my spikes fit only about 1/2 " lower than the ladder feet ,they are sharp so they get a good grip on the tiniest of cracks inbetween slabs . Since i put spikes on my ladders ive never had a ladder move on concrete or whatever .
the only surface i wont ever put a ladder on is a Quarry tile path ,thats what i slipped on when i was using a rubber mat . or green decking ,altho the spikes dig in the wood a tad,i wont take that risk
A few firms sell "ladder spikes" just google the words. they bolt thru the ladder and are easy enough to fit . i believe i could have made my own ,better quality,using a strip of hardened steel . the ones i bought are made of mild steel,not so good as hardened steel but they work o.k. , and they get a bit blunt after a few months and the tips need sharpening with a file.