Stinging Nettles

Not sure if this is the correct place to ask this question, but here goes.
We have three or four sting nettle plants growing on our roof parapet. We employed a Roofer to get rid of them last year as the sitting-room below them was getting damp walls. He is not getting on too well - he thought he had eliminated the plants by spraying them with weed-killer and then rendering over the holes; but they have reappeared! You obviously cannot dig out the roots, and I believe they are fairly deep into the cement of the brickwork. Any ideas greatfully accepted.

 

Hi and welcome to the forum

you need to spray when they are actively growing other wise they dont absorb the pesticide they should be dormant at the moment or maybe just about to re-sprout I will ask @Zigs to help as well on this one

Spruce

 

Welcome to Gardeners Corner

Spruce is right, they need to be actively growing, spray with a systemic weed killer such as Roundup, that'll kill the roots as well.

Make sure you do it when there's no rain forecast. Takes a few weeks before you'll see them drop