Many thanks for the replies that you have given me. I have to agree about the problems salt will cause. I had only done this for a short distance and have now removed it. I do not think there is an easy way to prevent the roots. I am sure that any barrier will not work too well as the roots just go underneath. My solution is to dig my trench around the perimeter about 4" x 4" and cut out every root I find and rip it back as far as I can. Then fill the trench with good soil. The roots will no doubt grow back in time but the good soil will be very easy to just hoe it out, cut the roots again and replace the soil. The effort is all in the initial work. The follow up will be easy!
