Do a good job on your gravel bed, reinforce the slab well, and it's highly unlikely any cracks in the control joints will telegraph their way through the 1" of bedding sand to the pavers. Just lay the pattern continuously. Don't bother caulking the joints, either.

Just be sure you have a way for water do drain out of the bedding sand layer in the event it gets in there.

Alignment of the pavers with the expansion joint is usually a concern when the pavers are mortared to the sub-base
If you are doing a dry lay ontop of the concrete sub base, which is somewhat unconventional , but can be done with a satisfactory outcome if strategic drainage planning is done.
Commonly pavers are set on a permeable layer of crushed gravel and a level thin bed of setting sand so that adequate drainage can occur.

By the way , stunning stone work on the building in the far background.

Thanks for the replies. Actually, I was thinking of laying concrete blobs (2 sand / 1 cement) under the stone. By paving stones, I mean, somewhat irregular stones of various sizes, and around 1 inch thick, but not all the same thickness, as in the photo. But do you think it would be better to lay these stone pieces on a 1-inch bed of sand, on top of the dried concrete? I take your point about leaving proper inclinations on the concrete bed to run off any rain water that might get in through the stone joints of 2 sand /1 cement. Also, do you think it would be necessary to leave a little space between the concrete and the wall of the house, say 3 mm? Or maybe better to leave that space the other side, next to the garden wall? By the way the building in the background is the barn, over 100 years old as far as I know. And yes, the idea is to remove all the existing concrete, as I also want to lower the general level, to make a higher step up to the main door. [quote]There would be no point at all to putting back any concrete if you're going to finish with pavers. You would only need the conventional paver installation. `[/quote] But in this case, wouldn't any rain water that gets in between the paving stones possibly get in under the house, or is there a way of laying waterproof material under the sand? And also, no iron mesh reinforcement would be necessary either? Thanks.

This post was edited by elbow on Sun, Aug 10, 14 at 10:58

One thing I see on a fairly regular basis in this forum is someone showing up with a preconceived desire to proceed with a faulty scheme, or one that is only partially "baked." I can't tell yet if this is what is coming. Elbow, why do you want to lower the grade around the back door and create more of a step? (Usually, people desire less steps rather than more so it would be good to explain the reasoning behind the desire.) In lowering the grade, it will become important to know how the grade outside of the project area interfaces with that which is within. Of course, we can't see ANY of that and it has not been explained to us. This is not a question about water which gets into the joints between pieces of stone. It's about the much greater volume of water that doesn't get in and must run off.

In the last picture, what you are showing is what most of us would call some sort of "flagstone" ... relatively flat, broad pieces of irregular shape. My own thoughts about this is that thin flagstone does not really make good paving when installed on a sand base (regardless of whether there is concrete under it, or not.) If you walk on packed beach sand, you'll notice that your feet leave an imprint because the pressure of your foot is enough to cause a shift of unprotected sand. While flagstone protects the sand layer SOME, it is THIN and CAN shift in small degrees (usually in a rocking motion) when pressure is applied and depending on the state of moisture content in the sand base. It can shift due to the action of freezing and thawing. Over time, these small shifts can add up to a cattywampus and not very pretty surface ... not the kind of thing you'd want, really, for a rock solid 100-year old barn and nice house. Thin flagstone makes a much better -- durable even -- paving surface if it is mortared directly on top of a concrete slab ... without any sand embedment layer at all. But this kind of installation, while not impossible as a DIY project, requires some learning and skill so is usually left to someone already with experience. Using this size project to "learn" on might be asking a bit much of one's self.

It would be MUCH easier -- much, much easier -- to install a normal thickness (2 3/8" or greater) regular paving unit on an aggregate base: bricks, concrete pavers, uniformly cut stones, etc., if that's the type of base one wishes and if their experience is limited. One would still need to learn something, but it is much easier to develop skill for that than it is to develop skills at mortaring in flat work.

You could use steel mesh in a concrete slab, but it would be pointless in paving laid on sand.

I'm going to guess that the appeal of the flagstone is that you have access to it at a reasonable price ... but the other end of the equation is that its installation will be expensive (even if only in your labor the amount of time you must put in to learn, and the risk-factor of a potential botch.)

As far as water that penetrates into the soil or below the paving ... consider that the soil is basically an ALWAYS DAMP medium. That's why the grade outside of a building cannot be raised to too high an elevation. If the grade is too high, the only solution is reducing it. Also, the grade around a building must be graded such that water runs away from the building (it is usually said for 10', but it depends to some degree on surrounding conditions.) If the grade elevation around the building is acceptable, then there is no reason to worry about the soil having some dampness. It's the way it is. What one should worry about is moving the excess rain water the heck away from the house ASAP. In the case of this paving, if you get the surface to drain properly, there is nothing to be concerned about with a little water entering between paving stones. The idea that you can put down a slightly subsurface waterproofing membrane to take care of water penetrating the soil is more likely to cause problems than solve them. It's not needed. And you wouldn't want to create a layer of water that has no real route of escape.

