Foundations | Building a wall | Piers | Damp-proof course | Retaining walls | Dry stone wall
Without some expertise, high walls of concrete blocks or bricks pose a difficult task to build, and badly built walls are always an eyesore! However, low walls are well within the ability of most people. A low wall, 30 to 60 centimetres high is a useful divider of space in a garden, and plants look well, set against the level uniformity of the wall.
Walls are prone to damage in a number of ways. If the foundation is inadequate, they break their ‘back’ and sag in the middle. They are very easily knocked over, even by wind, unless supported by piers, and if there are no gaps left for expansion and contraction, they can crack and become unstable.
Every garden wall must have proper foundations. These need only be 5 to 10 cm thick for walls up to thirty centimetres high, but generally they will need to be at least fifteen centimetres thick and about three times as wide as the wall. Foundations should be laid on firm subsoil, thirty centimetres or so below the soil surface.
Lay the foundation by digging out the soil to a depth of 15 centimetres – enough to support a couple of courses of blocks, deeper if the wall is to be higher. Fill the trench with concrete made of 4 parts gravel, 1 part sand and 1 part cement.
Building a wall
Give the foundation a few days to set, then make up mortar, 3 parts clean builders’ sand and one part cement. Lay the first block at one end, on a 1 centimetre bed of mortar, and level it, using a spirit level. Lay a second block at the other end, stretch builder’s string between the two, to give a straight line, and then lay the rest, making each one level.
The second course goes on top, in the same way, but ‘staggered’ to give strength. Block walls will need capping and the simplest way is to buy capping, and lay it on top. ‘Screen’, or open, blocks are made to a variety of patterns, and they make very attractive boundaries without the ‘closed-in’ feeling of a solid wall.
Walls built of 10cm thick blocks are not safe over 3 courses without piers. The maximum safe height recommended for 22.5cm blocks is 1.8 metres with piers at least every 3 metres. For safety, walls over 1.8 metres high must be designed and built by a professional.
Low walls supported by piers at their ends will usually be secure if they are not longer than about ten metres. Longer walls, and walls higher than one metre, must have piers every three metres , and at their ends, to prevent them keeling over.
Every six metres, walls need movement joints to prevent cracking. These can be provided at a pier by tying the wall to the pier with galvanised strips inset to the mortar joints and greased at the pier end to allow them to move more subsequently. Piers carrying heavy, or wide, gates will need to be reinforced with steel.
A damp-proof course may be built into the base of a garden wall to prevent rising moisture causing white efflorescence from the concrete and spalling of the concrete. But a damp-proof course can make a wall unstable and advice should be taken about the design of the wall and its piers.
A damp-proof tanking course of heavy polythene can be put behind retaining walls to protect them from dampness seeping through from behind. However, this effect might be sought after and low retaining walls without much weight behind them will not be affected by water, once seepholes are provided at ground level every two metres. Professional advice should be sought for walls retaining high banks because these will need steel reinforcement to make them safe.
A retaining wall with an earthen bank behind has to carry a very heavy loading. Simple walls of blocks are not able to withstand the pressure and may collapse, or be overturned. The design of these walls involves calculations of the amount and type of steel reinforcements.
These walls may need to be built of poured concrete. These operations are beyond the competence of lay people. Considerations of the safety of retaining walls are most important when these walls are accessible to the public.
Dry stone wall
A dry stone wall can be used as a low retaining wall, where a sloping piece of ground is terraced into two pieces of level ground, for instance, or as a free-standing wall. It is an attractive ornamental feature in itself.
First set about moving soil to change the levels. If the slope is steep, several terraces will be needed. Having no mortar, a dry stone wall is not stable at a height greater than 90 centimetres. Move soil down the slope to fill the lower level. When this is done, a face, or bank, of soil is left half way up the slope.
Begin building the wall about 30 centimetres from the front of this bank. Use the bigger stones at this stage, as they are easier to manœuvre on the ground. Fill in behind the first course of stones with soil and pack it tight. Then lay another course of stones, with more soil and firming.
When the height of the bank is reached, soil can be brought down the slope to fill in, raising the wall until the upper area is levelled. Place the stones with the broad end out and the pointed end into the soil, and put a slight backwards tilt on the wall to give it stability.
Plants can be put in the gaps between the stones. This type of wall is a lot of work, but worth the effort – a garden on a number of levels is always more interesting. A free-standing wall is built in the same way but with two sides and some mortar used in the middle to strengthen it.