Perhaps I am wrong - but it would seem to me the birds are attracted to a gravel base. I have seen many a roof in Winnipeg with ordinary patio blocks as a base. Per chance if you used those on that ledge, it would not significantly add to any cost, preserve the integrity of the roof and discourage future nesting desires ??
What about this idea, TPC? Seems like a good one... 
We have had peregrines nest on asphalt roofing, metal platforms, metal downspouts (really old large catchbasin types), bare concrete, on large stones, on astro-turf, in old crow's nests and in pigeon feces. Of those, we have only had 3 nests (9 chicks total) survive to fledge and none of those chicks are known to have survived. Sometimes it is just the location and not what is in it I'm afraid
That's why we try to find places they are interested in and find a way to make a nestbox work at the location - win-win for us as well as the birds.
As for putting patio blocks, they are quite heavy and we would need to put a lot of them there to fill the space - the weight could be an issue in the location because that roof was never meant to support anything so it may not be strong enough to support the blocks. They may also be an issue to get up the side of the building and then they have to be cut to fit (remember the ledge is a triangle and some customizing would be required. Then need to make sure that they won't interfere with drainage and then hope that the birds don't take a liking to the spot anyway. And plants are always an issue there - you will see after the chicks leave how much vegetation grows up, so there may be plant debris the birds would take advantage of as well as any prey remains - bones in particular. We clean out the nestboxes here, but if you watch some of the other webcams you can see how much stuff can accumulate in a nestbox over the course of a season, the feathers eventually mostly blow away but if they get combined with feces they make a pretty solid mass that isn't going anywhere - and there is a bunch of that kind of debris on the nestledge right now.
So, 1) can't interfere with building integrity, 2) can't be a potential hazard for peregrines, people & property, 3) has to be transportable, 4) can't damage building, 5) has to be a real deterrent to nesting otherwise the exercise/expense is wasted, 6) has to be affordable, 7) has to have longevity - don't want to have to redo it in a few years because it can't stand up to weather, drainage, plants growing in it, etc ... there is probably an 8th and 9th but I think you get the picture.
Relax folks, we are considering what our options are and if we don't find one this week, maybe next week or next month or the month after or the year after. Our last nest was here in 2009 and all went fine but still we've been looking and considering what we might do to interfere with the ledge as little as possible but stop the birds from nesting there without stopping them from doing other things - roosting, hunting, defending the building, eating, courting. And its not just adults, we want to make sure its safe for the chicks to use once they fledge.
