How do I fit Arris rails to concrete posts?

Firstly have a look at your posts on the fence line ie the face of the post that the arris rails wil be fixed to. If there are wire tensioner holes[ Used for tightening diamond style mesh fencing] then simply plug the holes with dowel and use them to screw a board 19 x 100 mm x ? ? being the height of the fence to each post then fix your arris rail brackets to these boards.
If there are no holes then you need to use a mains hammer drill with a good quality masonry bit around size 4 - 5mm or so, as these posts are hard. Check the size masonry bit against the rawlplugs you buy.

If you have a stud and wire locator commonly used in house to locate studs and metal pipes use it to locate the 4 reinforcing rods then drill out the holes for the boards in between the rods. Once the boards are up fix the arris rail brackets to the boards.
Wickes sell arris rail brackets or go to any local fencing supplier.
Forget making a hole for the arris rail you will destroy the posts and will pepper your face with concrete shrapnel, It's hard just doing the boards I know I've done many over the years.

DIY'er
March 2005