Okay aspley. Straight answer. Hope you understand it.
Yes it is now possible to lay block paving over either concrete or bitmac, providing good falls of at least 1 in 60 are already in place, either to a side to side camber or across the width.
However the block paving can not be laid in the traditional method. There must be a fin drain laid either side if the fall is a camber or one side only if cross fall. The fin drain, or drains must connect to a suitably designed soak away. There must be a geo-composite drainage core between the impervious bitmac and the sand bedding course. This core will consist of a permeable membrane, the drainage core and a bottom impermeable membrane. This composite must be taken down the fin drain trench to the bottom of the fin drain perforated pipe.
This system does work, but the disadvantages are.
1. If existing drive abuts house wall, lifting up new block driveway by approx 110mm may interfere with DPC
2. If existing drive marries in to existing back edge of footpath, then you have a lot of cobbling round to do at that point.
3. Additional Cost.
4. All of this drainage will be 90% redundant after two or three years when the paving has matured to being 90% impermeable.
There are cheaper ways of laying blocks over a impermeable sub base, that we have tried, but they did cause us a lot of problems in the initial maturing period, and if we did use these methods again, they would need a lot of improvement.
With regard to bitmac planning’s. Yes they have been used for years as hardcore, but would suggest you only, use them underneath concrete, new bitmac or old field tracks, and not underneath block paving as over time planning’s revert back to an impervious base.
We would not advise anyone to lay block paving over an impervious surface, unless they fully understand how to deal with the hydrostatic pressure from water in the sand bedding layer.
Your choice, Help this helps.