Soft pine, 3/4 inch thick about 24 inches wide by about 7 foot long. I add to it to make it wider when needed. I run the edge through the plainer and the new wood then use a biscut joiner and glue to widen. When the glue sets I run it through the over head sander to make sure it is level. I use several cross members screwed to the bottom to make sure it stays level and remove all the cut marks.
I may have to replace it because every few years I run it through the sander and it's no longer 3/4 inch thick. Either replace it or use shorter screws for the bottom braces?
I have had it for over 25 years and it's still a perfect building board!