I'm not sure how extensive is the damage caused by the skip wagon, but I'd guess that your best bet is to lift and relay whatever has been disturbed, if only for safety reasons, and then consider a full reconstruction next year, once you've had a chance to see how the rest of the driveway performs over the summer. It may be that, once normal traffic resumes, the drive copes adequately, and there would be no need to re-construct, or, if you're really unlucky, it might be that it gradually deteriorates as it's trafficked leaving you no real option other than to start again.

If you've any photos of the damage done, send them in to me and I'll take a look.

Laying the flags 6-10mm higher than the top of the kerb is not a problem - that's how they should be laid, although 3-6mm upstand is much safer than a 10mm "trip".

The paving is straightforward enough, and, from speaking with contractors throughout Britain and Ireland, the 'going-rate' for laying Indian sandstone is around 15 quid per square metre. So, your 84m² patio should cost something around 1250 quid to lay.

The walling is another matter, as the amount of labour involved depends on the type of stone being used - particularly whether its coursed or random, and how much dressing will be required. The price diff between an average 'builder' having a stab at building in stone, and the cost of a genuine stone mason is around 200%, but a stone mason will do a better job in a shorter period of time.

You have to go with your own intuition. Have you checked out previous work by these companies? Have you spoken to previous clients? Is there one that makes you feel more comfortable than the others, or is there one that gives you the jitters?

The proportion of cowboys in the construction industry is actually quite small - it's just unfortunate that one story about a cowboy is more interesting to the general public than 1,000 stories of honest-to-goodness builders who bend over backwards to fulfil the client's wishes. I would suggest that the best way to sort the wheat from the chaff in your case is to ask for a written method statement, in which each of the contractors tendering for the work sets out exactly how the paving will be laid and the wall built. This will give you a better insight into their working methods and, hopefully, will help narrow down the field.

It's not so much that the contractor cannot repair it, it's more a case of they can't be bothered to make it right!

The moral of the story is that, just because someone can plant flowers and lay turf, it does not mean they can lay paving. You're not the only one to make this error - I hear it time and time again..."Oh, they did such a lovely job of the garden , so I thought they'd be ideal to lay the driveway, but..."

If you employed a painter to re-gloss all the windows, doors and fascias of the house, would you use them to to plumb-in your new cooker, just because they did a good job on the bargeboards?

Each man/woman to his/her trade!  

The contractor is now proposing to grind the patio.  Is this the correct thing to do?

NO! It will make a right mess of the slate, rendering it useless and covering the garden in a dangerous, lung-clogging dust!

Can this solve the ponding problem?

Possibly, but it is more likely to cause more problems than it solves, as well as being very expensive, environmentally-unfriendly and somewhat hit-and-miss.

What will the slate patio look after the grinding?

The area that has been grinded will have a radically different surface texture to the rest of the patio, and that difference is permanent. The only way to hide the treated area, is to grind the entire patio, which is just being silly!

Should we insist him to relay the patio instead? Should the entire patio be relay or only the puddle areas?

Yes - re-lay the affected areas and see if that cures the problem. There's no need to re-lay areas that drain adequately at the moment, just the areas that pond and any other areas that need to be adjusted to ensure the surface water has a clear escape route to a disposal point. So, for example, if there was a 1 metre diameter puddle 3 metres from the edge of the patio, the puddle area plus the 2 metres between the puddle and the edge would need to be re-laid to ensure the water can escape in future.

This 'contractor' has already made a mess of the patio, and is now offering to turn it into a total disaster. They should realise and accept the limitations of their talents. They are out of their depth - they do not know how to lay paving properly, nor how to put it right when they've got it wrong. They would be best advised to bring in a specialist to correct the problem. In the long run, it's probably cheaper and quicker to rely on someone who actually knows what they're doing!

An 8:1 mix for bedding is fine. I normally recommend a 10:1 mix for patios, as I found that was always adequate.

I'm not sure what you mean by 'dry-mortared' but I'm assuming you buttered the joints with a mortar and then pointed them immediately, but because of the heat, they mortar was 'parched', ie, it had the water sucked out of it by the thirsty flags and natural evaporation before it had a chance to ensure full hydration of the cement content. This is why the resultant mortar is weak and crumbly, and the only remedy is to cut out all the weak mortar and replace it with fresh, prefeably on a cloudy or overcast day, when it's out of direct sunlight, and late in the afternoon.

