Tek, i used the foam on both my very large pillars, and i have 8 lb ledges overhanging in various directions. you have to dry out the rock you plan to create your arch out of, then drill tiny holes so you can zip tie the rocks together, you can use the pond foam as a glue or adhesive, then cover it in resin and sand, and it will hold up guaranteed. i used rod to pin things together in some places thinking the foam wouldnt be sturdy enough, but i must say now that it is done the faom could probably hold a house together