair mod?

Green

Wood need roof above it or the Health drops to 50% over time
And roof means roof and not a woodplate

Like it has already been said, you need to use actual roof pieces and not just using floor pieces on top. Also, you don't use any resources on repair, I don't know if you were mistaken about using wood to repair your houses or if you're actually destroying them and rebuilding, but either way the repair option in the build menu requires absolutely no resources whatsoever so there's literally nothing stopping you from spamming it if you have to mass repair a structure, which you shouldn't need to do all that often.

Use the dedicated roof for a roof. Thats what most people misunderstood if you but a floor piece it does not protect the stuff underneath from rain.

Best way is to build a frame first here's an example how I started my workshop use small ladders to get up higher craft a few ontop of each other then destroy.
There is lots of different methods but always try for a frame first then add the correct roofing and you wont have any deteriation.
https://steamcommunity.com/sharedfiles/filedetails/?id=2417628896
finished results
https://steamcommunity.com/sharedfiles/filedetails/?id=2609239264

Maybe have here a look
https://steamcommunity.com/app/892970/discussions/0/3102389184721098772/
older but still valid

I will implement the knowledge into my mod the next days. But thats the future