"Sand has smaller particles..." Well, can't say where Ink buys his stone dust, but here in Massachusetts, stone dust is by far the smaller particle. Perhaps Canadian dust is larger than sand, or sand up there is extremely small...:-) Around here, the dust is the smallest. Sand in these parts varies in size based on where it's mined and how it's processed (if at all), but always has a larger grain size than the dust. " As you don't want water draining THROUGH your patio but over it the drainage capability of sand/stone dust is a moot point." Not necessarily so. A well designed and constructed hard surface of pavers, or stone, depends on slope to evacuate rainwater, and the material within the joints if slopes are minimal. And most patios will have minimal slopes. It should be pointed out that the reason many patios fail is not due to poor surface drainage, it's due to poor subsurface and base preparation that doesn't allow for water in the soils to drain away from the area. Water draining through the joints should be able to do just that.., drain through a well prepared base and beyond. Stone dust is a commonly used material in both commercial and residential applications as a base material. Depending on the loading requirements of the surface, it can be used directly over a well compacted gravel base, or over a well compacted crusher run (processed stone) base. And a very important key to good base preparation is good drainage. If stone dust were impermeable, it wouldn't be used around here to the extent that it is. Just thought you'd like to know...

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.

That would be very difficult and costly to try and seal all those joints and when you get into joints that wide it makes matters worse. I wish I had some good advice to give you, but I am stumped. The water is no doubt flowing down that slope and infiltrating the expansion joints, washing out the soil under the boards. I'm not sure why it was constructed like that, I would think a continuous slab with contraction joints would of been the way to go. I'm pretty sure this is not the direction you are wanting to go, but I'll throw it out there just the same. I have seen people plant grass or some sort of vegetation between the joints, but this is a very distinct look and involves maintenace. Here is a link of something similar. http://www.troublefreepools.com/viewtopic.php?t=826&postdays=0amp;postorder=asc&start=60 I just thought of something else. You could pull out the board, level the soil in the bottom of the joint as best you can, then fill half the depth of the joint with a concrete or mortar mix (making sure to put a piece of fiber board in the side to allow for some expansion), then rip the expansion board to the remaining depth and replace it. This should offer some stabilization to the soil in the joint and keep it from washing out. You could probably fill the entire joint this way, but I think it might look better to keep the expansion boards visible. See the sketch below.

Your drawing more or less shows drainage running parallel to the house so this would be a flaw in grading that needs to be rectified. The drainage should run away from the house for a few feet, at least, and then run off in a direction that coordinates with the overall lay of land. The grading could direct water straight away from the house or somewhat at a diagonal. It could be a slightly warped surface that begins shedding water straightaway and gradually transitions to a diagonal, skewed toward the direction of overall flow. (A subtly warped surface would appear flat and the warping would not be perceptible to the eye. A possibility might look along these lines... I'm wondering why you would leave the gravel bed between the deck and patio. It seems like it would be better to widen the steps. They/it could be the entire width of the deck, if desired. The wider they are, the more of a direct connection from one space to the other and the better the visual and traffic flow. The step(s) also add the possibility of temporary, impromptu, extra seating facing the patio area. If the gravel space remains it seems to divide the two spaces for no good reason. It would be better to bring the patio all the way to the deck, even if the steps are less than the full deck width. If for some reason you are creating two separate "rooms" to be divided with a screen/wall so there is less interaction between the spaces, then you might leave the gravel bed/potential planting space. Note, what happens with grading is dependent on what the grading is doing in the rest of the yard. We have no information about that so I'm speculating in the above drawing.