The joint width is a problem. When laying these stone flags, or even the imitation stone flags that are popular, it's best to leave the joints unpointed until an area has been laid and the joint widths adjusted by levering with a bar or a spade to even out any differences. Ideally, no joints should be less than 6mm and none wider than 18mm, (12m ± 6mm), but there is always some variation.

If you have flags in direct contact with their neighbours, this is less than ideal, but, if it's only one or two, then it's not worth losing sleep over. However, if there is a lot of very tight joints, it may be better to re-lay a section to even out the joints. Without seeing it, I can't say which is best.

When it comes to filling those tight joints, you could mix a slurry mortar and pour that into the joints to find its own level and then top them up later, once the slurry mortar has settled - it always 'sinks' by 6mm or so as the water content drains out and is used up by the cement. For a slurry mix, use a 4:1 mix with a plasticiser and plenty of water to make it a soup-like consistency. Keep the slurry agitated by stirring and pour it into the joints from an old watering can or similar. Wipe up and sponge down any spillage immediately. Take your time - you will probably get some slight overspill and that may stain the stone slightly, but it can be washed down with an acid wash in 7-10 days and no-one will be any the wiser.  

An alternative, and in my opinion less successful method, is to use a dry mortar, ie a mix of very dry sand and cement with no added water. Brush that into the joints using a hand brush, and then pour water into the joint to wash down the dry mortar into every nook and cranny of the joint and initiate hydration. I find that this method always results in a lot of settlement in the jointing material as the water washes through the dry mix, and you end up having to repeat the process two or three times to get properly filled joints with no great gaping voids.

And then the gap in the rendering - using sharp sand will give you a coarser-textured mortar. Structurally, it's fine to use it to make up the 25mm fillet between the old render and the paved surface, and, if you're happy enough with the texture, then there's nowt wrong with it.
Building or plastering sand is normally used for rendering because it is finer and has a higher clay content, which means it makes a 'stickier' mortar and adheres more immediately to the substrate (it's all to do with grain size, the size of the voids between the grains, and capillary action), as well as giving a smoother finish when trowelled. But for a 25mm gap, at the base of a wall, that no-one will ever notice, using sharp sand will not be a problem.

The problem with the levels and your neighbour's DPC is more of a problem, though. It may be that some previous occupant laid the old patio and didn't understand the way that DPCs work, and so, in blissful ignorance, laid paving and constructed flower beds that breached the DPC.
Your idea about the render protecting the DPC is not right - a render would, normally, stop at or above the DPC, not bridge it, as seems to be the case here.

Does your neighbour have any problems with damp in their property? If they do, of if damp problems were to develop, and it could be shown that your patio was the prime cause, you could be left facing a large-ish bill for remedial work and re-decorating.

When you bought the property, did your surveyor not identify this problem of a bridged DPC?

It's essential that the mortar is pressed firmly into the joints and then tooled on the surface to create a full joint that bonds properly to the edges of the flags and forms an effective seal at the top. Simply brushing-in the mortar mix will, of course, fill the joint and help keep out litter and fag-ends, but it doesn't actually help reinforce the construction.

With regard to the mystery DPC, these older properties can often be very confusing as to whether they have a proper DPC at all, and, if they do, where it is and why. I'd suggest that, as long as you're no higher than what was there before, and there's no problem with damp walls for either yourselves or the neighbour, then forget about it and just enjoy your patio.  

what you call 'plates' are actually termed 'landings' - it's one of those things that you actually know, but that you can't call to mind on the only occasion in your life you'll ever need the information!

The loose flags can be re-bedded using a good quality, Class II mortar, with an added waterproofing and bonding agent - I'd be tempted to use SBR (Styrene Butadiene Resin), as it imparts excellent adhesive, water-proofing and strengthening properties to a standard mortar. It can be bought from most good BMs for around 20-30 quid for 5 litres, but it's money well spent for a job such as this.

Remove all the loose paving and any other stonework that needs to be re-seated. If in doubt, take it out - it's much easier to chisel out an 'iffy' piece of stone now, rather than wait and hope that it doesn't work itself loose next winter.

Clean off the old, degraded moratr and expose the main structure of the steps as far as possible. Don't leave any soft, degraded or suspect material in place.

Starting at the top of the steps, and working your way down, re-set the treads on a 25-45mm full bed of SBR-enhanced mortar. Spread the mortar with a trowel, ripple the bed to allow seating, then press the flags into the mortar and tap down to the required level using a small rubber hammer. Make sure they flags are well and truly held by the mortar, and point up any joints immediately. You need a fall of around 1:40, that's 50mm over the 1250mm depth of each tread, to ensure there's no standing water or chance of ice formation, so use a spirit level to guide your levels. There's no need for a cross fall if you have sufficient end fall.

