24 by 40 house on cement slab, twenty yrs old. Roof is well vented, (just added more vents) and two feet of insulation added on top of the original.
Frost forms on inside of roof on north side. The north and east walls have frost with changes to water and leaks out onto floor close to N and E walls.
The extra insulation seems to have helped, but frost still leaked down onto ceiling as soon as warm weather started.
If heat from the house is going up and hitting the cold air and creating frost, how can I insulate the ceiling more?

You need to figure out where the cold air is coming in that makes the frost in the first place. You also need to figure out where to funnel away the water out of the attic. Not a quick or easy fix here. Sorry. I don't think that adding more insulation will solve your problem as it is the outside air coming into a warm attic causing the condensation problem.

Thank you. I guess I assumed that the lack of heat and the neccesarry vents in the attic would create the cold air?

Where do you live? Two ft of insulation should be a pretty darn high R value, do you have a vapor barrier? You have some heated air leaking into the attic maybe from a trap door or bathroom exhaust fan. It is totally OK for the bottom of your roof to be freezing but there is too much heated, moist air getting in. What else might account for heated moist air?

Northern Minnesota. Moist is the key word I guess. We hadn't realized. I finally got the right guy at a lumberyard here and he said our bathrooms and stove need to be vented outside. Not into the attic. We hadn't figured out it was the humidity not the heat.
And you mentioned moist air also. We just thought heat, didn't think of the moist being the problem.
We'lll vent the bathrooms and stove outside, and use dehumidifiers. They say there's an air exchanger we can use but that it's pretty expensive.

Thank you.