Several questions re choice of slab

[split59]

Hi sorry for all the questions but would like to get this job right.
Just laid the base in concrete at 75 - 100mm thick. wih a healthy fall. Now i need to choose the right slabs and getting totally confussed with the choice. When we first discovered indian sand stone we were happy with the choice from our local stone merchant in uncalobrated sage riven. Since doing our research and reading this site and forum and looking around the net we have decided we like the sawn effect and seen so much differance in pricing from site to site. Do you still have to watch the quality of sawn slabs (would they bother working with cheaper stone in the first place?). Also one site mentioned you have to seal all sawn sandstone on both sides and use white sand and cement though none of the others mention this are they using a cheaper grade of stone? Can anyone reccomend a merchant and also anyone in Southampton area to lay it correctly?
Many thanks Malc

[PavingSuperstore]

Yes you most definitely do have to worry about working with the cheaper stone - there are some very poor quality sandstones out there, often too soft and porous for use in the UK. We often hear stories from customers who have purchased the cheapest paving from elsewhere and have then found a large proportion of it has to be discarded as it's too damaged to use. They often complain that it's almost like chalk as well.

We only offer paving from leading brands who offer CE/ British Standard quality approval for porosity, slip resistance etc, so this would certainly be something to look out for as it means the paving has been through rigorous tests to ensure it is suitable.

With regards to sealing - everyone on here has their own opinion on pre-sealing and using white cement etc, so it's worth forming your own view once you read through the answers, however our view is that none of the leading manufacturers preseal their paving including huge industry leaders like Marshalls and their technical department does not consider it the correct thing to do and in fact, suggests it could impact adhesion. We would recommend sealing the surface of the stone (with the appropriate breathable sealer) straight after laying the paving. As some lighter colour sandstone tends to draw the colour of the mortar through, using white cement can sometimes prevent this becoming a temporary eyesore. I haven't come across a situation where it ever lasts that long, so you'll need to decide whether it's worth the cost difference.

Sorry - can't help with a contractor in Southampton, but you're in the right place to find one on here.

[higgness]

Does anyone have a picture of a stone drawing the dark colour thru?

[London Stone Paving]

I hate to be the bearer of bad news but this project could already be off to a bad start.

I see from your post that you have installed a concrete sub base. The correct material for a sub base is MOT type 1.

MOT type 1 is extremely permeable so any moisture from the bedding layer will escape through the MOT. A concrete base is not permeable but the moisture from the bedding layer will still need to go somewhere. It will take the easiest route which IS straight through your stone. On its journey through your stone it will pick up all sorts of crap from the bedding layer and pull it through the capillaries of the stone. This process often leaves horrible staining on the surface of the stone which can be impossible to shift.

I've see this happen on many occasions when natural stone has been laid on a concrete base. I would urge you to consider ripping the concrete base out and replacing with MOT or taking other precautions such as:

1. Pre-seal any stone with dry treat. There are other sealants available but IME this one goes the deepest into the stone and will give the most protection in this situation

2. Drill holes in the concrete base at regular intervals

3. Use a permeable bedding layer.

I will try and dig out some pictures of stone that's been laid on a concrete base so you can see for yourself the potential problems

Will post them later

Steve

[split59]

If tha is the case London stone why does the info on this site sate concrete or mot1 sub base. And the advice for use on driveways or patios used for ocassional car access state a Concrete base be used
Totally confused now???/:(

[London Stone Paving]

Really sorry, I hate to add confusion to the situation but I can only give you advise based upon my own experience. Without wishing to blow my own trumpet I have vast experience of sawn paving and how it performs in different applications and with different construction methods. Myself and colleagues have experienced serious issues (when paving is laid on concrete bases) on too many occasions for it to be a coincidence.

Does the advice I have given make sense if considered logically?

Steve

[split59]

Im not doubting your experience or reply. Just peeved this was ot mentioned in any advice iv read to date. Does sawn act differently to natural riven then? If so will this be unlikely to be a problem if we used riven slabs would help us make up our mind and save afew bob as well

[London Stone Paving]

Sawn does act differently to riven. When stone is sawn the capillaries are opened up and this will make it more susceptible to water absorption. However you can still experience the same problems with riven as it still a lot more porous than a concrete base.

I did advise of a few precautionary measure that you could take. Are you able to consider any of these?

Steve

[split59]

Yes the base is a very weak green mix so not too much of a hardship to drill some holes. We have dogs so will be sealing It over anyway so happy to seal the underneath as well if really needed.

