# Forum Home Renovation Paving  Roadbase on concrete to level up, will it wash away?

## Rani

Hey guys,  
I have some very weird slopes in my sidewalk and backyard. 
The backyard will be paved after say 100mm of roadbase.
The sidewalk will be stepping stone pavers with pebbles surrounding. 
Question is, will the roadbase fines (especially under pebbles) wash away after a while? There is no footing or any barrier on the concrete floor right now. 
I've also read on this great forum that "crusher dust" or "blue metal dust" forms a layer like concrete, so I'm thinking have some of that over the road base to prevent water seeping into road base? 
Or should I just lay on top of concrete by sand cement?

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## cyclic

Yes, any bedding will wash out eventually, but if you can set up a timber /log/concrete etc border to hold it, then all should be good.
Crusher dust is probably the best bet but you will still need the border.

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## PlatypusGardens

Crusher dust will set "like concrete" if you wet and compact it right.
Wetting it on top of concrete is tricky though as the water has nowhere to go.
Compacting it will draw water to the surface. 
That said, there have been bad batches of the stuff with not enough dust in it so it doesn't compact as well as it could/should. 
Blue metal on its own won't compact.
Roadbase will have larger stones in it (10-20mm) which makes it a bit harder to compact with a small plate compactor.

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## Rani

Well guys, tell me what you think.  
We were originally gonna sand cement the pavers straight on the concrete. Now I'm thinking we raise to level needed with roadbase then make a bed of sand cement on top to stop water from getting to road base.  
Cyclic, problem (in my opinion) with what you're saying with building a barrier is that the water will have no where to go and because the bottom is concrete it might become a flood zone! Correct me if I'm wrong  
Another possibility is to make a French drain around the borders of brick and drill holes in the bricks for water to escape but then again these holes will get blocked in no time from fines in the roadbase.  
Again, please correct me if I'm wrong. I'm no expert by any means (IT guy who's never punched a nail in his life till this recent reno)

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## cyclic

> Well guys, tell me what you think.  
> We were originally gonna sand cement the pavers straight on the concrete. Now I'm thinking we raise to level needed with roadbase then make a bed of sand cement on top to stop water from getting to road base.

  (A) You will still need a border so any bedding does not undermine with rain, or you can make a 100mm bed of bedding sand and cement mix and lay the pavers as you go, that way you will have a solid base for the pavers which will not wash away.
You don't need to do the whole area at once, just mix and do a section you are comfortable with.   

> Cyclic, problem (in my opinion) with what you're saying with building a barrier is that the water will have no where to go and because the bottom is concrete it might become a flood zone! Correct me if I'm wrong

  (A) Some electrical conduit pieces around the perimeter will drain any water off providing the conduits are above ground.   

> Another possibility is to make a French drain around the borders of brick and drill holes in the bricks for water to escape but then again these holes will get blocked in no time from fines in the roadbase.

  (A) No, any holes will allow seepage to drain off but not heavy downpour rains.

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