Yes it is possible to install slabs or tiles on top of an EPDM flat roof. There are a couple of options here depending on the type of decorative finish you require and the design of slab.
1) A slab support can be used underneath the concrete slabs placed directly on the EPDM rubber membrane. These paving slab supports are a 15mm high circular plastic disc that supports the corner of 4 slabs in a standard check board pattern. Water will pass through the gaps in the slabs and safely away to the gutter.
2) If you are installing a ceramic or decorative tile on the flat roof that requires fully bonding then subject to roof loads it is perfectly acceptable to install a minimum 70mm screed on top of the rubber membrane (using an underlay fleece to protect the rubber from sharps). Tiles or slabs can then be bonded and grouted on to the cement screed.