Building a patio

[arty7774]

I am building a patio and want to kill the ground beneath it. Can anyone tell me the best method or weed killer to do this.

Thanks in advance.

[heathfield]

Just put down a plastic sheet of heavy grade DPM, (damp proof membrane). Available from builders merchants. Nothing will grow through it.

[missieb]

If you are going to use a membrane you should ensure that it is porous otherwise you could get a build-up of water under your patio. Ask for a heavy duty weed-suppressing membrane at the builder's merchants or garden centre.
Most weeds will not come up through your new patio - Japanese knotweed is an exception. If you do get a weed problem it is likely to be from annual weeds that seed themselves in the pointing gaps. You can stop this for a whole season by watering in March with Pathclear.