Believe 32c's fitting on a Roubaix is dependent on rim width.
OP, if you insist on 28's....not sure why you would want them...not ideal for road riding unless your road is quite broken, you may want to consider a gravel bike like a Diverge which can fit a much larger tire but has essentially the chops of a Roubaix. A slightly more robust Roubaix if you will...exact same geometry but can fit wider tires. In effect a drop bar 29er..or a tall cross bike. These types of bikes are quite popular for both on and light off roading like ride dirt/gravel, single track etc. You can then keep a second wheelset with skinny tires for more dedicated road riding and not give up much in terms of speed to a Roubaix. I would own a Diverge or a bike like it but I don't ride dirt and single track any more. I prefer smooth road riding but I have spent a lot of time riding the rough stuff in my past.