I'd try formply as well Lou, pretty sure that's what the kickboards were on SWMBO's mum's arena and what I've always used when needing something strong and relatively weather proof.
As mentioned, you need to seal where it has been cut.
big guy, you need something that'll cope with a horse kicking it, I don't think rendered straw bales would.

It also needs to be solid, ie, no rails so they don't cast themselves either.
Lou, the other style I've seen (OK, I've seen several) is to use hardwood boards placed horizontally inside a channel at either end.
This way you can cut the rotten one out at the base and slide the rest down to replace it and install a new one at the top.
I've also seen pre-cast concrete panels used.
Can't ask the boss ATM, she's already hit the sack.
Bookmarks