Re using existing sub base for new patio?

[johndeere]

Hi

We currently have 40sqm of block paving in the back garden which we want rid of, it doesn’t look great and it’s a pain to keep weeds and other gunk off it.

To that end, we want to remove the block paving and replace with sandstone paving.

I am hoping we can pull up all the block paving, remove all the sand to leave the sub base and then pave straight onto that? 

Unsure of the existing sub base thickness but it’s held the block paving for 10 years without sinking anyway so assuming it’s adequate for slabs

Cheers

[Temp]

Yes If its not moved in 10 years should be fine.

[Gus Potter]

4 hours ago, johndeere said:

To that end, we want to remove the block paving and replace with sandstone paving.

Good question.

 

My thoughts and a bit of tecky stuff.

 

The capacity of the ground to bear the load is probably not a major concern. I think this hinges on the thickness of the hardcore you have underneath and what is below that. Monoblock is quite forgiving in that if one or two blocks move about you won't see it. But when you start using large format tiles they can protest.

 

There are two things here to consider.

 

Does the underlying soil dry out in the summer, say if you have trees near or just live down South. If the soil dries unevenly (say at the edges of the patio what happens? Is it clay that shrinks, chalk that shrinks much less or open textured gravel that is good in this context? 

 

Next is frost.. where do you live and how cold does it get and for how long does that cold persist. Persistant cold can result in quite a lot of frost heave in clay type soils, a bit in soft chalk and less in gravel. Say you have your new tiles.. they may well be laid on mortar dabs, then some hard core.. say all of that is 30mm  for the tile + 40 for the dab (average) and 100 hardcore that is not succeptible to frost. That gives you 170mm before the heave potential ground.

 

Now the NHBC require founds to be 450mm below ground level for frost heave for a house designed for a 50 year life span.

 

But this is a patio.. what next?

 

Have a look round and see what vegitation / trees you have. Are the roots going to be growing under the patio? If so they will increase in size.. cause heave and dry the ground between winter and summer, shrinkage and swell. Solution.. manage the garden.

 

Frost heave. At shallow depth you can get differing amounts. But if you have say an open gravel ground, low / minimal ground water flow small ice crystals will form and then stop. But if you have ground, say a clay / silt mix that has a slow flow of water this allows the ice crystals to grow larger as they are fed by the water and cause a lot of heave. The solution to this is to provide drainage around the outside of the patio but not too deep as you don't want to suddenly do something that will upset the house founds.

 

Normally we would never have bothered about this but modern patio finishes are now a big investment so they need a bit more thought.

 

Hope this info helps.

 

 

 

 

 

 

 

 

[saveasteading]

I have learnt that slabs need to be bedded solid, and that the dot and dab method (which prevails) will fail.

 

Being one who looks in excavations and watches roadworks when on holiday...

I have noticed that Spanish paving is laid on a thin layer of sharp sand to get the levels dead on, but beneath that is reinforced concrete. Even in domestic situations. It doesn't budge of course.

That may be excessive, but I would suggest running a vibrating plate over the exposed sub base, adding at least 50mm more and compacting it, then laying the slabs on a 100% bed of mortar, as if tiling.

 

[Temp]

3 minutes ago, saveasteading said:

I have learnt that slabs need to be bedded solid, and that the dot and dab method (which prevails) will fail.

 

Being one who looks in excavations and watches roadworks when on holiday...

I have noticed that Spanish paving is laid on a thin layer of sharp sand to get the levels dead on, but beneath that is reinforced concrete. Even in domestic situations. It doesn't budge of course.

That may be excessive, but I would suggest running a vibrating plate over the exposed sub base, adding at least 50mm more and compacting it, then laying the slabs on a 100% bed of mortar, as if tiling.

 

 

Just make sure the sand can't be washed out at the edges?

[saveasteading]

33 minutes ago, Temp said:

make sure the sand can't be washed out at the

Agreed. Mortar better for the bed, or a sand cement mix, and an edging.

[Gus Potter]

35 minutes ago, saveasteading said:

I have learnt that slabs need to be bedded solid,

Yes agree but when the customers back is turned.. ?

 

In some ways dabs are not that bad as it allows the dabs to dry more so when you get frost there is less water for ice crystals to expand?

 

 

 

 

 

 

 

 

[saveasteading]

Just now, Gus Potter said:

In some ways dabs are not that bad 

You should see my patio here. I may post a photo. Not by me but groundworkers we used once only.  Water runs through it, then it moves, and rocks and more water gets in. 

 

But speaking as an expert crazy paver ( why I don't know)  total mortar bed is the way. I've even done some as steps into the sea that survived a storm with a name. Did it for a builder friend and even he was impressed.

[Gus Potter]

16 minutes ago, saveasteading said:

But speaking as an expert crazy paver ( why I don't know)  total mortar bed is the way. I've even done some as steps into the sea that survived a storm with a name. Did it for a builder friend and even he was impressed.

Ok, thanks for that.. every day is a school day.