Hi, I m new to cake central. I've always Google answers and usually find them but getting stuck on this one. 

I've been asked to make a 8" rainbow layer cake covered in fondant. Usually my cakes are baked in 3" cake tins and I can bake one deep 3" cake and torte it into 3 layers or bake two 2" deep cakes to get 4x1" layers. How tall can I go without needing dowels and support? I've done double barrel cakes where I stack two 3" cakes on top of each other with support and that seems sturdy. I worry that a rainbow cake covered in fondant without support would be too heavy. 

Obviously I would need to bake each layer separately to achieve the rainbow colours but how thick should the layers be? I've seen 6 and 7 layer rainbow cakes on Google but they are covered in buttercream not fondant. 1" layers would mean the cake is 6/7" tall with no support. Is that how it's done? Any ideas?! 

Because rainbow cake has to have 6 layers I would make them no more than 3/4th inches tall each. With filling it should be about 5.5 inches tall. It should be fine without double barrel support, (depending on your cake density ).

If you make it usuall thickness like your other cakes and do double barrel it will loose the whole point.

Dowels are recommended on cakes over 6" in height.  If you follow Natka81 suggestions above, you should be fine.   If you are concerned, just place on a covered foam-core base board and whack a central dowel through the middle.

Have you seen Jennifer Bratko's Rainbow cake tutorial here?

http://www.cakecentral.com/tutorial/34565/rainbow-ombre-cake-tutorial

The only step difference for you would be that you would cover your cake with fondant around Step 17

Thank you for the replies. I thought the maximum height was around 4" without support but then I've seen naked wedding cakes with 5 layers or 3 x 2" cakes (at least it looks like it from photos!) which must make them around 6". I haven't seen many pics of rainbow cakes covered in fondant  so wanted to be sure it was ok to do in case the fondant weighed it down too much. 

I think you are right in following a tutorial as it tells me how much mix I need to get the height of each layer right. I think I will fill and cover in white chocolate ganache though just for extra support as it stays firm. I usually use smbc but that might be too soft and bulge out under the weight.