There are many types of Carpet grade stairs. The most common type is the closed tread. These have Ex 250mm x 40mm Pine Stringers that run down both sides of the stair. Then there is the under stringer stair. Also known as a saw tooth stair. This type of stair does not have the stringers visible from the top. Carpet grade stairs generally have a 25mm nose on the treads, however some people prefer the raking riser. The treads and risers are made from plywood. Plywood is a lot lighter making it easier to install.