[London Stone Paving]

Don't seal the underside as it will prevent the bedding layer from bonding to the slabs. If you use a quality impregnator like dry treat sealing the top surface will be adequate to prevent the markings coming through. Dry treaty penetrates very deeply into the stone and this will form a barrier that prevents water coming through from underneath. There will be a Dry Treat stockist in your area, go on the Dry Treat website.

I don't know many contractors in Southampton to be honest.

Have you made any decision on the paving yet. Sawn or riven?

Steve

[split59]

So just to be clear if I choose either sawn or riven I need to use a impregnator such as dry treaty an this will stop the chnce of staining. Now a dumb question do i do that before the slabs are laid Ie lay them out and give the a few coats?

[PavingSuperstore]

Some people prefer to pre-seal, however we recommend sealing straight afterwards with a product such as Pavetuf invisible or Pavetuf satin sealer depending upon the look you prefer. These are both breathable sealers so will allow efflorescence to come through and dissipate as normal. They last around 8 years, so you should get a good length of service from them.

[London Stone Paving]

So just to be clear if I choose either sawn or riven I need to use a impregnator such as dry treaty an this will stop the chnce of staining. Now a dumb question do i do that before the slabs are laid Ie lay them out and give the a few coats?

That's right, a quality impregnator will prevent the moisture coming through the slabs and causing the unsightly marking on the surface of the stone. The moisture will eventually escape in the form of vapour.

Its not always possible to pre-seal due to space and storage restrictions on site but if you do have the space on site its highly recommended to pre-seal. There are a few sound reasons why:

-Stone can often get grubby during the installation so, there is every chance it will need thoroughly cleaning after installation and prior to it being sealed. This will involve it getting wet and the stone needs to be bone dry before it can be sealed, so you could get held up sealing it. Also while you are waiting for it to dry it can get dirty again and there is the potential for it to become a viscous circle

-British weather could keep the stone damp for days or even weeks, again preventing you for sealing

-Cement spills are much easier to clean from sealed stone

[PavingSuperstore]

I refer back to my original post - you will get different opinions so you need to decide which is best for you! There's pros and cons with each method so weigh it up with your own circumstances. With regards to the concrete bed, this is not recommended by any paving manufacturers - they all suggest Type 1 as the sub-base, however that said, we hear of several customers who adopt the full concrete method as they perhaps feel it is a more robust paving sub-base. They certainly don't all go about ripping it up again, so as much as it's not recommended, not everyone has problems, so forewarned is forearmed as they say, you can perhaps prevent the situation from becoming an issue by using more forgiving materials such as the denser sandstones in raj for example.

[Tony McC]

I can't agree with the comments regarding concrete bases and their unsuitability for use beneath pavements finished with sawn stone. The permeability of otherwise of the concrete is irrelevant, because the mortar onto which the stone is directly laid has, more or less, the same recipe as a concrete, so it, too, is relatively impermeable. If surface water is going to get backed up in the pavement structure, the first barrier it meets is the mortar bed, not the base or sub-base.

If we look at using trass bedding over an unbound sub-base, then yes: we do have a completely permeable sub-structure, but mortar over a sub-base is no different to mortar over a solid base.

Further, work done at Heriot-Watt and Nottingham Uni pavement engineering departments shows that paving laid on a solid bed of mortar over a solid base of concrete or macadam is to be the prefered method of pavement construction for the forseeable future based on perfomance over a whole life cycle. Personally, I prefer trass bedding, but the price is still inexplicably prohibitive.

Whilst casting no aspersions whatsoever on any suppliers contributing to this thread, the biggest contributory factor to staining of sawn paving is the use of cheap, soft stone. Too many exporters opt to saw and/or secodary process the softer stone becauses it's cheaper, it's easier on the blades, it's quicker....and then when problems occur, we hear these stories about unsuitable bedding or inadequate sub-bases, or improper jointing, or lazy installation.

True: it's not always the stone at fault, but there are more instances of softer stone than there are of unsuitable sub-layers.

Again, this is a personal view, and I don't expect everyone to agree with me, but too many of the imported sawn sedimentary stones are not best suited to the damp climate we have in Britain and Ireland. Once again, we have let fashion overrule common sense.


Turning now to pre-sealing, it has its benefits, but it also has its problems. At the moment, I'm sitting squarely on the fence - I really can't decide which is the lesser of two evils. A huge part of the problem is that we face so many variables with every paving project. It may well be good practice to pre-seal a sawn sandstone on such-and-such a particular job, but that does not mean it's right to pre-seal a sawn granite on a job down the road, or on the other side of the country. Every job has to be judged on its own peculiar circumstances.