Root Killer around toilet flange?

[FLBiker]

Hi folks,

We had some roots come up over our toilet flange in one of our bathrooms.  Yesterday, my FIL (a plumber) and I pulled the toilet, removed the old flange, and cleared dirt and concrete away from the cast iron pipe, exposing about 6 inches.  When we put on the new flange, we're going to pour concrete in the gap, in order to prevent this from happening again.  I've read online that you can put an herbicide under the concrete to make it doubly secure, but I've also read that some of these chemicals can corrode cast iron pipes.  Anyone have any experience with this?

We've got some Root Kill and some Roundup in the shed.  The Root Kill was inherited from the previous owner, and the Roundup was bought at the advice of the plumber who first came out.

Thanks!

[FLBiker]

Just to add a bit more info -- it seems that Root Kill (which is copper sulphate) isn't a good choice (based on this link: http://pmep.cce.cornell.edu/profiles/extoxnet/carbaryl-dicrotophos/copper-sulfate-ext.html)

Still not sure about Roundup.  My concern is that, since I'd be putting it around the outside of the pipe, it will just sit there.

[MetalCap]

Better to use a hard root barrier poly or fabric prior to the concrete than chemicals.  It also depends on how long you need this to last for (5 yrs vs 25 yrs would be different methods)

[FLBiker]

OK, thanks!  That sounds like a good idea.