Loose paving slabs - How to fix newly laid loose paving slabs

[milleniumaire]

For 6 weeks we've had a builder landscaping our garden and he's done a great job building a number of new random stone walls and setting the levels for 3 different patio areas totalling around 142 square meters.

The slabs are 22mm sawn Indian sandstone with a rough sandblasted finish and all are 600mm x 900mm. The slabs have been laid on a full mortar bed on top of compacted hard core. To my untrained eye the mortar looks to be a slightly moist gritty bed. He has spent a lot of time getting the levels, laying one slab at a time and the gaps are 4mm, giving a modern looking finish.

He's almost finished putting the slabs down, but has been away for a couple of days. Some of the slabs have now been down for a couple of weeks. We have had a feeling that the slabs haven't been sticking to the mortar bed and determined today that most of the slabs are loose and even putting a foot on them will cause them to move (slide slightly into the next slab). Some of the slabs rock slightly.

None of the edging slabs have been fixed in place using mortar up their side edge.

We plan to use vdw 815 plus jointing compound to fill the small 4mm joints.

When he comes back on Monday we need to address this issue with him and I would appreciate any advice on how to resolve our situation.

Are we unduly worrying i.e. once the jointing compound slurry is brushed in and set, the slabs will no longer move? I've read (on this site) that it's acceptable for the slabs within the patio to move, but the edges should be secure, then when the jointing compound is added, everything will be solid i.e. similar to laying a block paving drive on a bed of sand.

Or, should we raise this as an issue with the builder and get the flags re-laid prior to the jointing compound being used? If so, what is the best way of doing this? I wondered if a bonding agent (SBR?) could be brushed onto the underside of each slab and a very thin layer of wet mortar laid (mixed with the same bonding agent). Essentially, raising all the slabs then "gluing" them down again.

Thanks for your advice.

[lutonlagerlout]

there should be zero movement
did he use a bond bridge?
what stone was it?
LLL

[milleniumaire]

If by bond bridge you mean something like SBR, which I've read about on this site, then no they haven't used anything except sand and cement.

Sorry, I should have said the slabs are Indian sandstone. (I've updated the original post).

[milleniumaire]

Hi everyone, any feedback on what I can do to resolve this issue? I have the builder coming back from holiday on Monday and will raise the issue of the slabs being loose. One of three things is likely to happen:

1. He will throw his teddy out of the pram, curse and walk off site never to return. At which point I will refuse to pay what he is owed and use this to engange someone to fix the issue, however, I still have the problem which needs to be resolved.
2. He will try to persuade me that when he fills the joints the slabs will no longer move. In my opinion, the joints are not the means of securing the slabs so this is unacceptable.
3. He will offer to lift the flags and re-lay them. This is what I need feedback on as I need to understand what will/will not work.

I'm tempted to have a play with some off-cuts over the weekend and some Cementone SBR admixture, which I can purchase from screwfix, 5 litres for £20. I will try mixing a very wet (slurry) of cement and SBR and stick 2 small pieces of paving together. I may even lift one of the smaller edge slabs that he has put down and which is completely loose and try painting on some of the SBR to the underside of the slab to see if that sticks without adding cement. Alternatively, I could try re-fixing another of the slabs using a slurry of cement and SBR mix, but my concern here is it may raise the height of the existing slab slightly and I don't want to do that until I'm sure it will work and will be done to all the slabs, resulting in them all being raised by the same height.

Any thoughts? I assume using it neat isn't likely to work, but what about making a slury mix and using this as a very thin "adhesive" between the existing hard mortar bed and the slab?

I also wondered if I could use external tile adhesive to stick the slabs down to the bed? The adhesive could be put on the back of the slab using a tile adhesive spreader as the backs of the slabs are smooth.

I would really appreciate some feedback from someone. Anyone?

