Author Topic: Laying A Patio Using Concrete Products  (Read 2536 times)

Alan Sargent

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Laying A Patio Using Concrete Products
« on: February 26, 2016, 09:46:23 pm »

(‘Trade Tricks’ denote Hints and Tips that you may find extra helpful during your works programme. They are designed to save time, effort and money, helping you to gain maximum benefit from your project.)

Planning is the most important part of any garden project. Planning is the key to the success of Hard Landscape schemes in particular, and the benefits of careful forethought will make things run more smoothly from the outset.

The face work or finished slabs are only one element of a project, and each stage, from excavating the foundations to completion should be carefully laid out as areas on a plan, and to a schedule of costs. You need to decide how much you want to spend on the job – some costs will be the same no matter what type of paving slab you choose. When choosing your paving, be careful to consider your choice of colour. What may appeal to you when it is dry, may not be acceptable when seen wet (and vice versa)

Excavating to levels and falls for example, will require careful calculation. The finished level of any paved area against a building must be 150mm below the damp-proof course, thus the volume to be excavated will directly relate to the height of the existing ground, together with the thickness of the slab plus foundations. When calculating the amount of material to be removed from the area to be paved, you should allow for the ‘bulking factor’ – the amount of increase in volume between the original compacted ground and the loose material once excavated. This volume factor may lead to additional costs in skip hire and should be included in your calculations.

Plan ahead and assess how you propose to carry out the work. Do you have sufficient dry storage space for those materials that need to remain dry e.g. cement? Can you receive crane off load vehicles to site, or is your driveway too narrow? Are there any over-head power cables that will affect delivery vehicles (including skip lorries)?

Draw up a scaled plan of your proposed patio or pathway. It is easier to use the metric scale, as the products you select will be described using centimetres. Once drawn, it is a good idea to peg out the proposed area, with wooden pegs and canes laid out, leaving the layout in position until you are happy

Trade Tricks  -  Having laid out your proposed patio, live with the idea for a few days, noting how the project appears in scale to the rest of the garden; check that the patio is large enough to accommodate the required number of tables and chairs; does the patio catch the morning/evening sun and is it perhaps in a wind tunnel. Make a careful note of all of these factors before finalising your plans.

When drawing up your plans, you will need to find a visually dominant line; a line that will ensure that your project appears to have been designed as part of the house and garden, and not simply ‘added on’ without thought of overall balance. The dominant line is normally the longest and therefore most important visual line in sight, often the main wall of the house or building.

This dominant line will become and remain your base line. All other measurements will be taken from this line. It is perhaps worth noting that not all buildings are in fact ‘straight’, especially those that have had extensions and alterations built over the years, and this is the primary reason you need to rely on your guide line – a builders string line firmly secured at each end and tightly stretched between two pegs.

The dominant or primary line should be set at finished level of works i.e. 150mm (minimum) below Damp Proof Course (DPM). This line may also be used as The Datum Level from which all levels and falls are taken.

The Datum Level is that point chosen to be the finished level of all works, and is usually taken from a fixed or constant point – often a door thresh hold – with all other levels being fixed in relation to that point, either as matching heights i.e. levels or as cross-falls, ensuring that rain water does not lay against the house.

Tools & Equipment

For all hard landscaping projects you require a basic set of tools. These include a laying trowel (usually 20cm long) a spirit level (not less than 90cm for paving works), a pointing trowel (125 or 150mm), a rubber or nylon mallet, a shovel and spade. If the project is fairly large, consider hiring a concrete mixer (electric machines are quiet, although on most sites, you will need to hire a transformer to reduce the power from 240v to 120v) and perhaps a stone cutting angle grinder. Always ensure that you wear protective gloves and glasses when using these tools, and never ignore safety instructions. You will also need at least one Builders wheelbarrow for use on every aspect of the scheme.

Most Builders Merchants supply a wide range of paving materials, in various sizes and weights. Care must be taken when carrying or moving these products, and steel toe-capped boots are essential, together with a pair of tough gloves. Wherever possible, slabs should not be stacked laid flat, as the weight of the upper slabs may cause damage to those on the bottom of the pile. If possible, store the slabs in a fairly upright manner against a wall or other substantial strong surface. If you suspect that a slab may have been damaged (hair line crack) it is better not to use that unit as the fault will remain and appear as a damp line for hours after rain fall has dried the rest of the paved area.

Preparing The Ground

The success of the project will be greatly improved if careful thought is given to the preparation of the site. The old adage that Preparation Is Everything is true.

In order to achieve the required levels and cross-falls, the main Datum peg should be identified with the top painted in a bright colour. A series of other pegs, set into the ground and driven in to the same level as the Datum peg, using a length of straight timber and a spirit level to ensure that the whole of the project area is delineated with a number of pegs, each painted with the same bright colour to indicate that they are Datum pegs.

Trade Tricks – Having set out the level/datum pegs, place a second peg against each one and drive it into the ground either higher or lower than the datum pegs, dependent on which way you want the rain water to discharge. These secondary pegs become the finished level pegs, the height above excavated ground of which will depend on the thickness of the chosen paving product.  You should aim for a cross-fall of a minimum of 1 – 100 (If you set pegs two metres apart, and raise/lower the secondary peg by 20mm, this will give you a 1 – 100 fall). This is the minimum fall for smooth slabs and should be increased if an irregular faced product is selected.

