new floor for old garage

[cupoftea]

I have a precast garage (3m x 6m) built on a 6 inch concrete base. When it rains the floor gets wet around the edges. I've sealed where the precast slab meets the base so I guess water must be making its way up from underneath the floor. I reckon I need to raise the floor by 60mm to resolve the problem. I have 3 ideas not sure which is best:

1 Polythene sheet and then screed.

2 Polythene sheet and then concrete.

3 Polythene sheet and then ply sitting on pressure treated timber.

Also, I've seen some black bitumen type stuff used in the place of polythene sheets, is it any good?

Thanks.

 

[DAZB]

Have you checked the joints of your precast panels because that is the most likely point for water coming in and running down onto the floor?

 

[nstreet]

How have you sealed around the edges, with silicone or something similar or do you have concrete filets around the edges.

 

[cupoftea]

I have double checked all the joints and the water is defo coming from the floor. I've sealed the bottom edges with sand & cement and still it keeps coming.

Is there a maximum depth that I can screed to?