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Depends on the connection.  Pvc to Pvc--tape only.. Pvc to Stainless or Bronze---Paste

Here is some good stuff on using teflon tape to seal threads. I am sure this applies to roof cleaning.

I’d like to share a few useful tricks, although I can’t take any credit for them: I’ve learned them from the mechanics in my own life. Today, how to apply Teflon tape.

Teflon tape

Teflon tape is applied to threaded fittings so that liquid or air does not leak out of the threads. It’s a white tape, simply wrapped around the fitting. Not hard, right?

Teflon tape and fitting

But there’s a trick. You want to wrap that tape so that when you screw in the fitting, the tape doesn’t UN-wrap itself. Think about it: if you screw in a fitting “righty tighty, lefty loosey” you want the tape to be wrapped in the same direction so that it tightens on the threads as you tighten the fitting.

But it’s hard to think in three dimensions like that while you are standing there with a roll in your hand. So I learned to do it the same way every time. I’m left handed, so I hold the fitting in my left hand, threads out, thumb on top.

Hold fitting in left hand, threads out, thumb on top

Hold Teflon tape in right hand, twist fitting to the right as you wrap the tape.

Start tape holding in right hand, end under left thumb

Twist fitting to the right as you unroll tape

Following that procedure, the tape comes out wrapped the correct way every time. If you are right handed, adjust to make it work for you. You can use trial and error to figure it out the first time, since no matter what configuration you hold things, in the end your only two end results are tape wrapped counter- or clock-wise. The key is, once you find a way that works, stick with it!

Tape will not come unwrapped as you screw in fitting

I also use Tape, and just put it on the threads however I can.  I found the pictures I posted about the "Right" way to put it on, so it don't "Unscrew" itself.

Perhaps they are right, but I never worried about all that before.

I think the main thing to watch is too MUCH Teflon Tape put on the Threads, especially plastic ones. That seems like a sure way to split and crack things.