Paving is a vital – and potentially costly – aspect of garden design. Andrew Wilson explains the pros and cons of different materials and gives some useful advice on how to get the best results
Paving is forever the bridesmaid of the garden, taken for granted as we clamour for plants; yet take it away and our gardens fail to function.
The big issue with paving is cost. It is usually the most expensive element of any garden design. The most costly materials tend to be stone such as granite, limestone and sandstone, or more unusual stones such as slate, quartzite or basalt. Most of these materials can be finished in a variety of ways.
Gravel and concrete
The cheapest natural alternative to stone paving is gravel. Less natural but also cheap is concrete in its pre-cast form. Many concrete slabs are designed to imitate stone, and do so more or less successfully. They sometimes include dyes, which may fade over time. Other mass-produced concretes include aggregates that give the finished slabs a more interesting texture.
The costs of concrete paving rise dramatically when concrete is poured on site. Concrete itself is not necessarily expensive, but the work in creating moulds or formwork can be pricey.
Stone and brick
If you want to use imported stone in your garden, it’s worth bearing in mind certain ethical issues. Many stone suppliers obtain materials from India and China, where some quarries have been accused of using child labour.
Putting this issue to one side there are also practical problems for British gardeners. Some imported paving slabs aren’t fully hardy in UK conditions. Water-absorbent materials, for example, may crack in freezing conditions.
One paving material often prone to frost damage is brick. Most house bricks will absorb moisture and shatter in freezing temperatures. Instead, make sure you lay paving bricks, which are so dense that they will repel most water.
All paving, except decking, needs a ‘sub-base’ or foundation – a stable raft that prevents sinking or cracking. Traditionally, hardcore was used – broken brick, stone or concrete combined with finer aggregates. Other graded stone materials are now more common. They are compacted to a layer 10-15cm deep, on to which mortar and paving is laid.
Digging down to this depth over the whole area of a patio can leave you with a big pile of soil. The ecological option is to use this soil elsewhere in the garden rather than taking it off to the tip, from where it may end up in a landfill site.
Landscape contractors and builders usually use mortar as a ‘bed’ for paving, fixing the individual units in place. They seal the surface by using more mortar to backfill the joints between slabs. The resulting paving is rigid and strong. Brick and stone were traditionally laid in this way.
Over the last 30 years compacted sand has become more popular as a bedding layer. The paving can be laid on to this, and joints backfilled with sand. When compacted and vibrated together, this combination is strong yet flexible, making it suitable for drives and parking areas. In general, smaller and thicker blocks or slabs are better for this kind of application. The larger and thinner the slabs are, the more likely they are to crack.
Many DIY handbooks show paving laid on to dabs of mortar, usually one at each corner and one in the centre. This is generally considered bad practice as it leaves a high proportion of the slab unsupported and liable to cracking. A continuous bed of mortar under each slab is better.
Sand bedded paving needs a solid edge, without which the bedding material will wash away. Solid edges can be built in mortar-bedded paving on a concrete foundation or supported by poured concrete ‘haunching’ – a continuous strip of concrete just below the soil surface. Metal strips are increasingly common, too.
Paving laid on sand needs no mortar grouting or jointing, but other methods do. Even if you use good quality slabs, their appearance can be wrecked if grouting is applied poorly, or they are laid in an unsuitable pattern. Remember that mortar can be dyed or coloured to suit the slabs. As for patterns, joints running along the length of a pathway will give a sense of direction; joints crossways will slow this sense of movement.
Some materials do not have a unit-based pattern. Aggregates can be bound with clear resins, so they look like gravel but are solid and weed-free. These materials can be laid to any shape or form.
A paved area made of poured concrete should be designed with temperature change in mind. Large slabs of concrete paving will heat up in summer and cool in winter, expanding or contracting accordingly. Flexible joints between concrete slabs must be designed into any scheme to absorb this movement.
The largest slab possible in concrete is probably 6x6m before expansion joints are needed. Large slabs such as these should be reinforced with steel mesh, or glass fibre.
Paving needs to drain water effectively, or puddles will form on it. Slabs are usually laid to a ‘fall’ or gradient, a normally imperceptible 1:60 or 1:100 slope. When laying slabs against a building, paving must slope away from it, ensuring that there is no water penetration. Be aware that solid paving must not breach the damp course and should be 15-20cm below the internal floor level. In older properties make sure the level of the paving is below any air bricks.
New materials and methods of construction create porous surfaces. These allow water to sink into the ground beneath, an option preferable to the non-porous surfaces used for growing numbers of paved over front gardens. These create rainwater run-off, that increases the risk of flooding.
The selection of paving materials is complex, involving cost, colour and texture, quality of finish and appropriateness to location. Giving them all some thought along with the function of your paved areas is essential if you are to get the best out of your investment.
KEY POINTS TO TAKE AWAY
1 Unless you are a keen DIY enthusiast, you must consider the cost of labour when planning to lay paving, as well as the cost of the material. It takes time and work to prepare the ground, make a sub-base and lay the paving. This can double the cost of the paving materials.
2 Supply and demand has a bearing on the cost of materials. Second-hand or salvaged York stone is now often as expensive as freshly quarried York stone.
3 Always create drainage gradients that fall away from the house or any other building. The subsequent drainage or holding of this surface run-off should also be a major consideration.
4 Paving surfaces should be below damp-proof courses and air bricks. Where gardens slope towards the house it is important to remove enough soil to create a sloping surface that falls away from it. Drains can also run along the threshold of the house. These are covered by open grilles and will carry excess water away from the house, especially during periods of heavy rainfall.
5 In the case of imported stone, ask the merchant where it comes from and how it will perform in the British climate. There should be a paper trail from the quarry to the retailer and you need to look for ETI (Ethical Trading Initiative) approval.
Stone merchant for natural materials from UK, Europe and Asia with a wide range of manufactured concrete materials.
Stone merchant specialising in wide variety of natural materials from the UK and overseas.
Suppliers of natural stone but with a huge range of pre-cast concrete slabs, paving bricks and other garden products, including edging.
Suppliers of huge range of concrete-based paving and garden products.
Practical site with lots of useful advice, often illustrated, and well explained by former landscape contractor Tony McCormack.
This article first appeared in December 2007, issue 132.
Andrew Wilson is a garden designer, writer and lecturer, and an assessor and judge for RHS show gardens.