Of all the materials you can use to build a path, slabs are among the simplest to lay.
The large size of the individual units means a path should not take long to complete and the range of slabs available gives you a wide choice when deciding how your path will look.
Paths are made for going places and while their function might be to prevent mud being trampled into the house or to get a wheelbarrow to the garden shed without making furrows in the lawn, how they look in relation to the garden and your house is also important.
A wide range of attractive paving materials is available for you to choose from. This section deals with the techniques for laying pressed concrete slabs.
Planning a path
Any path should have a purpose. There’s little point, for example, in laying a path that skirts the garden and then seeing it ignored as short cuts are taken across the lawn.
You should also make sure your choice of materjal blends with the surroundings. Concrete slabs, for instance, can look out of place if you have a lot of brick walls, whereas crazy paving might complement them. If the lawn is large and a path straight across it, an unbroken length of slabs might look too prominent and it might be preferable to use stepping stones with areas of grass in between (remember to relate the spacing of the stones to a normal walking pace or you will defeat the purpose of the path). If the garden is dotted with trees and shrubs it might be more eye catching to curve the path around them so that it doesn’t dominate the setting.
When you’re designing, think of the width as well as the length — a path that’s too narrow to walk on easily will remain a source of irritation. If you make it too wide it might give the garden an unbalanced look, though a wide path can look very good if flanked on both sides with an array of shrubs or flowers.
WHAT SIZE SLABS?
Square and oblong slabs come in a range of different sizes. The commonest are:
– 225 x 225mm (9 x 9in).
– 450 x 225mm (18 x 9in).
– 450 x450mm (18 x 18in).
– 675 x 450mm (27 x 18in)
Some slabs are based on a 300mm (12in) unit, so squares are 300 x 300mm or 600 x 600mm (24 x 24in), and rectangles 600 x 300mm (24 x 12in) or 900 x 600mm (36 x 24in). Slabs over 450 x 450mm (18x18in) are very heavy. Hexagonal slabs are usually 400mm (16in) or 450mm (18in) wide. Half slabs are also made, either cut side to side or point to point.
Both rectangular and hexagonal slabs are generally 38mm.
Light-duty paths – for walkers only – can be laid with slabs bedded on sand about 25mm (1 in) thick. The joints should be filled with sand or soil, not pointed with mortar.
Heavy-duty paths – for wheelbarrows, rollers and heavy mowers – should have slabs bedded in stiff mortar (1 part cement to 5 parts sand) and mortared joints.
TIP: STACKING SLABS
To avoid chipping corners and edges and marking the slab faces, stack slabs on edge in pairs, face to face, against a wall, with their bottom edges on timber battens.
The best way to start planning is on paper. Use graph paper to make a scale plan of the garden, marking in any fixtures such as established trees and a shed or greenhouse and obvious targets for the path such as a gate or the washing line. Draw them in ink and use pencil to plan in path shapes — they can always be rubbed out if you change your mind.
The plan will give you something to work to as well as a method of calculating the number of slabs and the amount of sand and cement you’ll need. But first you’ll have to decide on the pattern you want and the type of slab (home-made or bought), whether you want grass to grow between the cracks or whether you prefer the overall look that formal pointing will give.
The most common concrete paving slabs are square or rectangular in shape, though you can also buy them circular or as parts of a circle (called radius slabs). These are useful for curved or meandering paths which are difficult to make with formwork. Hexagon-shaped slabs look good, too, and these can be married up with half hexagons which give a straight edge for the path’s borders.
Concrete slabs can be bought in a variety of colours — anything from red, green and yellow to brown and the ordinary ‘cement’ grey. Some concrete slabs which are patterned to look like brick or natural stone are finished with a blend of two colours — grey over deep red and grey over buff. But the important thing to remember about any coloured slabs is that the colours won’t always last. The pigments are added to the concrete during manufacture, and in time they will fade with the effect of sun and rain. In damp shady spots under trees, lichen will grow on the surface and diminish the original colours. Some slabs may also show signs of staining as a result of efflorescence — white powdery deposits brought to the surface as water dries out of the concrete. Brushing will remove the deposits temporarily.
Patterns of laying
You may decide on a simple chequerboard pattern using one size of slabs or a pattern with staggered joints as in stretcher bond brickwork. Alternatively you can create a more decorative path using different sized slabs. Riven surfaced slabs can be particularly effective if two sizes of slabs are used with the larger slabs set to radiate around the smaller ones, producing a square which is repeated down the length of the path.
If the pattern you’ve worked out requires cut slabs (it’s helpful and certainly more easy if they’re half sizes), the cutting is relatively easy. After you have marked the cutting line all round the slab you place it on a bed of soft sand or even on the lawn (anything to absorb the shock) and cut a groove along the cutting line, using a bolster chisel and a club hammer. You can then split the slab by tapping the bolster with the hammer along the groove. Cutting sections out of paving slabs is not so easy. Chipping to shape is time consuming and cast slabs are likely to fracture anyway. It’s worth considering filling L-shaped gaps with two separately cut pieces, or leaving out the paving slab altogether and infilling with pebbles, stones or even cobbles set in mortar, or simply finishing off with bricks.
If you want a perfect finish for cut pieces and have a lot of cutting to do, it is worth hiring a masonry saw from a plant hire shop. Although it’s possible to fit masonry cutting discs to an ordinary drill, with a lot of cutting you run the risk of burning out the motor.
