Hello, I have a pipe knocking issue. Pipes behind the wall of the bathroom knock [U]only [/U]when someone turns on the sink faucet (hot or cold) while the shower is running. It does not happen when only one or the other is running. Turning off the sink faucet does not stop the knocking - it will continue as long as the shower is running. I can make the knocking stop if I turn the shower water more toward cold (or turn it off). There are two bathrooms with showers on this floor but the issue is limited to only one of the showers. However, turning on the sink faucet in either bathroom starts the knocking. Any help is appreciated!

I’m in the process of building a 40 x 60 pole barn. The building is up and now it’s time to pour the concrete slab. Before I put the stone in and Stego vapor barrier, I need to dig in the plumbing waste lines. I’ve located the primary vent and waste stack which will be 3” and extend from ground to roof. So I feel good about that. I’m a little concerned with the shower, getting the 1.5” pipe in the exact spot. I’ve seen where guys actually build a 1 foot square form with 2 x 6 to leave themselves some “wiggle” room. Is this a common practice or is there another way to accomplish exact pipe location. Also, I’m wondering the best way to drain the sinks. I can only assume “S” traps where pipe goes into floor is best (just seems to make sense to send water down opposed to horizontal), as opposed to “P” trap where pipe enters wall. Is my assumption correct? If so, do I plan for pipe to come up in center of cabinet, or do I offset to allow for trap?