If the ground is reasonably solid, the best method of providing sufficient foundations for a domestic hard wearing patio is to lay a foundation mix of one part  Ordinary Portland cement to six parts of sandy ballast, to a depth of 75mm and consolidated using a straight length of timber, with no low or high spots, all laid to levels and falls. Leave this concrete to harden before continuing with the project – usually 48 hours is sufficient.

The slabs are then bedded onto the concrete foundations using a mortar mix of one part Ordinary Portland cement with six part of soft builders sand, to a bed depth of 25mm laid as a full bed – not ‘spot bedded’ as the resultant air gaps may cause problems with frost damage or allow ants to colonise the spaces – and the thickness of the slab (usually 37mm – 50mm).

The make-up of the patio structure is therefore 75mm concrete, 25mm mortar and 50mm slab giving a total thickness of 150mm between the depth of the excavation to the top of the secondary pegs.

Trade Tricks – If you think you may, at some future stage, wish to expand the area of paving, push a length of steel reinforcing bar into the concrete base by around 90cm, and leave 60 – 75cm extending beyond your current work. These bars should be below ground, and will provide a very strong ‘key’ between the present scheme and future works.

Laying The Slabs

The laying mortar mix should be one part cement to six parts of soft builders sand, mixed in uniform batches, each the same strength as each other to avoid any variations in porosity in the mix. The mixture should be sticky, not too wet, nor too dry.

Lay the first slab in the corner of the area to be paved, using five or more trowel spots of mortar, depending on the size of the slab. There should be sufficient material to ensure that the whole under surface of the slab is covered once the slab is tamped into place.  The slab may be bedded into position by gently tapping with a rubber mallet, starting from the centre outwards, not corner by corner as this will result in a ‘domed’ shaped mortar bed.

Using taut builders string lines to ensure accurate levels and falls, carefully checking dimensions especially when using more formal types of slabs, designed to be laid either butt jointed or fully pointed, progress the laying of the paving.

If you are using a random pattern with multi-sized slabs, ensure that a correct balance of sizes are used, and thus avoid having too many of one size in a given area. Mix the various shades of slabs, even drawing from different packs as the work progresses, to ensure that there are no areas of one colour or hue, thus creating a visually uniform finish. Too many small slabs, too many large or same size slabs in one area can also spoil the effect.

Some regular sized slabs lend themselves to the use of spacers  to ensure that all joints are uniform. These may take the form of (say) 13mm battens or plastic strips, in the same way as glazed tiles are set in an indoor bathroom or kitchen.

If you are laying a large area, take the slabs from one or more pack to ensure colour variation, even if nominally the same product. Batches can vary very slightly due to age and length of storage/manufacture. Once an area is complete, seal it off completely to prevent anyone from walking on the work – including dogs – and if you think that any slab may have become loose, re-set it immediately whilst the mortar is still soft.

Trade Tricks – A Conduit for potential future lighting or perhaps an irrigation system should be placed in or under the concrete foundations, and a draw string (strong builders line is ideal), colour coded if more than one string is placed, and left exposed and secured at either end of the conduit. Normally, a 50mm plastic waste pipe is used for this purpose, available in three metre lengths. If the project is wider than three metres, and you require a longer conduit, a 50mm pipe inserted into a 75mm wastepipe much as a telescope, will increase the length without the need for any gaps.


For pointing, a mix of one parts Ordinary Portland cement to four or five parts of soft builders sand is suggested. Once again, the mortar must be carefully batched to ensure a uniform colour. The pointing mixture should be moist enough to roll into a ball, but not too wet that it will not crumble when rubbed in between your fingers.

Ensure that the paving is dry before commencing work to prevent staining. If the joints contain water, this must be allowed to dry before continuing, otherwise you will create more staining.

Place a small amount of pointing mix onto a piece of board (45cm x 45cm) and fill each joint with a pointing trowel, ensuring the gap is fully filled with no low or soft spots (another advantage of a full mortar bed under the slabs). Press the mortar into the joints using a piece of rounded metal, such as a bucket handle, and ensure the joint is filled with well compacted mortar to prevent weed seeds from gaining access.

With some products, you may wish to use a coloured mortar. These dyes are available in liquid or powder form. It is essential that you experiment with these colourants, and a wet mix will appear to match, yet when dry will turn into a vastly different – much brighter – colour than you wish. Try a small sample first, away from the site, noting the amount you have used and adjust to suit once you are pleased with the results.

Summary  Careful planning is the key to success. If you choose more than one type of slab, check the actual measurements of each size. What is listed as a size may be only a nominal description, and if your design is produced with little or no tolerances, this may prove problematic. Some products are nominally (say) 45cm x 45cm are actually 44cm x 44cm or 46cm x 46cm, and whilst the difference may sound small, it may have an overall impact on the scheme over a larger area, with each slab losing/gaining one centimetre.

Alan Sargent
February 2016