Advice on laying carpetstones

[jone]

I'm building a raised patio and adjoining paths using Bradstone random carpetstones and need some advice -- I've scanned your excellent website, but found little specific advice for carpetstones. I assume the same basic techniques described for flags apply, but I thought I'd check with the experts.

The raised patio is constructed of a square slab of concrete 80mm thick upon which a low coping-topped perimeter wall approximately 215mm high has been built. I then filled the square void with a further 125mm of concrete using a thin section of specialist fibreboard between the concrete and the wall to absorb any expansion. This then left 90mm for the bedding mix and the carpetstones such that the carpetstones would finish flush with the coping.

I made a screeding bar with notches cut out at each end to sit on the coping such that the bedding mix would sit about 5mm higher than necessary on the assumption that the stones would bed in this distance. However, after I'd laid the semi-dry bedding mix (to Bradstone's recommended 6:1 sharp sand to cement ratio), I found that many of the larger stones (300mm x 200mm) refused to bed in properly using a rubber mallet (they're a little thin for anything more heavy-duty and I even managed to break one). A few days later and hey presto, over half the stones sound hollow when tapped and quite a few have a noticeable rock.

So here are my questions:

a) Did I add too much or too little water to the bedding mix? I know you're not supposed to add any, but the sharp sand was bone dry, the weather was quite hot and I was concerned that laying the mix onto a concrete slab wouldn't allow any moisture to be absorbed from the ground.

b) If I added too much water, could the bedding mix have started to set before I had chance to get the stones into position? It took a good 2-3 hours to get the mix in place and properly levelled before we started laying stones. Would I have been better adding no water at all? Would the mix have still set properly?

c) Could the bedding mix have been compacted too much? I doubt this is the case, since all we did was lay and level it. It would have been partially compacted during the process, but we didn't perform any additional compacting.

d) Would I have been better using a wet mortar mix? I know you don't generally recommend this, but would I have had more luck bedding the stones in? If making a wet mortar mix for bedding do you use sharp sand or soft sand and is the correct ratio 4:1 sand to cement?

I'm now worried about the best way to lay the adjoining paths (using a single line of carpetstones). Unlike the patio which at least has coping to constrain the edges, the paths will simply be bordered by a lawn on one side and a flower bed on the other. I don't intend using any edging since I want a flush finish on both sides.

To help me get the levels right, I'm building temporary shuttering to contain the concrete sub-base, bedding mix and carpetstones and arranging it so that the carpetstones will finish flush with the top of the shuttering (I know a sub-base isn't strictly necessary, but it will help me correct the slope of the garden). Since there won't be any permanent lateral support for the carpetstones (other than the lawn and flower bed), should I persevere with the semi-dry mix method (taking on board any responses to my earlier questions) or adopt the wet mortar method?

Apologies for the long description -- any help would be very much appreciated.

Jon.

[Tony McC]

The reason there's no specific advice on laying CarpetStones â„¢ is that they are a manufacturer-specific product. If Bradstone want to sponsor a page showing how their product should be laid, that's all well and good, but I'm reluctant to provide what would be free advertising for one particular company. If I do it for Bradstone, why not do a page for Marshalls, for Brett, for Tobermore, Roadstone, Formpave, etc? So far, I've tried to make the info on the site as 'generic' and non-specific to any manaufacturer as is possible. Hopefully, there's enough info to enable the general princioples involved in laying any paving materials to be gleaned. However, as I said, if Bradstone want to cobver the cost of a sponsored page detailing the use of Carpet Stones, I'm always willing to listen.

I can't say whether your bedding mix was to weet or too dry - I didn't see it. A 'good' consistency would have enough water to bind the sands and cement, but not so much that water would dribble through your fingers when you squeeze a handful. Besides, it's hard to have it too wet, unless it was completely dry like a powder, or too wet, unless it was sloppy.

With only a 5mm surcharge, I'm surprised to hear that the CSs wouldn't 'settle'. The few times I've worked with CSs, we've literally plonked them down, covered them with a decking sheet of 15-18mm ply and run the plate compactor over them a couple of times. Tapping down the "Stones" individually seems like an awful lot of work.

How 'compact' was the bedding layer before you placed the CSs? If you stood on it, would your boot/shoe/clog leave an imprint?

[jone]

Tony,

Apologies -- I didn't realise Bradstone were the only manufacturer to produce a "sheet" of connected concrete slabs. I wasn't after any kind of endorsement of the product, just some advice from the experts.

Anyway, the bedding mix would certainly have left an imprint from a boot. I didn't have any clogs to hand, but I imagine that would have left an imprint also.

I assume you would not recommend the wet mortar option?

Jon.

[Tony McC]

Bradstone have the licence for strung-together-sheets-of-concrete-blocks, but there are similar products that use stone, usually porphyry, and there was some...err...heated discussion a few years back about whether the stone versions were infringing the copyright of the concrete CarpetStones, which was, at that time, held by a Dutch company. At the mo', Bradstone are the only company supplying/manufacturing the concrete block type product for use in Britain and Ireland.

It's not that I wouldn't "endorse" CarpetStones - they are good for what they are - it's more a matter of not actively promoting a partcular product without there being a sponsorship arrangement in place. That way lies madness...

So: back to the matter in hand - we've not resolved why it is that your installation of CarpetStones would not consolidate as desired. Use of a wet mortar/concrete might make bedding-in, but it's awfully bloody messy. I can't understand why the semi-dry mix presented you with such prroblems, unless, of course, you have compacted to refusal prior to placing the CarpetStones.

[jone]

Tony,

Thanks for your continued help with this.

Good news to start with -- the carpetstone patio seems to have settled in somewhat after the pointing and even though they still sound hollow when tapped, there's no discernable rocking. I've decided to leave it for now and see how it goes.

Now for the paths. As I described in my previous post, there won't be any lateral support to the path and I'm using the carpetstones in single file to produce a 60cm wide path. Obviously the fact that adjacent stones are connected will provide some support, but to be on the safe side, I'm considering using one of the techniques described on your "laying flags to edges" page, namely using a wetted 10:1 bedding mix or instead using a class II wet mortar mix. Which would you recommend? I assume the mortar mix should contain soft sand rather than sharp sand, but can you confirm this, please.

Also, one final question. When pointing, do you always use soft sand in the mix or is sharp sand ever used?

Jon.

[Tony McC]

If I'm not too late, I'd recommend using sharp sand for the bedding mix. Normally, I recommend soft/fine/building sand for jointing, but because the joints on these CarpetStones are relatively wide, I'd probably use a sharp sand for that, too. Yes: I know it gives a coarses looking moretar, but I quite like that - I think it looks more 'authentic' than a neat and precise smooth joint as created with a soft sand.