Post category: Construction


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.



Piers And Walls
Click here to view bigger sized image

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.



Piers And Walls
Click here to view bigger sized image

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.


Damp-proof course


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.


Retaining walls


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.



Piers And Walls
Click here to view bigger sized image

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.


Laying tarmac and concrete accurately is skilled work, best left to a professional. But gravel and paving slabs are relatively easy to lay. For a path, choose the obvious route through the garden. If there is no obvious route, create one by using obstacles or barriers.



Paths and Paving
Click here to view bigger sized image

Put a paved area in a position of full sun, as far as possible. The area will be used more often and stay cleaner. A paved area must be situated so that it is private, or can be made private by screening. Otherwise, there will always be a feeling of discomfort.

There are three ways to lay paying – sand bed, dry sand-and-cement bed, or sand and cement mortar. Laying paving on a sand bed is the easiest and most commonly used method. To lay gravel paths or paving, start by removing the top 10 to 15 centimetres of soil, or more if the ground is soft. As the site for paving must not be soft, drainage might be necessary first.

Pack the remaining soil well. Lay 5 centimetres of hardcore for gravel; 5 centimetres of sand on top of 10 cm of hardcore for paving slabs. Gravel laid on hardcore lasts longer than if laid on soil as it tends to sink into the soil. Firm the foundation well.

It is essential to get an accurate level for paying slabs, especially for areas wider than a couple of metres. To achieve this, a ‘screed’ should be set up. First, drive in short pegs to set the level of the foundation.

Allowing for the thickness of the slab to come on top, use a long, straight piece of timber with a spirit level, to get the tops of the pegs to the correct height. A slight slope, to throw off water, should be built in at this stage – 1 centimetre in 1 metre is enough.

Lay two straight pieces of 5 x 5 cm timber beside the pegs and level with their tops. Fill in the sand accurately and firm it well, levelling all the time with a third ‘straight edge’ laid across the two pieces of timber.



Paths and Paving
Click here to view bigger sized image

When the sand is level and evenly firm, lay the slabs in position, leaving a very small gap to allow for manœuvre. Level and lay the slabs by degrees. Some cement – 1 part dry cement to 8 parts sand – can be used to give a firmer base. The cement sets in time.

If a kerb is to be used to retain the sides of paved or gravel areas, especially paths, it should be put in place before laying foundations. Stand the kerbing on a bed of sand and cement (3:1) and allow it to set. It must laid level, or evenly sloping. Rough kerbing can be put in place without fixing in concrete.

A useful kind of informal kerb is a mowing strip of bricks laid level with the soil of the lawn. It reduces the amount of lawn edge cutting and can be continued around the lawn edge – not just at pathways. The mowing strip need not be set in concrete.



Paths and Paving
Click here to view bigger sized image

Slabs can also be laid on a bed of wet sand and cement mortar (5:1) laid under each slab. The amount used should be enough to allow the slabs to be lightly tapped to their correct level. The level of the foundation needs to be accurately alid out with pegs beforehand and these pegs constantly checked off as slabs are laid. This is the best way to lay natural stone paving, which is often of varying thickness and very difficult to lay on a level sand bed.

Measure the area to be paved and work out how many slabs will be needed. One square metre will take 6.25 (40cm x 40cm) slabs, 4.2 (60cm x 40cm) slabs or 2.8 (60cm x 60cm) slabs. One tonne of sand covers about ten square metres to a depth of five centimetres. Measure the layout of slabs carefully to reduce the number of slabs that must be cut.


Garden steps should be low and wide in the tread. Start by measuring the fall of the bank with a spirit level and length of timber. Divide the ‘drop’ into a whole number of steps. For example, a fall of fifty centimetres will make five 10 centimetre steps, or four 12.5 centimetre steps.

Remember to allow for mortar between courses of bricks or paving in the ‘risers’. The same thickness of mortar should be used in each joint, otherwise the steps will be of slightly different heights and the flight of steps will look uneven. A very slight ‘fall’ forward on the step treads will throw off water.



Steps and Platforms
Click here to view bigger sized image

A grass, or gravel, platform can be made with a course of bricks or one 10 cm ‘step’ of paving slabs. Even one course of brick or paving will need a small foundation strip of about five centimetres deep. Otherwise, the line of brick will eventually sag and spoil the level of the platform.

A wooden platform or deck lasts better if it is set up on blocks of concrete formed in position using wooden shuttering, or built like a gate pier with concrete blocks. Small wooden decking platforms are within most people’s capabilities to build, but a large platform, or one raised higher than one metre, carries a lot of weight, may need steel supports, and should be designed by a specialist contractor or engineer.

A sunken garden needs retaining walls to hold back the soil and may need drainage pipes as well, especially if it is at the foot of a slope. It will need one or more flights of steps for access.


Wooden fences can be supported by wooden or concrete posts. Wooden posts should be heavy enough to support the weight of the fence and the pressure of the wind. They should be long enough to allow about one quarter of their length to go into the ground.



Click here to view bigger sized image

Concrete posts are much stronger and last longer. They suit wooden panel fencing but not other fence types, trellis for example. Wooden support posts are best made of hard wood such as oak, although these can be difficult to get, or pressure-treated softwood. Soft wood posts simply painted with wood preservative will last only two or three years. 

