Is there any other way to place a div across the top of one or more divs using float on all divs, other than applying margins to the top div?
Don’t know about others, but I’m not clear on what you are asking. Could you maybe post an image of what you mean?
z-index was what I needed.
Glad you found the answer but your question was so vague we would never have guessed that as an answer 
If you had been clearer with the question then Ralph would have given you the answer at 4.48 this morning and maybe saved you some time.
Glad you sorted in anyway.