Pretty sure it's Homasote board.
It's a paper based product. Used sometimes for sound insulation
It's sold in generally in 4" x10' sections and 4'x8' sheets. It bends round turns. This stuff shown looks about 6" wide. Guess it varies where you buy and it's availability.
We've used it for expansion joints when pouring concrete slabs.
Don't let it get wet or it will swell up.
Thanks for the reply Kevin, do you/ anyone know of anything better to use for the same purpose? Ideally something I can re-use again for multiple pools. I was thinking of trying Stegmeier Slip-N-Join Lumber but at $405 for 144 feet seems expensive. Any other methods or ideas for coping installation would be greatly appreciated.
We use 11/32" plywood-it's about 3/8". It's bends easier then 1/2"
We cut it into 6" strips. Some local lumber yards will cut it for you. We usually cut it ourselves on our table saw. Home Depot will it into 12" strips and then we just need to cut it in half.
We fasten it to the pool wall with 1/4" Tapcon concrete screws and fender washers. Mason nails are tricky and don't work that well on gunite. Same for the Ramset .22 powder gun. We also re-use the concrete screws.
We re-use the plywood as long as we can. At 6" strips, you'll get (8) pieces total 64' for about $20.
Lot cheaper than Stegmeier. which are good for concrete deck/walk pours.
Great! thanks for the advise I really appreciate it
He says in the text, answering the same question "It's a piece of siding"