Any vertical stonework that needs re-fixing can be stuck on using the same mortar, although you'll need a thinner 'bed'. Again, work from the highest step downwards, so that you're not having to stand on newly-finished work to proceed.

SBR hastens the initial cure of the mortar, and you'll have only 20 minutes or so of working time, so don't mix too much at once. Follow the guidance on the container for dosing quantities, and don't be tempted to overdo it, or add a bit extra just to be safe - it's a waste and will only reduce the working time without improving the end quality of the mortar.

A building sand is fine for making the mortar, and you can use an ordinary portland cement - the SBR acts as its own plasticiser, so no need to add any or to buy pre-plasticised cement. Keep your measures fairly accurate - X ml of SBR per litre of gauguing water, 4 empty paint tins of building sand to one empty paint tin of cement, that sort of thing, and then don't mix the mortar too wet or it starts dribbling everywhere. Keep it stiff but workable, and don't rush!

Good luck!

Q2 - for the joints between treads and risers, I'd use the same mortar, but, for the joint between the top landing and the side of the house, an elastomeric sealant would be used as long as there no direct tie-in of the step structure to the house structure, i.e., they are two, separate entities.

Q3 - If you use a SBR-improved mortar for the joints, there's no way a power-washer is going to loosen that mortar! We've successfully used SBR mortar to point setts and stone pavings in city centres that are subjected to very high-pressure jet-washing to remove chewing gum, and to the little motorised street cleaning vehicles that use vacuums to collect up any litter and detritus, and the mortar stays put!

Leave out the selant at this stage - I'm pretty confident that you'll never need it.  

Apart from that, your plan sounds feasible. Gradient for drainage is as stated on the site, particularly the Drainage for Pavements page - 1:60 is about right. Bear in mind that there has to be somewhere for the water to drain to.

Have a read of the Land Drainage pages of the site. They answer your questions.

Now that you've Aqua-sealed the surface, I would suggest that you lay a damp proof membrane over the concrete (Visqueen 1200 or similar), then lay the flags on a full bed of Class II mortar, with mortared joints, and adequate fall to a drainage point. That should be fine.  

There's no doubt that timber edgings are easier to install, and cheaper than concrete-bedded edgings, but they aren't as secure and tend to have a shorter lifespan.

The cost of a concrete bedded edging has two components. The first is the cost of the edging itself, which can be anything from a few pence per metre if you use old, relaimed bricks or setts, up to several pounds per metre, if you opt for a fancy manufactured unit. Secondly, there's the cost of the concrete needed for the bedding and the haunching - this varies slightly, depending on the dimensions of the edging being used. A 200mm wide brick paver used as an edging needs a bed around 300mm wide, but the haunching will only be 40-50mm high, whereas a 200x50mm concrete edging kerb needs a bed only 150mm wide but the haunching will be three times that needed for blocks, at 150mm wide.

So, I'd suggest you first decide whether an edging really is necessary, and then, if it is, decide on the type.

To calculate the volume and cost of the necessary concrete, take the width of the edging, add 100mm and then multiply by 100mm. That gives a rough quantity of concrete, per metre, needed for bedding. Next, take the height of the edging, subtract 25mm and multiply that by 100mm - that's gives the volume of haunching required. Add the two together, multiply by the total length of edging required (30m in your case), multiply by 1.15 (for wastage and spread) and that gives the total volume of concrete needed to do the job. To find the cost, multiply the volume, in cubic metres, by 75 quid, which is an average sort of price for site-mixed concrete.

How's that?

Having said all that, if you want to lay an edging to your flags, then go for it. There's nowt wrong with doing it that way, but it's not essential, which is what you seemed to think originally.

If you're just about to plant these fruit trees, I'd suggest you use a root barrier membrane to line the planting pit and keep roots away from the paving. You only need place the root barrier between the tree(s) and the paving, not all the way around, and it need only extend down around 600mm or so, as once roots are deeper than that, their effect on the surface of the paving is hardly noticeable.

You might like to take a look at Peter Scott's site, as they specialise in just that sort of thing.

The acid has reacted with the iron oxide based dyes and adversely affected the colouring, although I've never seen a bleaching effect before - the usual reaction with the yellow/buff dye is to darken it to a rusty brown sort of colour.

