I don't know what I'm doing wrong, but everytime I put a cake together the edges of it sag or droop. I use a 'bakery' buttercream or a swiss meringue... fill in the edges with the BC and it still droops, the weight of the fondant pulls down on the cake, but shouldn't the BC be strong enough to keep the cake up??? HELP!!!!
Are you allowing enough time for the icing to set up before putting the fondant on? I put my cakes in the fridge until the icing has completly firmed up. About 10-15 minutes minimum. Than you can cover your cake, and it shouldn't sag on you. HTH.