I recently did a wedding cake, the bottom tier was 16 inches and when I put the fondant on top most had cracked. Why does this happen and how can I prevent this in the future. I put some vegetable shortening, but the cracks are still showing.

Are you talking about a kind of alligator skin look? If so, that is caused by using too much PS or CS when rolling out. If you are talking about actual opened cracks then I can't help you, don't know what would cause that. HTH

Here are some tips I'v found that help when working with fondant.
1. Warm about 10 sec. in micro (1/2 batch of MMF) - to help soften
2. Knead really well - with slightly greased hands on well greased mat.
3. Don't roll too thin - will cause it to crack more easily
4. Apply onto cake with wonderful blue fondant mat from Atelco - I haven't had a crack since using this!
5. Trim excess as soon as you get it on cake - to ease the weight and prevent cracking.

I hope this helps....

Thank you sweetcakes. Yes, mine did have the alligator skin on it and I did not knead it well. What is too much CS or Ps?? I do have a fondant mat, but I'm not too crazy for it. Thanks for your help.

Too much cornstarch or powdered sugar will give the alligator skin...I actually prefer to use shortening when rolling out fondant. Sweetcakes is right about kneading out the fondant very well...it should be very pliable and not sticky before you begin to roll it out. I don't use a mat just roll out, pick up (roll onto a rolling pin for large pieces) and place.

I agree with lorijom - once I started using shortening instead of cs or ps I started to enjoy working with fondant. I have also found that if the fondant dries out, I can knead a small amount of shortening in without any effect to taste or looks.

LOLOL, I understand, everyone seems to find their "comfort zone" on applying fondant. I envy anyone who can roll fondant onto a rolling pin without issues....just doesn't work for me!
If I'm applying the fondant for the overall "base" of the cake, I don't put CS down, just the shortening, that makes it release real easy from mat.

However, If I'm doing boarders etc., After I've worked the fondant well with shortening/kneading, its THEN I dust board with CS, and start working my boarder/designs so they will roll up or whatever I want them to do, but they will become more dry and workable to apply to cake.
I hope that makes sense?