This is why I always urge extreme caution when using brick cleaning acids, and, to be fair, many of the 'trade' brands do carry a warning that they can affect coloured concrete, but some of the budget 'patio cleaners' can contain up to 2.5% Hydrochloric acid, which is enough to alter the dyes used in concrete paving, but they make no mention of this on the packaging.

Is there a lot of flags affected, or would it be feasible to yank them out and slot in replacements?

It can be temporarily removed by scrubbing with a brush, but it will return until the supply of salts responsible for the bloom have been spent, an event that occurs as part of the weathering process.

1 - No - see the adjacent thread about choice of edgings. It covers this very topic.

2 - yes! because the Indian sandstone varies in thickness so much, it's more or less impossible to lay on a pre-screeded bed, and so you have to use individual bedding, and a cement-bound bedding material (the 10:1 mix I keep banging on about) is the best option.
Just mix enough of the bedding to keep you going for an hour or so, and then mix more as and when you need it.

3 - If you have a sub-base AND a cement-bound bedding material, the weed membrane is a complete and utter waste of time. Treat the existing ground (asphalt???) with a good weedkiller before starting work. I prefer Sodium Chlorate, but I have a feeling it's not generally available to the public, so summat like Round-Up or PathClear will have to do. Once the ground has been doused with that, and overlain with cement-bound bedding and the flags themselves, the weeds will not come through - but see the newly uploaded Membrane FAQ for more info.

Good luck!

The membrane - my mistake, I didn't read your post properly. Yes, you could use a membrane beneath the sub-base for the gravelled/pebble-covered area, as that will be prone to weed colonisation. If you level out the sub-grade, compact it and make sure there no sharp stones or bits of glass to puncture the membrane, that will be adequate preparation. These weed membranes are not intended to be 100% weed-proof, but to reduce the opportunities for weeds to get a foothold, so any minor tears or areas not completely covered are not critical.

I don't have an online calculator for jointing mortar; perhaps I should write one, as I have them on my system here. Your calculation is basically correct except for one very important factor - wastage. You need to add 20-60% for wastage, and the reason why there's such a relatively large amount of wastage depends on just how the flags are being jointed.

If you're buttering and pointing, a significant quantity of the buttering mortar is trapped beneath the flags or is squeezed up and lost. With square-edges flags, this might be as little as 20%, but with riven stone flags or those wet-cast flags with imitation fettled edges, the amount lost to wastage can reach 60% or even more.

If you're pointing only, then allowing 20% for wastage is much nearer the mark.

Anyway, to complete your calculation....

170m x12mm x 40mm x 1.4 (wastage) = 0.11m³

0.11m³ @ 2.4T/m³ = 275kg @ 4:1 = 55Kg cement plus 220kg sand

Plasticiser for such a relatively small amount of mortar is negligible. If you have lots of other mortar work, it might be worth buying a 5 litre pack, but, if this is your only requirement, then the old trick of using a squirt of wash-up liquid to the gauging water is probably a better idea.

As you've experienced, if you use a wet mortar it slops everywhere and stains, but if you use a dry 'brush-in' mix, it doesn't set as well as it should to give a good joint. There's an old trick of brushing in dry and then wetting the lot via a fine rose affixed to a watering can, but this still splashes some of the mortar onto the surface of the paving. Further, a dry mix doesn't 'colour' properly. The cement dyes used to make coloured mortars rely on water to disperse the powder and activate their colouring power, so you don't get a good depth of colour with a dry mix.

One method we've used successfully in such situations is to use a semi-dry mix, adding just enough water to bind the mortar and bring out the colour. It's damned hard work mixing a mortar in this way - you have to keep the water content to an absolute minimum, which means it's very, very stiff, but you also have to mix thoroughly to prevent any 'streaking' of the dye within the mortar. Once mixed, the semi-dry mortar can be fed into the joints from the blade of a brick trowel and pressed in with a pointing bar. If the mix is just damp, it tends not to stain the surface of the paving if accidentally spilled, but it gives a good, hard joint of full colour. Any accidental spillage should be sponged down asap. This method is shown on the Pointing: Case Study page.

Other methods you might use include...

- masking: you can use masking tape to protect the surface immediately adjacent to the joint

- retarder chemicals: Grace Pieri have an interesting substance by the oh-so-catchy name of NMP01 that they claim can be painted onto the surface of paving to protect it from mortar stains during pointing. I'm not sure how readily available it is for DIYers or what it's cost might be, but it's certainly made me sit up and pay attention.

