Creating Stone Paving
There are two acceptable methods of laying flags - individual bedded and screed bedded. There is a third method, spot-bedding, but this is not considered suitable for a professional paving project. The choice of laying methods depends on the personal preference of the contractor/specifier, the type of bedding that is chosen, and the types of flags being used.
Individual bedding
This method is more common used with larger flags, where handling and manoeuvreability is a major concern, with natural stone flags, where there can be significant difference in flag thickness between adjacent units, and for many of the mould cast riven or decorative flags, which tend to have a variable thickness ranging ±15mm on individual units - this being a result of the mould not being perfectly level when the wet-cast concrete is allowed to set.
Whilst obviously more efficient that the individual bedding method, this method is only suitable for the smaller flags, generally those less than 600x600mm, which are small and light enough to be manhandled onto a screeded bed. It is essential that the flags be of a constant thickness, hence only the small element paving and wet-press manufactured patio flags are suitable candidates.
Spot Bedding
Note: The '5 dollops of mortar' method, known as spot bedding is not recommended. On public and commercial works, it is rarely, if ever, allowed. If your contractor is using this method, ask would they like a bucket of oats for their horses.
Regardless of what you may have seen on TV DIY and Gardening programs, spot bedding is not an acceptable bedding method because...
1) It leaves voids beneath the flags, making them more likely to fracture when loaded.
2) The voids can allow surface water to accumulate, leading to subsidence or instability.
3) The voids provide a ready-made home for invertebrates, particularly ants, which love to mine an unbound bedding and/or sub-grade.
4) The solidified mortar spots settle differently causing the flag to 'rock' when trafficked.
5) Correcting any rocking flags requires the old mortar to be broken out and disposed.
6) It is more expensive than solid bedding.
7) It does not comply with the relevant Code of Practice (BS7533:Part4) which requires bedding to provide "uniform support".
When asked to inspect a flagged pavement which is troubled by a large percentage of rocking and/or broken/spalled flags, in over 70% of cases, we have found that spot-bedding is the prime cause of the problems. The two most common faults we encounter are lost bedding, because the voids left between the mortar spots provide a channel into which surface/ground water can wash-out the bedding or subgrade material, and excessive breakages because the flags are not evenly supported across their entire base.
Whilst we can accept that these problems are less likely to occur on a small residential patio, we cannot and will not endorse spot-bedding as a suitable method for laying flags, regardless of the location. Some publications may depict spot-bedding as an acceptable methodology for diy projects, because it makes flag-laying appear simple, but, for a professional contractor, it should never, ever be considered.
There are only 2 layers to flagged light-use paths and patios, while drives, forecourts and public footpaths should use the thicker type of flags (50 or 63mm) or may use a concrete sub-base and lay the flagstones directly onto the concrete before it sets.
Preparation
Decide where the paving is to run, and mark out with sand guide-lines, if required. It is assumed that the path is to be flush i.e. level, with the existing ground. It is a good idea to dig wider than the planned path width, allowing an extra 50-75mm at the edges makes handling much easier.
Sub-grade
The surface needs to be dug off to a depth of approx. 90mm, if no sub-base is required. The depth of dig can be calculated as...
(Average depth of flagstones + 35-50mm bed + any sub-base required)
Remove all weeds and other unwanted organic matter. Excavate any soft spots and fill with compacted sub-base material or a 10:1 grit sand/cement mix. If the area of the path is troubled with weeds, you may treat the excavated sub-grade with a general weedkiller such as Sodium Chlorate.Weed or root barriers are not essential unless there is known problem with roots or invasive, deep-rooted weeds, but it is unlikely that any weed will be able to penetrate the upper layers.
If an edging is required, it can be constructed at this stage or left until the rest of the paving is in place. Brick edgings, plain or decorative edging kerbs, kerbs or cobbles laid lengthways are all suitable.
Sub-base
This will not be required for a path or patio. However, any low spots can be filled with crushed hardcore, crushed stone or compacted bedding material - see below. If the area to be paved is such that it requires raising or elevating to a higher level, then the elevation should be constructed by adding to the sub-base layer, not by increasing the thickness of the bedding layer.
