Problem with Pigeons on roof

My son has purchased a top floor apartment (not a new build). He has
discovered a big problem with pigeons roosting on one section of the
roof. The gutters are completely clogged and dirt from the pigeons is dropping down on to his patio. He has advised the management company and they are going to clean the gutters but they don't know how to get rid of the pigeons.
The apartments were built close to an old grain store which is no longer is existence. The area developed very rapidly over the last few years. Any advice welcome (apart from pigeon pie!)

you can get pigeon guards which cn be placed on the gutters or areas where they ladn. They are plastic but look like nails stuck up in the ky. they work for all birds and you can get them in any DIY

You can get a bird of prey model which works for a while but the pidgeons can then get used to it. I have found that nets stretched over the surface in question works well. Another option is to try a chemical tretment on the roof. see http://www.absolutebirdcontrol.com/birdgel.htm and http://www.birdx.com/products/roostinhibitors.html

Thank you for your replies. The Management Co. says the problem is coming from a multi storey carpark beside the apartments. The pigeons are nesting there. They have in place the pigeon guards (the spikes) which are not working so they are now looking at putting in netting. They may put netting on the roof of my son's apartment. (This will cost them a lot of money). I will pass on the details of the chemical deterrent to them.