- wet grout: this method is illustrated on the website, and it works quite well with 'awkward' pavings such as you have, as long as you are absolutely, totally, 100% methodical in washing off all of the mortar slurry. Could be a good method to combine with the retarder idea mentioned above.

- polymeric compounds: there's a good few of these on the market, and they are becoming more readily available in a range of colours, although I've still to see a genuinely dark-grey/charcoal/black. All of the so-called dark colours I've seen have actually been medium grey at best. Some are dry-brushed in and then cure by exposure to the atmosphere, while others are mixed to a wet consistency and poured into the joints from a watering-can or can be pumped in via specialist mortar guns.

I can't tell you which would be best for your project. You need to think about the skill levels available, the size of the job, the budget, how much time you can afford to spend, the end use....and then decide which is best suited to the job in hand. If it were me, the very stiff mortar would be my choice, but then, I've been pointing flags since I was three years old and have almost got the hang of it now.

I'm not sure which would be the best way to work with your sett rectangles. Ideally, you'd use full setts and full flags and avoid all the cutting, but it's not always possible. In your situation, I'd do a dry test panel first to see which looked best.

Tobermore only serve Norn Iron, don't they? I've never seen their products on this side of the water. I think they use a shot-blast texturing process which is essentially the same as Marshalls (and lots of other manufacturers) but the big diff is the starting material. The aggs and dyes that Tobermore use are different from those used by Marshalls, which is why the end product looks so different.

I'm surprised to hear you've not much praise for the Tobermore customer services. I was talking with a contractor from Derry last time I was over, and he said they were a great firm to deal with. Maybe they treat private customers less effectively than their trade regulars.

Top it up as required with extra sub-base material, re-compact and all will be well (touch chipboard!)

2 - the bedding layer should be at least 25mm thick. Now, if you're using sandstone flags, you'll probably find that you have a bed of variable thickness to accommodate the difference in thickness of the sandstone flags, but, in all cases, 25mm should be the minimum to aim for.

3 - Depends on the height of the walls and the condistion of the flags, as noted in (1) but I always prefer a separate foundation for walls, even dwarf walls on gardens. I know some contractors and certain publications advocate building dwarf walls on top of paving, but that assumes that the paving is sound and will not move at all during the life of the wall - how does that tie-in with your plans?

the recessed covers do allow some water to penetrate between the frame and the cover, unless you splash out extra bobs to buy one of the double-sealed units that are normally used on internal applications. Therefore, if you use a standard tray and cover, you'll need to make some allowance for drainage of your fire pit.

I've no experience with fire pits. I know they are popular in America, because I keep getting emails from there asking about them, but I've never educated myself about their possibilities within the patios of these soggy islands as I assumed we had far more sense. I know that intense heat and concrete/porous stone are not a good mix, so I think some form of refractory or heat-resistant lining would need to be used, but, as I said, I've never really considered them to be a worthwhile feature, so haven't any real advice to pass on.

I look forward to the pictures from your new toy!

2 - Laying method for a patio? Depends on the materials being used, but for flags/slabs, I'd nearly always go with a rigid construction because it offers better stability for larger paving units that are bed-dependent for their load-bearing capability.

3 - If the old patio comes out, then you can make up the levels using a cement-bound material (your 6:1 ballast mix) or you could just use granular sub-base material. No need to remove existing sub-base material, if it's in good condition.

I'm not sure how the 'hardcore' (as you call it) can 'spill out' unless this is an elevated pavement, in which case you need some form of retainer at the edges. Under normal circumstances, the sub-base is buried, so there's nowhere for the sub-base material to spill.

Once it's out, the sub-grade can be re-graded in a few minutes, by raking over it with the back of a spade or a rake, and then re-compacting the lot with the vibrating plate.

The low spots are probably of long standing and occurred when there was some minor settlement of the sub-base or inaccurate laying of the crazy paving.

Hardcore is a term I prefer to avoid because it means different things to different folk. Quite a few one-handed surfers end up on this site, looking for hardcore, and are somewhat deflated when offered pictures of crushed rock or broken bricks. Strange men!  

Hardcore can be anything from half-bricks, to slate-waste, to gravel, or, as you believe, a lean-mix ballast-based concrete. On this site, I try to use accurate terminology that refers to specific materials, so that there is less chance of confusion amongst the site-users. Obviously, I can't (and don't) expect everyone else to be as specific as I try to be, but if I slip into generic or loose terms, then the accuracy of my information is reduced.

