Hi there 

Over the last few years I have been baking cakes for my children's birthdays.  I tend to use faily large tins and have found of late that the middle rises and the side sag in the tin.  Does anyone know why this happens or how I can avoid it?  Does this mean that I need to add more mixture and cut it down to size.  I find I come undone when i am applying fondant to the cake and the bottom sinks in not making it a very good look. 

Thanks for your response!

Depending on the size of the pan, it is always good to use a heating core for anything 10 inches or larger so it will bake evenly and eliminate the dome that sometimes happens.  You can buy heating cores at any cake decorating supply shop or I just use my metal flower nails coated with the same thing I used on the pans.   Such as flour and a little shortening.  Place the nails, top side down in the pan after you have filled it with the batter.  Example, if I am baking a large sheet cake, I will place them about 2 to 3 inches apart lengthwise down the pan then bake as normal.    I have also found that if you use baking strips on the pans this helps eliminate the dome and I get a taller cake.  These can be found a cake supply shop or online as well. Hope this helps.