Welcome to the Cake Decorators Q&A

I am doing a 5 tier fruit cake for my daughters wedding.
When stacking it, is it better to put it on boards, then stack on doweled lower cake, if so what thickness board.

Fruit cakes are normally strong enough to be stacked without dowelling but for the sake of peace of mind for a five tier wedding cake it’s best to dowel.
Place the base cake on a 15mm cake drum and the rest of the cakes on their corresponding sized single thickness cake cards. Don’t muddle these up with the cut edge cards which are really flimsy. The size drum you place the base cake on is up to you, some people have a 12″ cake on a 14″ drum or they may use a bigger one, it depends on the look you want to achieve. Dowel and stack as shown in Paul’s tutorials which you’ll find here:
http://www.cakeflix.com/free-cake-decorating-courses/stacking-cakes-overview
If you have a little gap between card and cake it can be filled in with a line of royal icing and washed away with a damp brush. For easy royal icing make a paste with a small amount of your sugarpaste and a little cooled boiled water, add water little by little until the paste is piping consistency.
Hope this helps.