Stair treads
Dan,Stair tread stock is available at any lumber yard or builder supply store. It is what you step on when you go up or down the stairs. It is usually 11 1/4" wide and comes in either precut lengths from 36"-48" and longer if special ordered, or in long lengths of 12'-16' to be cut to length. The materials can be yellow pine, oak or maple to name a few examples. It is usually designated 5/4 which is actually 1 1/16" thick which makes it stronger than 3/4" but not as heavy as 1 1/2" stock.Now, as to how to use it... Stair tread stock has a round edge called a bullnose on one edge of the stock and that must be trimmed off and a rabbet or bevel should be cut on one edge of the top and bottom pieces and on both edges of the intermediate pieces to keep the water from leaking at the seams. How many pieces you will have to cut depends on the height of your companionway. Thickness could also be a problem if your companionway slats have to slide into slots at the edges of the openings. 5/4 stock is probably too thick to fit the slot. You will have to rabbet the edge on a saw or with a router. Cutting and fitting should not be a problem if you're handy with wood. That's probably more than you ever wanted to know about stair tread stock, but I teach carpentry/cabinet-making, and you just got a dose of my classroom spiel.Good luckTom