Using Lag Bolts in Treehouses
Lag bolts usually have a hex head for driving with a typical socket wrench. The shaft is the nominal thickness, and the threaded part is a little thinner. Some, like the GRK fasteners to the right, do not require pre-drilling and are quite strong for their size, but they do not have the same sheer strength as the large galvanized lag bolts to the bottom right of this page. They are better applied with washers so that you can tighten them more without the head or nut sinking into the wood. You never want to crush the fibres of the wood by overtightening.
I used the top style of lag bolts on my treehouse primarily for fastening the rafters to the top plates, some of the railing parts, and for the tops of the knee braces. They are amazingly easy to use. I used the larger lag bolts on the bottom right for attaching the bottoms of the knee braces to the tree and for a couple other attachments at the floor level.
To install a lag bolt like the ones on the bottom right, you should drill a hole 1-2 sizes smaller than the nominal thickness of the lagbolt. For instance, for my 3/4 lag bolts, I drilled a 1/2" hole as deep as I needed the bolt to go. Then, I widened the first part of the hole to 3/4". I do this because the first part of the shaft is 3/4" thick and it doesn't need to be squeezed in, but you want the threaded part to bite into the target wood (or tree) so that it doesn't pull out. Then I tightened the hex head with a socket wrench.
I didn't think my treehouse needed it, but I found these 1" lag bolts and 1 1/4" lag bolts for sale. So if you are building a bigger treehouse, there are bigger lag bolts for sale there. You can also special order big lag bolts that size from Fastenal, Home Depot, or Lowes, but they will take some time to make and ship them.