Buying paving slabs
Visit local garden centres and builders merchants to see what sizes, colours and textures they have in stock. It’s always worth shopping around. Your supplier should be able to give you helpful information — for example, some coloured slabs are more colour-fast than others, and he should know which ones. If local suppliers don’t have what you want, remember that the cost of transporting heavy slabs over a long distance is high, so it may be better (or at least cheaper) to choose from what is available.
Prices will obviously vary depending on the type of slab — for example, hydraulically pressed slabs are more expensive than cast slabs. And when ordering, allow for a few more than the exact number required for the path; you may crack one or two during laying so it’s better to have spares handy.
Preparing the base
Making a flat base is the single most important step in laying the path. And to do this you will usually have to dig out a shallow trench along the line you want the path to follow.
Digging out the topsoil, roots and any organic matter needs to be done carefully and you should dig the trench to a depth just a little deeper than the slab thickness. As well as being flat, the laying surface must be firm and compacted. At this stage, an easy way of checking that the trench is flat is to use a length of straight-edged timber—a plank or a fence post, for instance — to indicate hollows or bumps which might not be obvious to the eye.
Once the trench is dug you may find that because of the type of soil, the surface is still soft. The answer is to dig out another 75mm (3in) or so and then fill the extra depth with a layer of broken brick (rubble) or broken concrete (hardcore). This layer has to be well compacted before a layer of sand or fine ash (called a ‘blinding’ layer) is spread on it to provide a smooth surface. If you don’t want anything to grow up through the path, saturate the trench with a powerful weedkiller.
Laying paving slabs
The easiest way to control the line of a path as you begin to lay the paving slabs is to set up string lines to mark out the edges. How the slabs are bedded — whether on sand or pads of mortar — depends on the weight of traffic the path will carry and whether you intend to point the gaps between the slabs with mortar.
As you lay the slabs, use a timber straightedge to check that each slab sits flush with its neighbours across and along the path. On level ground, you must lay them so that there is a slight slope across the width of the path — a drop of about 25mm (1 in) across 1 metre (just over 3ft) will be sufficient. Check the slope by placing a 25mm thick block of wood under one end of your spirit level or batten; the bubble should then be in the ‘dead level’ position. On sloping ground, the slabs can be laid dead level across the path width to achieve the same effect.
Slabs can sometimes be butt jointed tightly together. However, because there is often some slight variation in the sizes of slabs, it makes sense to allow for a joint of about 9mm to 12mm (%in to 1/2in) to take up these minor inaccuracies. It’s important that the joints be kept even — for they act as a frame for the slab shape. Spacers cut from board will give you the desired joint thickness and will also prevent the newly laid slabs closing up as you lay adjacent ones.
When you have positioned all the slabs you can fill the joints. If you are not pointing them, simply brush a mixture of soil and sand into the gaps. Where you want a pointed finish there are two methods you can use — and both require care or mortar stains will mar the slabs.
One method is to mix the cement and sand dry — the sand needs to be very dry — and pour or brush this into the joints. You then sprinkle the joints with a watering can fitted with a fine rose, or wait until it rains.
A better method is to mix up dry crumbly mortar and press this into the joints with a pointing trowel. Any mortar crumbs falling onto the face of the slab can easily be brushed away without staining. You can also use a piece of wood or a trowel to finish the joint so it is slightly recessed.
After pointing is completed don’t walk on the path for a few days — if you tread on an edge of a slab you may loosen it and you will have to lift it (using a spade) and lay it again on fresh mortar.
To finish off the gaps at the edges you can point them where they adjoin masonry or a flower bed; where they run alongside a lawn, fill them with soil and let the grass grow back, or fill space with gravel which will drain away excess water.
HOW MUCH SAND?
For a 25mm (1 in) thick sand bed, ½ cu metre (% cu yd) of sand will cover about 20 sq metres (215 sq ft) – a path 20 metres long and 1 metre wide (22yd x 3ft 3in).
HOW MUCH MORTAR?
One 50kg bag of cement, mixed with 24 2-gallon buckets of damp sand will make enough mortar to lay about 30 slabs using the ‘dab’ method. One quarter of this mix will fill the joints between about 60 slabs measuring 450 x 450mm (18 x 18in).
GETTING THE RIGHT LEVEL
A path alongside the house should ideally be two courses of brickwork below the damp-proof course (dpc). If this is impossible, make a shallow, gravel-filled gutter about 150mm (6in) wide between the path and the wall to prevent rain from splashing the wall above the dpc.
If the path runs next to a lawn, the finished level should be below that of the grass so you can mow over the edge without the blades catching the masonry.
ALLOWING FOR DRAINAGE
Horizontal paths should have a slight slope across the path width to allow rainwater to drain off the surface. A fall of 25mm (1 in) per 1 metre (3ft 3in) of path width should be sufficient: use a block of wood under your spirit level or batten to check that the fall is correct.
TIP: HANDLING SLABS
Slabs are heavy. To avoid injury.
– grip the edges of the slab firmly.
– lift them with your knees bent and back straight, or
– hold the top corners of the slab, and ‘walk’ it on its bottom corners
– wear heavy shoes to protect your feet in case you drop a slab on them
– wear gloves to protect your hands when handling slabs with a very rough surface.