philip wrote:

Hi,
I have just cut down a wisteria, to prevent any further damage to the
roof. What could be done to stop it from growing again, for good? A friend
suggested salt but I am sure there must be a right chemical for the job.

If I understand you correctly, you want to kill it at root level. Then
if you don't want to dig it out, though they're not to difficult to
remove, then you could go for one of those stump-kill type products.
Sulphate of Ammonia being the usual ingredient.