For driveway or other heavy use use, the sub-base layer is the load bearing layer, and should be at least 100mm thick, minimum. For public footpaths, this may sometimes be reduced to 75mm, or increased to 150mm or more to give added protection to underground services. Alternatively, the flags can be laid directly onto a mass concrete sub-base, but be prepared to work fast to cover the ground before the concrete sets! The finished surface level of any sub-base, should be the (flag+bed depth) ±10mm below finished paving level.
Bedding layer
The flagstones can be bedded directly onto a coarse, grit sand bed, approx 40mm thick. DO NOT USE BUILDING SAND - it is to soft and can become 'fluid' when waterlogged, consequently moving beneath the flags, causing settlement.
The bedding material can be stiffened by the addition of a small quantity of cement, around a 10:1 mix is adequate. This is useful when working in wet areas, or where the movement of ground water could lead to bed migration, and we find it useful beneath the thinner patio flags as it gives the finished pavement a bit more solidity.
The bedding material should be spread to the area and compacted either by trampling or with a couple of passes with a plate compactor prior to bed preparation, which will be one of the two methods previously mentioned.
The flags should be laid to a taut string line, to aid coursing and levelling accuracy. Basically, a length of string is stretched from one known point of the paving to a projected point, and used to guide longitudinal line/level. A straight-edged timber or long spirit level (1000mm or more) is used to check transverse accuracy.The distance between consecutive line pins should not exceed 10 metres.
Individual bedding
Use a spade or large trowel to level out the bedding material, which should have the top 15-25mm loosened to allow the flag to be bedded down - this is usually acheived by 'rippling' the bedding surface, creating small troughs and peaks with the blade of a trowel. The bed should be 5-8mm high to allow for consolidation. Use the taut string line as a guide - use a piece of broken flag or a piece of timber cut to the correct thickness to judge the level of the bed adjacent to the line.
Make the prepared bed spread 100mm or so further than the flag will cover - it is better to have the bed to big than too small, leaving an edge of the flag unsupported. The true art of flag laying is to get the bed right on the first attempt, so that the flag can be laid and consolidated without having to be repeatedly lifted while bed adjustments are made. If you have 100 flags to lay, and it's taking 3 attempts to get each bed right, you've laid 300 flags in actuality, but have only 100 to show for your efforts.
Screed bedding
The bedding material is spread out over the area to be paved, compacted lightly with a plate compactor (around 2 or 3 passes) and then screeded to level 5-8mm high
Flags at a free edge
Where flags are laid to a free edge, that is, an edge where there is nothing firm or solid against that edge, as is the case with a garden path and some patios, the edge flags can be bedded using a mortar in order that they are held firmly in place. As the mortar (or a suitable concrete) cures and harden, it holds the flag fast and prevents lateral movement.
If using a mortar joint, the 'receiving edges' of the previous flags should be 'buttered' with a generous amount of mortar, taking care not to get any mortar on the surface of the flag. The next flag to be laid is then presented to the receiving edges, squeezing against the mortar that has been buttered thereon.
With dry jointing, the flag is positioned and pushed tight against the preceding units. For dry-sand joints, the units should be as tight against their neighbours as possible, and no more than 3mm - we call this 'butt jointing'. With the dry mortar joint method, where the flags are laid without mortar and a dry sand/cement mixture is brushed into the joints on completion of the actual laying, it is advisable to use small pieces of plywood between the flags to maintain regular 8-10mm wide joints.
The smaller paving units, such as those shown opposite, are often light enough to be lifted directly into place, but the larger units, 450x450mm and above, need to be lowered carefully into position.
There are two ways of laying the bigger, heavier flags - from the paved surface and from the leading edge. Note that when using the screed bedded method, work must be done from the paved surface to avoid disturbing the prepared screed bad.
Laying from the paved surface.