[AFKMatrix]

Hi there,

I am no expert and you have probably seen my post a bit further down, with pretty much same issue. I have a slate patio and slate wall cappings that have detached from the walls. So on the advice on my thread I bought some SBR and some cement and mixed up a wet slurry and tested it on one slate step to see what would happen. It has stuck like concrete! And just today I have done a load more of the wall cappings and patio edging (bloody hard and hot work!) and after around 15mins they were all sticking like nobody's business! If you can convince your guy to relay the tiles with a thin layer of SBR/Cement slurry on the tiles then I think that would work.

[milleniumaire]

Hi AFKMatrix, thanks for your response and good to know you have resolved your issue.

I'm planning to try your suggesting and yesterday I bought some Bostic cementone SBR from Screwfix and will mix a slurry today and try it on one of the paving slabs. If it sticks okay then I will then try to convince the builder he must raise and relay ALL the slabs.

Out of interest, what mix did you use for your slurry? In the tech spec of the SBR it says use 2 parts cement to one part SBR for a bonding slurr, then paint this on. Obviously this is going to be thin enough to avoid the slabs being raised, which I would prefer, as the alternative is to apply a thinker layer of mortar (mixed with SBR) and this will raise the level of the whole patio. The existing mortar bed seems fine and is very flat and as hard as concrete, although it is not smooth like concrete due to the gritty mortar mix they used. The underside of the Indian sandstone flags is very smooth.

What mix did you use for the tiles?

[Tony McC]

2. He will try to persuade me that when he fills the joints the slabs will no longer move. In my opinion, the joints are not the means of securing the slabs so this is unacceptable.

This is the bodge-it-and-cross-yer-fingers method of fixing loose flagstones.

The technical explanation is that flags, as bed dependent paving, *must* be securely held by the bed as the jointing is there, primarily, to separate the individual pieces and prevent them spalling against each other, and also to accommodate any dimensional differences. It is not there to secure the flagstones.

Jointing alone is never enough to secure loose flagstones.

There are ways of jointing, using proprietary jointing mortars with a fine texture, that can fix *some* loose flagstones, but it has to be judged on a case-by-case basis. The premise is that a fluid mortar flows into the joint and into any voids beneath the paving, holding it all firmly in place, once cured. It depends on the fluid mortar being able to get beneath the flagstones, which is not always possible.

The only sure-fire way to fix loose flagstones is to lift and re-lay. This is best done by providing a whole new bed, but, in some cases, using a sbr-based bond bridge can help re-seat flagstones onto the original bed. However, even the slightest bit of extraneous grit or detritus can completely undermine this technique, and you have to be thoroughly consistent with the slurry to ensure that each flagstones is, effectively, lifted by the same amount, so all are up by, say 1mm, and not have some that are 3mm up or worse.

I would prefer your "builder" (a professional paving contractor would know all this) to lift and re-lay the lot on a new bed, but, if they can convince you, and provide you with a written guarantee that, should the sbr/cement fix not fully work they will then lift and re-lay, then let them have a go.....at their own risk!

What you need to avoid is providing them with an excuse to leave you in the lurch by claiming it was you that wanted the bond bridge fix, and when that didn't work, you expected even more remedial work....all of which could, if ever put before a court, make you seem unduly awkward.

[milleniumaire]

Hi Tony,

Thanks for the feedback. I agree the best solution would be to re-lay the bed, unfortunately I'm not convinced they would do a better job laying them a second time! I'm begining to have suspicions they haven't laid Indian sandstone before based on some of the comments over the last couple of weeks, basically calling it rubbish stone.

Today, for my own benefit, I lifted one of the edge half slabs (60cm square), cleaned it up (there wasn't much on the slab to clean except a bit of edging mortar), wet the slab, then painted it with some cementone SBR mixed with cement to give a slightly stiffer than liquid mix. I aso brushed away any loose grit from the base and poured water on it, then used up some of the left over mix by painting it on the base. It will be interesting to see what happens. Not the best day to be doing it as it's 26 degrees outside!