Not a problem for me to sit these irregular flagstones onto a few blobs of fresh mortar and levelling them on top of the dried concrete base containing the gravel and wire-mesh. Obviously, I know the importance of a sloped surface, so that rain-water runs away from the house.
I want to lower the level of the ground outside the front door because the step is only around an inch higher, and other parts of the present concrete are higher than the doorstep level, and dirt tends to accumulate on top of the step,
I might be interested in installing regular sized pavers if I could find them around the area of Spain where the house is situated, in which case, i think I would go with the gravel base, plus an inch of sand. I assume that some rain getting down through the pavers wouldn't lead to water getting under the house wall?
But the present plan is to do the job in parts: first install gravel, wire-mesh and concrete base, with appropriate control joints, and when that is fairly dry, lay the flagstones in sections, aligned with the control joints in the concrete base. At the same time, it's interesting to consider other approaches, such as not using any concrete or wire-mesh, as with the regular pavers...the only doubts I had about that were whether some water may seep through and under the house, or that this water might possibly even accumulate and freeze in cold weather, causing upward movement of the pavers. The other option I'm considering would be to lay the gravel bed, wire-mesh and concrete, and when that's dry, just add the 1 inch of sand and regular sized pavers on top, in which case, I assume I wouldn't need to worry about aligning the pavers with the control joints in the concrete base? But, more than anything, I'm wondering which of the above options would be better in terms of water drainage etc. Thanks.

"I want to lower the level of the ground outside the front door because the step is only around an inch higher, and other parts of the present concrete are higher than the doorstep level, and dirt tends to accumulate on top of the step." Were this mine, I wouldn't be so interested in creating a higher step by lowering the grade, but instead directing the flow of water so that the entire patio surface was self cleaning. (Consider why some paved areas are completely clean after a storm and other areas seem always clogged with debris.)

"Not a problem for me to sit these irregular flagstones onto a few blobs of fresh mortar and leveling them on top of the dried concrete base." I don't suppose it would be. But when you set the stone on 400 square feet of blobs, will it end up looking like a professional job ... or something a later owner will want to get rid of ASAP. I've seen nice flagstone patios and really crappy ones. The nice ones have the stone edges cut to mate with the adjacent stone so that joints are uniform and narrow. The mediocre ones have wide, irregular gaps between the stones and poorly finished joints. One reason I'd lean away from the flagstone is because it will introduce a busy, alternate texture to compete with the nice looking stone of the building ... like wearing two different plaids at the same time.

"I assume that some rain getting down through the pavers wouldn't lead to water getting under the house wall?" Being in Spain, I would think you could see countless examples of paving stones set on sand over a gravel base, many of which will date back eons, and that abut buildings while not destroying them. Pavers as such are widespread throughout Europe, long before they ever appeared in the US. The base drains such that water does not accumulate under the pavers, freezing and uplifting them. If that were the case, they would be an unpopular product and no one would want them.

The picture you offered does not explain where water would go. The existing paving looks like it is not sloped to drain water away from the house, but along side it (of course, creating problems.) We can't tell where the water would go. If graded properly, I'm sure you would have no trouble with pavers. (Without ANY concrete!) Why don't you consult with someone locally -- a supplier or installer that knows something about pavers and see if they could not give you some advice about this?

So, what I've learned from the replies is that the better solution would be to use ordinary regular shaped pavers, sloping away from the house, on top of about 1 inch of sand and a layer, of say 4 inches of gravel (compressed) below that. No concrete, or wiremesh. I understand that using this system, it would be considered normal for some rain water to filtrate down through the pavers, but I'm still a little concerned at the prospect of some of this water seeping under the house, though am I right in thinking the quantity of water seepage through the pavers would be absolutely no problem for the house? Or that, in any case, some kind of sealant could also be added to the fine sand between the pavers? Thanks.

" ...am I right in thinking the quantity of water seepage through the pavers would be absolutely no problem for the house?" If your house was built correctly and the grade around it prepared properly, it's hard to imagine it could be any problem. I can't see your neighborhood but aren't buildings there surrounded by properly graded soil and doing fine? In the US, the foundation of every home is made to tolerate the normal dampness of soil, including periodic rains. The wood portion of each home is raised up enough that the dampness doesn't affect it. (Of course, there are shoddy homes built that don't comply and those can eventually have problems. But based on appearance, I'm thinking your house does not fit into that category.) Find a knowledgeable person locally who can verify that the grade is acceptable ... and then forget about the idea of installing concrete below pavers and construct the paving in the conventional, professional way. If you are directly mortaring thin, random-shaped stone to the surface of a concrete slab, it will be more costly, troublesome and probably not look as good.

I believe your choice of paver is your issue and it's driving your solution to the base. You're going to have an interesting problem when you remove that concrete out from under the stucco and I suspect, if there is wood adjacent to joint were stucco and concrete meet there will be rot. There is a lot of irregular stone already in that area and it would be nice to have a contrasting material to differentiate between all this stone other than the stucco.

my suggestion is to consider the stucco first. you might have to sawcut 12" (xxcm) away from the wall to save the stucco, hard to say.
Then, I'd evaluate if you have the option to sub-drain or surface drain only. If you put pavers in, and it becomes porous can the water get out and if so, is the "out" your basement :-) if it is, then you might consider 2 options
1. pervious material with underdrain similar to aforementioned suggestions
2. impervious material (mortared pavers, tiles, concrete, stamped concrete, stenciled concrete, stained concrete, etc.