Relaying flagstones on solid concrete base

[old wrinkly]

Some years ago I made pathways around my garden by laying flagstone/paving slabs on top of a previosuly laid solid concrete base (which was used as a temporary path)using the spot dab method which I had been told was the correct method to use.

Unfortunately, I have now learnt to my cost that this is not very effective as the slabs in some areas are rocking badly and very loose due I believe to the fact that the dab method gives inconsistent levels and provide a moisture trap. Further, the slabs themselves are B&Q riven type and the mortar I applied to the joints dried out and fell through to the void beneath becuaause there was nothing underneath to support it..

I have tried to rectify the problem a number of times over the years but the same problem happens in time, and I realise that it is time to take drastic action and rip up all the slabs and lay and mortar correctly.

I have a couple of people who are prepared to do the work but I want to make sure that when done it will last.

One chap said that he will run a bead of mortar on each of the long side and the centre so that when the slabs are pressed down this will spread out and give a base for the jointing mortar to fix to (hope this makes sense).

I assume that this method will support the slabs much better than the dabs method although there may be better alternatives. I then come to the jointing and again looking at various articles it seems that gun jointing is recommended by many and also that proper joint mortar should be use.

Before I decide what to do would welcome any advise on the best options available so I make the right choice and know the job I pay for will last. I should mention it would not be practical to rip up the concrete base.

Many thanks for any help.

 

[r896neo]

They must be layed on a full bed of mortar, no patterns no lines or spots or anything a FULL bed. As for the pointing, different people have different methods, i think that by buttering the edges and doing it as you go you get the best joint because then it is continuous with the bedding mortar.

Failing that they should be pointed up with damp mortar and then ironed/ struck. Anyone that tells you they will simply brush it in dry and the rain will harden it should be laughed off your property.

Full on polymeric resin type jointing can brushed in but this is done after mixing with a hardener and not something that you should try if you haven't done it before and is a very specialist expensive product more suited to very labour intensive pointing such as setts/ cobbles.

 

[old wrinkly]

Thanks for the reply, very useful. Will know now what questions to ask if I go ahead.

 

[LooPrEvil]

A good dab in each corner and one in the middle should be more than adequate. Its used for vehicle carrying drives so is more than sufficent for a path.

I think it may be down to your mix, you want a 3:1 mix with a plasticiser to give flexibility to it. Your mix was possibly too firm, or insufficent dabs used which does not give a good bed of mortar.

As for the pointing, once you have bedded the slabs with your paving mallet, the mortar spreads which gives a good base and protudes to the ajoining slabs. Did you use a dry mix brushed into the joints as this is a very weak due to it not being compacted, and eventually due to expansion and contraction it will just break up. Always use a wet mix (just sufficent water to bind the cement and sand).

Relay the slabs that rock and repoint where necessary, as above, and I am sure you will get years of trouble free use. Dont forget to use fresh cement!

 

[r896neo]

Don't listen to this guy, I don't know anyone in their right mind that would lay a drive that carries vehicles using the 'Dabs' method.

Do it right and do it once or you will be posting the exact same thing in another few years. After all you have seen first hand how the dabs method will fail after a while.

 

[LooPrEvil]

Old wrinkly, there is always more than one way to do a job. The way I have told you will work well for you. The way rneo has mentioned will also work well.

You can google subject and decide which method, or indeed any other, you prefer to adopt.

I am sure you will know that no one way is best!

 

[LooPrEvil]

I am in the process of extending my drive, and as previous done, I am using 5 dabs of mortar.

But I must add that I only buy British Standard 50mm thick paving stones for driveways. If you are using economy paving (35 mm thick) then I agree, you would need a full bed of mortar for driveways (if for foot traffic only, the 5 dabs will still do).

 

[imamartian]

For what its worth (i'm a DIYer, not a pro), i have layed many patios in my time, and the the idea of dabbing just seems daft. I've read so many articles which says you should put dow a firm layer of sand and then drop in a dab of mortar at the corners and the middle but surely these just stick to the slab? so it's as though the slab was cast with dabs, and you lay that on the sand !!

In your case, you relying on the previously laid slabs.... but dabbing just introduces gaps... and gaps are the enemy of the patio.... your patio needs to be a complete unit (slab) that works together against the forces you will later apply...

These gaps can fill with water and freeze (which can break bonds and concrete etc) plus having a space next to a dab means it can break, whereas if it's in a slab it won't

No discussion here for me.... dabs don't work.

 

[LooPrEvil]

A 2" thick paving stone correctly pointed will have a job freezing underneath it. Besides, when the paving stone is tamped down with a paviers mallet the dabs spread giving virtually total contact area!

 

[imamartian]

A 2" thick paving stone correctly pointed will have a job freezing underneath it. Besides, when the paving stone is tamped down with a paviers mallet the dabs spread giving virtually total contact area!

well i won't argue with the pro's but my gut feeling says, there aren't many patios pointed so acccurately so as not to have at least one hole or crack where water can get in...

and the tamping down of the dabs won't work on sand. Plus if you tamp the dabs down that much, it'll only be a few mil thick, and will potentially break up where there's excessive weight bearing, or impacts?

Just my thoughts.

 

[LooPrEvil]

It all depends on the quality of the workmanship. If you brush dry sand and cement into the joints and spray, you will get weak pointing.

Tamping will work on a sound foundation, and the dabs have to be sufficient volume, say 1/2 a garden spade per dab (not an egg cup full !).

Your sand base should have cement mixed inotherwise you will end up with worms living in it, moving it, undermining the slabs etc.

 

[DominicJ]

The five dabs method done well sounds like it uses a lot of expensive labour to save a few pounds of materials....
Dont badly, its sounds like a bad way of saving a few quid.

Just saying...

 

[LooPrEvil]

Horses for courses, don't lose site of what the OP was planning to do!

 

[WabbitPoo]

No, don't go for 5 blobs. And make sure the concrete is clean before applying mortar, and make sure you prime it first with a neat cement slurry.

Learn from your mistakes!