When I poured water on the gritty mortar bed I was suprised how the water just soaked into it, rather than ran off it or pooled!

The plan is to use vdw815 plus for jointing.

I understand your comments about suggesting solutions to the builder and will take that point on board and ask them what they recommend doing rather than suggesting a solution.

[milleniumaire]

My experiment over the weekend seems to have had the desired effect on the builders who are laying my patio. Unfortunately, the main man isn't due back until later in the week so no decision about how to fix the loose slabs can be made until then.

Yesterday I made two Indian sandstone slab sandwhiches using some off-cuts; one using a moist, gritty mortar similar to the one used for laying my slabs and the other using a wet mortar made with building sand, similar to the one used for building the wall. On each sandwhich, one of the slabs had SBR brushed on and the other side of the sandwhich there was nothing.

Although it hasn't been long enough for the mortar to set, when I was showing the builders my experiment this morning I picked up the one using the gritty moist mortar by the SBR end and the slab on the other side just dropped off - clearly there was absolutelty zero grip - just like our patio. The SBR side of the sandwhich could not be removed from the mortar! What was interesting was that for the other sandwhich, both slabs were stuck well, so it appears a wet mortar mix using building sand would have stuck the slabs, but not the gritty moist mix they have used.

The half slab I re-layed using an SBR paste on the back of the slab is absolutely solid. Even tapping it shows it has a strong bond compared to the untreated slabs around it which all sound hollow.

The builders were very impressed with the SBR, but told me they'd never had a need to use it before.

[AFKMatrix]

Hi milleniumaire,

Sorry for not replying to your question earlier, absolutely killed my self relaying my wall cappings this weekend (done about 30 of them and still have some to do !).

So I did one cup of SBR to two or two and a half cement and this seemed to create a good mix.

If your builder is going to go the SBR route then please tell them to have a bucket of water and a rag to instantly wash off any spillages. As I found out from the bits I missed, it is impossible to remove it once it has dried!

Good luck and hopefully they'll relay all the paving but use the SBR or wet mortar mix.

[milleniumaire]

Hi AFKMatrix, thanks for the update.

The Cementone SBR Admixture I bought from Screwfix over the weekend states to use 2 parts cement to 1 part SBR by volume to create a slurry, so that matches what you have used. However, it also says "mix to a paste" and brush on, but mixing 2 parts cement to 1 part SBR doesn't give much of a paste - it is very runny, which potentially means it will get everywhere. I also found that when wetting the back of the slab I had to brush off surplus water otherwise that made the SBR mix even more runny! I suspect there's a technique to getting this right without making a mess of the slabs.

[AFKMatrix]

It is quite a runny mix, we bulked it up if it was too runny by adding more cement. And I only put water on the back of the tile and the tile was so hot it was drying out rather quick so didn't effect the mix much. The mix is kind of a gloopy texture (not much help I know) and we had a big thick masonary brush we used to apply it. It will drip etc hence the need for the water and rag. It is a 2 person job though as I was picking up and laying the slate while the other person was sat mixing which stopped the mixture going off quicker. Just be thankful you won't have to do this lol!

[milleniumaire]

The builder has kindly agreed to try re-laying some of the slabs using a 1" wet mortar mix on top of the existing mortar bed.

Fingers crossed this will work.

[Tony McC]

....but how will that affect the clearance between paving and the DPC?

[milleniumaire]

HI Tony, that's a good point. On the lower patio it makes not difference apart from to reduce the height of the step from the lower patio to the higher patio, which is not ideal as the step wasn't that deep to start with.

On the higher patio, which is the one that meets the house and runs from the back to the side of the house, I'm not 100% sure and would have to check. I'm fairly sure the patio upto the house is lower than the old patio used to be and even with the old patio I could see the DPC, so hopefully it may not be an issue.

If it does take it higher than the DPC is there a way of working around this, possibly by fitting an impermeable membrane between the house and the patio? The paving slopes away from the house, which is built from random stone so is damp likely to be an issue?