Garage floor and waterproofing garage walls and floor

[PedroS]

I have an existing garage floor which is very rough. The walls are single thickness brick.
I would like to create a smooth surface which will withstand a car.
a) If I create a wooden frame to contain the screed, how deep should it be?
b) Do I need to leave a gap between the surface and the new screed as the DPC is visible above the existing concrete.
c) Should the floor be sealed with e.g. Thompsons or bitumin before laying the screed and if so how far up the wall should it come to create a single damp proof layer.
d) should the oil stains be removed first.

The garage is very damp. Cardboard boxes go soft in a week or so and the freezer is dripping with water on the outside.
a) Should the walls be sealed on the outside with e.g. Thompsons?
b) Must the walls be dry externally before painting the outside.
c) Is there an advantage to painting the inside as well?
d) How long between painting outside before painting the inside?

[Perry525]

If the floor is strong and sound, then after cleaning and removing the oil etc;
you can paint it with waterproof paint to make a dpc, then lay a self leveling cement that will be strong enough to drive fork lift trucks over. Take a look at Whatco's site.
Regarding the damp. You mention that you have a dpc in the walls. That should stop rising damp, that would only rise about four feet.

To have condensation, you need some warm damp source, like a person or tumble drier in the garage for some time. That doesn't seem likely?
Is it the roof? My garage has a steel roof that collects condensation that drips all over. I don't consider it a problem.