Originally Posted by
digrat
Pressure treated 4' x 4' x 10' boards at my local home center are $13.
Pressure treated 2' x 4' x 10' boards at my local home center are $4.
I strongly suspect that the cracking at the holes could be remedied (or improved at least) by using two 2' x 4' boards glued together. Use one lag bolt just in front of and one just behind each hole for strength.
I'm no engineer, but my reasoning is this: I am an avid New Yankee Workshop watcher, and Norm goes on and on about today's modern glues. He says that the wood will usually fail long before the joint does, especially if you're using something like Gorilla Glue (a waterproof glue--important for outdoor woodworking!). That's one.
Two is: If you have two different pieces of wood, you'll have two different grain patterns back-to-back. If you just so happen to get one with a weak grain where you've drilled, chances are the other board can take the weight.
Third, a tighter-tolerance hole will also help prevent cracking, in my experience. But that goes for using 4' x 4' boards too.
That's my $.02