How Lay Paving Slabs

Choose your paving according to type of construction, colour, surface finish, shape, and size. Hydraulically pressed slabs are about 37mm thick and are strong enough to support the weight of a car provided that they are laid properly. Cast concrete slabs are about 50mm thick and are heavier but their method of construction makes them weaker and suitable for paths and patios subject only to foot traffic.

Sizes and finishes

Paving slabs are available in a good range of colours so there should be something to suit all tastes. There are four common surface finishes. Smooth slabs have a non-slip finish; textured slabs have a relief surface and the aggregate in the surface is exposed; patterned slabs have a surface giving decorative brick, tile or cobblestone effect; riven looks like naturally split stone.

Most slabs are square or rectangular but there are circular, hexagonal and other geometric shapes available. Sizes of slabs vary. A popular size is 450x450mm because this can be easily handled.

Planning on paper

Especially where an ornate-shaped patio or drive is being laid, it is best to plan the project carefully on graph paper. Having selected a range of slabs and noted the sizes available it is then easy to create an accurate plan showing the exact number of slabs and sizes needed. Where a multi-coloured design is required, make use of coloured pencils to reproduce the final effect and so avoid the design becoming gaudy and overbearing. Try to design the project to full size slabs to avoid having to make lots of cuts. If you do have to include a lot of part-sized slabs then the job is going to be more time-consuming. You should order a few extra slabs to allow for breakages while cutting. Most manufacturers offer suggestions for patterns in their brochures and one of these might appeal.

When the slabs are delivered you will have to carry them to the storage area yourself. Be careful when handling them since even individually they are awkward and heavy. Stack them on edge in a row on a hard surface and leaning against a solid wall. Ensure that the row is stable.

Preparing foundations

It is important to provide firm and level foundations. Strip away any vegetation and topsoil and replace it with well-rolled hardcore topped with a layer of ballast. When laying a driveway, dig down to a depth of 100 to 150mm; the shallower depth is suitable for firm, gravel soil and the deeper one for soft clay. For a path or patio you need only dig down to a depth of 75 to 100mm.

Roll or ram the site soil until it is firm before adding hardcore. Place larger pieces of hardcore at the bottom of the layer and spread smaller pieces on top. Finally, spread a layer of ballast over the surface and roll or tamp it down to compact the foundation work thoroughly.

You must always take into account the house damp-proof course (DPC) when laying a path, drive or patio which butts up to the wall. The surface layer should be at least 150mm below the DPC. This might mean having to dig down a little further when excavating for foundations. It is also important to ensure that the paving is laid to a slight fall away from the house so that rainwater drains readily away. A fall of about 25mm in 3m is sufficient.

Laying slabs

First set out taut string-lines tied to pegs as a guide to laying the edge slabs in straight lines and to the correct fall away from the house. Lay each slab on a ‘box’ of mortar; with larger (over 450mm square) slabs lay an extra band in the middle. If the -paving has to support the weight of a car then spread an overall layer of mortar about 25mm thick under each slab. Use a mortar mix of 1 part cement to 5 parts sharp sand.

Position each slab carefully on the mortar and, where it is intended to use definite joints, leave a gap of about 8 to 10mm. Uniform joints can be maintained by using pieces of wood as spacers.

Tap down each slab using the shaft of a club hammer or similar implement until it is level, at the same height as neighbouring slabs and shows no tendency to rock about. When all the paving has been laid, the joints should be filled with a dryish, crumbly mortar. Press this well into the joints, keeping it clear from the surface of the slabs. Only do this job when the slabs are dry and do make sure any surplus mortar is brushed away before it rains.

Cutting to fit

When you need to cut a slab, first mark the cutting line by scratching the slab with an old sharp tool held against a straight-edge. Lay the slab on a bed of moist sand and cut a shallow groove using a bolster chisel and club hammer. An alternative is a masonry-cutting disc fitted to a circular power-saw. Whichever method is used, ensure that the score line goes all round the slabs, including the edges.

Lay the slab face downwards on a timber batten and tap along the score line with the club hammer to make the break. Alternatively, work continuously with the bolster chisel and club hammer.