For your bedding, I would suggest that you use my standard recipe for patio paving, which is a 10:1 mix of grit sand and cement, with no added water, as is described in some detail on the Laying Flags page, which I heartily recommend to you as essential reading.  

Hope that's cleared it up for you.  

Normally, any slab longer than 6m would have a movement joint of some description, even if it were only to act as a crack control joint. If you have concerns abouit the existing slab, then you could saw a control joint into the surface before laying the paving. This would act as the slab's weak point and then, if it was going to crack, it would crack along that line. However, even if you did this, I wouldn't worry about incorporating a complementary joint in the patio paving - if the slab cracks, the crack will be reflected through the bedding but it would then find a route along the jointing between the flags, which can easily be re-pointed.

I think you're worrying unduly about things that might never happen, and, even if they did, are very easily rectified.  

I think we are going to see a lot of silicosis-like problems with construction workers in the next few years. We had many years from when power saws first became popular when wearing a dust mask was seen as being 'girly' or 'soft'. Even now, I can go onto a site and see pavings or roof tiles or kerbs being cut by an operative with no dust mask, because they seem to think a few gobfulls of dust won't do them any harm, and the mask is buried under a pile of wet-weather gear at the back of the van, and by the time they've trudged across the site, they could have had the cut done....but they don't seem to realise that it's not just a few gobfulls: it's a few gobfulls on a regular, daily basis, and it's nasty, abrasive dust that will irritate their lungs and trachea.

In these litigious times, when employers are being sued for all sorts of minor transgressions, such as shouting at numpties, making the tea too hot or not providing moisturiser, I find it surprising that many employers and site supervisors do not insist on full protective gear when operatives are using a cut-off saw. Even for occasional use DIYers, I would compel them to wear a mask and goggles when using a saw - it's just not worth the risk!

I'd normally bed them onto a 100mm layer of concrete, but laying them on mortar is fine, as long as the sub-base is sound.

Paint Stripper is the way to go, combined with a power washer, perhaps, or a wire brush. Depending on the type of paint and area of spillage, you might be better off ripping out the stained flags and replacing them with new.

Once that's done, you can use 20-25mm of the 10:1 mix for bedding.

2 - Oh the joys of terminology in the building trade!! To all intents and purposes, grit sand is concreting sand is sharp sand. There are minor differences between them, but, for the type of project you're undertaking, those differences are totally irrelevant and you can use grit sand OR concreting sand OR sharp sand, or even a mix of all three. What you DON'T want to use is building sand, which may be known as soft sand or plastering sand. Confusing, innit?

3 - You could use Geo-Fix but I still have reservations about its use on the wide joints you'll end up with using the Bowland flags. I still firmly believe that you can't beat a mortar joint with that type of flag. Butter the receiving edges with a Class II mortar as you lay, point up the joint asap to keep out other crap, and then, after an hour or so, strike the joints to finish them. Leave them for about 4 hours and then you can brush off.  

There's a new page dealing with pointing 'in production'. I took the photies a couple of weeks back, and drafted the text over the last few evenings, so it's just a matter of coding it all into html now, and slotting it in to the rest of the site.

Bowland are not the only manufacturers to have these weird moulds that make the flags have splayed-outwards edges, contrary to natural stone, which always has the edges splayed inwards. However, you should endeavour to keep the flags far enough apart to ensure they are not quite touching, but that the top of the joint is not excessively wide.

Using a 10:1 semi-dry mix means you can walk on the patio a mere 24 hours later - in fact, if you're careful, and the flags are securely bedded with no movement or rocking, you can walk on it immediately. At this time of year, the bedding mix will be reasonably 'hard' after about 6 hours, but, if you're pointing as you proceed, best to leave them for 24 hours, just to be sure.

The new page on pointing stone flags has now been uploaded and you might find it to be a useful read.

The dry-grout method never gives a really solid joint unless it is brushed into freshly-buttered joints and tooled to a firm finish, but I wouldn't worry too much about it just yet. The "mortar" is not going to get noticeably harder (it does, actually, but you'd need a laboratory full of equipment to prove it) but it may well be hard enough for your purposes. After all, it's only a way of filling the gaps between the flags, so, if it's hard enough to stay put and resist colonisation by weeds, it'll do, but if you find it starts flicking out, then a re-point may be necessary.