The Brew Cabin

Hi Tony,

I’ve reviewed your very informative site, and read through many of the forum discussions – I know that you are primarily U.K. focused but I was hoping you could provide some advice for someone living in the frost belt of the US (Minnesota). We normally have 4 to 5 months of winter where I live with the ground freezing many inches (frost line of 1+ meters). The climate is extreme here with winter temps as low as –30 degrees Celsius and summer temps as high as 40 degrees.

Now to my quandary:

I have an existing concrete patio, 30 years old and 100mm thick. It is reinforced, and also has movement joints (expansion). It also has one 5 mm or so crack across the width of one section. It is about 9 meters by 5 meters. Other than the crack, it is in decent shape, with little surface deterioration, etc. Between the sections (areas separated by the expansion joints) there is little difference in height. We would like to install a stone patio, something more attractive than the raw concrete. We have tentatively selected a type of flagstone quarried hear in Minnesota that is sawn on two sides to different thickness, but for the patio flags would be 25 mm thick. It is a type of limestone called dolomite, very fine grained, resistant to weathering, and used widely as a building material in this area (http://www.uwm.edu/Dept/Geosciences/Urb ... ption.html).

This flagstone is an even thickness, since it is sawn on 2 sides, but would be irregular in shape (crazy paving). Ideally, I would like to install this new patio over the existing concrete, yet have it last for a number of years (20+). But if this does not make sense (to use the existing concrete as a subbase), I would take the existing concrete out and start new.

If I keep the existing concrete subbase – should I

1)bed in the 10:1 grit sand/cement bedding at 40 mm thick, and then grout with type II mortar? Since my flags would be regular thickness, I could use the screed technique.
2)bed in type II mortar 20 to 25 mm thick (with the same mortar pointing between the flags)?
3)Or should I treat the flags almost as a thick stone tile veneer and use a premixed tile mortar made for thick tile called medium bed thinset tile mortar? From what I gather, you spread this special type of thinset on with a notched spreading trowel – it is formulated to retain its strength even when spread thick (hence the name medium bed). Since my existing concrete has the proper fall, and the flags are even thickness, I think this may be the easiest to lay and possibly the strongest.

With any of the above methods, I don’t want to have major cracking – and I also don’t really want to cut through the stone at the existing concrete expansion joints. Is this the recommended way if I lay across existing concrete with movement joints? Would the 10:1 bedding prove more tolerant of sub base movement than the solid mortar bed or medium thickness thinset?

4) Or finally – would you recommend removing the existing concrete, and starting over with laying 100mm of dpt1 (I think class 5 crushed limestone here) subbase, then the 40 mm bedding?

Your advice would be greatly appreciated.

Hi,

I'm usually very cautious about advising on paving in the USA as the methods and materials used are totally alien to us in Britain and what works for us is not necessarily the best thing for you, and vice-versa.

Anyway, for what it's worth, I'd offer the following advice...

Bedding - the existing concrete base sounds fine, so there should be no problem re-using it, provided that the extra height created when you lay the new paving will not compromise floor levels or any damp proof measures you may have. I can't see any point in breaking out the existing base and replacing it with a flexible sub-base - it's a poorer construction, it involves a lot of extra work and it's unnecessary, as far as I can see.

For the actuual bedding, the tile adhesive sounds ok. In UK parlance, anything less than 30mm thickness is classed as a tile rather than a flagstone, but that, in itself, is not sufficient reason to use a tile adhesive for a job such as this. The thin nature of your chosen stone means that it must be laid on a full bed. I'd be worried about using my usual 10:1 bedding mix, as it's not intended to be subjected to repeated freeze-thaw cycles. A mortar would be ok, I suppose, and would be my choice if there were any deviation in thickness of 5mm or more, or an uneveness in the base, but, as all that seems to be fairly regular, then a proprietary adhesive would be a better choice, as long as it is approved for external use in the climatic conditions you have to endure.

But is this bedding material suitable for use as a jointing material?


Movement joints - you cannot bridge the expansion/movement joints with your stone paving. If you do, it will probably crack. Ideally, the joint is carried through to the surface of the paving, so you'd have to align your random-shaped stone in such a way that the underlying joint could be extended upwards, preferably with a flexible jointing compound (I can't suggest one as I don't know the US-ian market, but in the UK, we'd probably use Thioflex or Nitoseal).

Although you might not notice it, there will be some movement of the slabs either side of the joint, and, if you use a rigid medium, such as stone, to bridge the joint, then the shear forces will be transferred from each slab into that fragile, 25mm thick sliver of stone, and, as stone is absolutely useless when in tension, it will simply crack. If you had used a mortar or a low-strength bedding medium, it might be that the 'crack' would develop along the interface between the flag and the bedding, but with a decent adhesive, such as we described above, then the stone itself is the "weakest link" and the crack will appear as a fracture in the stone.

However, there's no need for you to have a perfectly straight line 'joint' in the stone paving, you can create a gap over the line of the joint that varies in width from, say, 5mm to 40mm, and fill that with the flexible joint sealant.....



I hope that answers most of your immediate questions, but if you've any more, I'll try to help, and I'd definitely be interested to hear how you get on with it all.