Hi, reading this site and thread with interest. Wish I had have read it a few weeks ago. I'm building a cubby/tree house for my kids - one corner is supported by a horizontal beam attached to a tree, the other 3 corners are on 4x4 treated pine posts. I'm in Western Australia. The 'soil' in our backyard is pure sand and so drains very well, but doesn't give a very solid base / sides of holes. I am following a book on tree/cubby house building and in line with that have set the posts into concrete by putting the post in and pouring pre-mixed concrete into the hole then plumming up - exactly as this thread recommends not to do! I did this around a few weeks ago and have been up and down on the structure, hammering away, putting in a decking etc. The concrete around each post has cracked in a diagonal from the corner of the post, in a couple of them there are 2 cracks, on around 2mm wide, the other more hairline. The structure feels solid, but it will be heavy - has a floor area of around 5m2 and sits about 8ft off the ground. At that height I didn't want to take any chances with the kids and so have braced the floor to the post at 45 degrees at each corner. Anyway, seeing the cracks and reading this I'm worried whether the structure is going to be safe. I don't mind if the posts rot out over several years - the kids will be largely done with it by then anyway. What I don't want is a catastrophic failure within that time. Any feel for how quickly the concerns raised here could create a rot-out / failure? Also, wondering if there is something I could or should perhaps do now like (1) trying to put some form of concrete sealant into the cracks (bondcrete?) and or running beams around the lower section of the posts to further brace it - is this necessary in the short-term? Any advice greatly appreciated. Have attached a picture. Download Attachment: Treehouse.jpg 566.06 KB Best Regards,
