I'm not sure as to how they would be repaired, but a soldier might not even bother doing so. I was able to find references to tourney records from 1278, and a plain wooden shield was only 5 pence. That's about what the average late medieval soldier would earn per day.

https://books.google.com/books?id=1fPDAgAAQBAJ&printsec=frontcover#v=onepage&q=5d&f=false