Walk the flag into a position where the bottom edge is hanging over the lip edge of the preceding unit, roughly halfway, as shown in the diagram opposite. This ensures that when the flag is lowered into position on its bed, it remains relatively 'tight' to the preceding flag.
Make sure you have the flag in the correct orientation, so that it will not be upside down when laid. Holding the top edge of the flag with both hands, squat down behind the flag and lower it carefully into position, allowing it to pull you up out of the squatting position. This technique places less strain on the back muscles than lowering the flag from an upright or stooped stance.
Always lay flags the 'narrow' way - ie, if laying a 900x600mm flag, maked sure the 900mm long edge is on the ground and that you are only stretching 600mm to lay the flag.One technique we use to avoid excessive strain on the back, is to use the pavior's maul to take the bending motion out of lowering the flag. The head of the maul is used to hold the 'top' edge of the flag, which is then lowered by the tradesman who controls the flag's descent via the maul.
Laying from leading edge
Walk the flag to be laid into position so that the bottom edge of the flag is aligned to deliver the flag into the correct position. Keeping one foot against this bottom edge to prevent the flag from slipping backwards, grip the flag on the open side and carefully lower it into position on the bed. Make sure you do not get your fingers trapped between the flag you are laying and the preceding unit.
It is not possible to use the maul assisted lowering technique shown above when laying from the leading edge, as there is no space for the head of the maul between the top of the flag being laid and the leading edge of the existing paving. This means extra strain on your back, so extra care must be taken.
Once on the flat, the flag is nudged into alignment with the maul, and is then tapped down to the correct level with the maul until it is 'flush' with the surface of the preceding flags and/or the string line guide. Use light taps on the areas indicated on the diagram. If you tap too near a corner, the flag might break, if you tap too hard over a hollow in the bed, the flag might break, so remember, light taps and plenty of them. Concentrate your 'taps' on the four main areas indicated.
Some of the small element paving is suitable for consolidation by running the plate compactor over the surface, rather than individually consolidating each flag. Do not attempt this method with patio flags - they are not strong enough and were never intended to be plate compacted - they are almost guaranteed to break and, on some of the decoratives, the surface detail will be spoiled.
Straddle the flag diagonally and shift your body weight about to check that the flag isn't rocking. If the flag is rocking when you straddle corners A and C, then areas B and/or D must be high - tap them down until the rocking stops. Conversely, if the rocking is along the B-D axis, areas A and/or C must be high. Use a spirit level to check that the flag is falling in the required direction for drainage.
Oh Flip! It's too high!
The flag can only be tapped down a limited amount. Beyond a certain point, no futher compaction is possible and if you persist in hammering it with the maul, the flag will break. There is only one remedy, the flag will have to be lifted and re-bedded, but before you do lift the flag, make a mental note of where the flag is high and by approximately how much, then, when you've lifted the flag and stacked it safely while you adjust the bed, you can use a spade or a trowel to reduce the bed in the necessary areas. Once again, looosen the top 15-20mm of the bed and try the flag again. Hopefully, it's right this time.
Oh Sugar! It's too low!
If the flag is too low once you've consolidated it with the maul, it will have to be lifted and the bed added to before relaying. Simply lifting one edge of the flag and packing bedding material in as far as your fingers will reach is not a good idea - it can leave a void in the centre of the flag, meaning the flag is only supported on the packed edge and the opposite edge. Again, make a mental note of where it is low and by how much, lift and stack the flag safely out of the way, and then add bedding material to the entire bed, re-levelling with a trowel or spade before re-laying the flag.
This is not strictly necessary, as the flags can be laid 'butt-jointed', meaning the edges are directly in contact with the adjacent flags. Dry joints should not exceed 3mm in width and should have the joints filled with dry silica sand, as used in block paving applications. Dry jointing works well with true-edged flags, i.e. those that have perfectly straight sides. For those flags with rough edges, especially the riven copies with 'authentic' or 'fettled' edges, mortar pointing is preferred. As a general rule, if two adjacent edges are more than 5mm apart, a mortar should be used to point them.