...I have to add a plate and pillars? I have seen YouTube videos where people stack 5-6 sheetcakes on top of each other before sculpting, and it doesn't appear to have any support between the layers. What is a safe height to stack without adding any support? (And I mean the same sized cakes, like 3 11x15 sheetcakes on top of each other.)

TIA!
<S>

You need support for every 4 vertical inches of cake.

From what I have read/seen/heard from other decorators one can go up 4-5" safely before needing to dowel.
You say 'add plate & pillars' - but doweling is a bit different. The cake that is going on top needs to be on a cake board (not a plate) the same size of the cake. Dowelling (no matter what you use - wood/straws/hidden pillars) go in the cake on the bottom, then the cake w/the board under it goes on top of that.
How many dowels to use depends on how much total is going on top. If it's one 5,6 or 8" then 3 dowels placed in a triangle is fine; if a 12 or 14" is going on top, then 2 or 3 more tiers on top of that you will need 9 dowels spread evenly around a circle of 10" (for a 12" cake).

Thanks, Leah, that's what I usually figure on, but when I see some of the things people "construct" I wonder if I am just over-engineering things or something!

Kakeladi, forgive my poor explanation. I did mean doweling the cake (using a cake board) to ensure support for the layers of cake above it.

Also have to remember that those videos might not show everything they actually do.

When I do a tall sculpted cake I will stack my 4-5 layers of sheet cake while frozen and do all of my carving. I will then mark a couple of sides (so I can line up my layers correctly when I restack )and unstack. I then add in filling, and boards (every 4" of vertical cake) along with the proper dowels and support needed. Then I start doing the crumb coat and finish decorating as normal.

HTH

Edited for some spelling/grammical errors