Posts must be put in the ground firmly, to hold the wooden fencing panels. Measure out accurately where the posts should go, marking each spot with pegs. Dig holes about 30 centimetres square and 50 to 70 centimetres deep.

Some big stones can be used to part fill the hole and help to retain the posts in position. Next, three-quarters fill the holes with concrete made of 4 parts gravel, 1 part sand and 1 part cement. Using a concrete footing is the most secure way to set up a fence and it is easier than driving the stakes, which is difficult to do accurately.

The stakes are stood in the holes as described in a straight line and at the correct spacing and are made exactly upright with a spirit level, using three or four temporary props of light timber nailed lightly to hold the post in position. The hole is filled to the surface and gently rammed in position. Check the props to make sure the stake is straight.

Allow the post foundations to set for a day or two and then nail the wooden panels into position, with constant checking that they are straight and in line, or use metal clips.



Click here to view bigger sized image

Rustic fencing can be tackled in the same way, except that forest poles are used instead of panels. The base of the poles in the ground should be treated, but the rest need not. 
Specially made metal footings can be set into the concrete to get over the problem of wooden posts rotting just at ground level.

A wooden fence does not last as long as a wall but the useful life of a fence can be extended by painting it with wood preservative or paint. Supports for plants should be capable of being dismantled to allow painting. Most climbing plants are flexible enough to be leaned away from the fence during painting.




Most rockery plants are sun-lovers – do not place a rockery in the shade of buildings or trees. A rock garden should resemble a natural rock outcrop and because rock normally outcrops from sloping ground, it helps greatly to have a rock garden built into a slope.



Rock Gardens
Click here to view bigger sized image

The rocks must not be simply dispersed over the area of the rockery but seem to actually come ‘out’ of the ground. They must be in the ground, not on top of it. Some of the rocks should be almost completely buried, as they would be in a natural outcrop, giving the impression of underlying rock.

Choose suitable stone and do not mix different types. Start by clearing the site of all existing vegetation, especially perennial weeds. On a flat site, build the rockery in stages – do not just heap up the soil and dot it with stones. Lay a tier of stones with the big ends outwards and a slight backward tilt to trap rainwater.

Fill in behind with soil and firm well. Mix some broken bricks, small stones or rubble into this fill-in material. Lay another tier of stones in the same way and fill in behind – repeating the procedure until all the rocks and soil are used up. Remember to hold onto some nice stone for the top of the rockery.

On a slope, dig out a low terrace, about 20to 30 cm deep and set the stone against the earth. Fill in behind and then move up to the next terrace. When placing rocks, try not to make the rock structure too regular. Group some stones together to give bulk and space them out a little in other places, as they would occur naturally.

Before planting, it is a good idea to make a special planting mixture for the top 10 centimetres, into which the plants will go. Even if some soil has to be moved to make room for this layer, it is worthwhile doing so. A mixture of 3 parts soil (2 parts, if the soil is heavy), 1 part coarse sand and 1 part peat is ideal. Use a 2.5 centimetre layer of shingle or stone chippings over the soil as a mulch to prevent weed germination.


Alpine bed


A raised alpine bed is an ideal way to grow alpine plants, giving the excellent drainage that they need. The bed is raised by at least 30 centimetres and looks best when the supporting sides are made of rock. A concrete block wall can be built first as support for the natural stone.



Rock Gardens
Click here to view bigger sized image

Stone walls can be built up without supporting blocks behind but they tend to ‘belly’ out and can even collapse in time, unless they are well built. But this irregularity can be part of their charm, and unsupported stone walls have the advantage that plants can be grown between the stones of the walls, rooting back into the soil behind.

If this attractive effect is required in an alpine bed with a concrete backing wall, some gaps will have to be left between the blocks during building. A drainage pipe should be laid beneath the bed to remove excess water, then a 10 cm layer of stone for drainage and finally the soil mixture when should be roughly 3 parts soil, one part shredded leaf mould or peat and one part coarse sand, grit or fine gravel. After planting, the surface can be dressed with 4-5 cm of grit.


Scree bed


A scree bed is built in the same way as a raised bed; it needs good drainage and it not simply a question of spreading gravel over the existing soil. A scree bed can be level with the existing soil or slightly raised above it to improve drainage. It can have a slight slope across it, but should meet the existing ground level and not have kerbs or raised sides.

Dig out the area to at least 30 cm and lay a base of rubble or coarse rock, use the soil for making the rockery soil mixture and use the surplus elsewhere in the garden. A layer of Plantex or similar material can be laid down to keep the rubble free of fine material. Then a layer of gravel about 5 cm deep is laid over and then the soil mixture of roughly 3 or 4 parts soil, one part shredded leaf mould or peat and one part coarse sand, grit or fine gravel.

A light coloured gravel, grit or coarse sand looks best in a scree bed in a layer of about 5 centimetres. One or two carefully placed large stones nicely set off the sand or gravel surface. They should be set well down into the surface layer.

When planting a scree bed, or indeed a rock garden or alpine bed, try to avoid planting too many plants. Alpine plants can grow quite large in a few years, spreading out by means of overground, or underground, stems. When the surface of a scree bed, rockery or alpine bed becomes cluttered with plants it loses much of its attractiveness.

It is the combination of plants, rock and sand or gravel, that is interesting. Plants are less likely to smother a rockery than a flat scree bed, but this problem should be guarded against by restricting the number of plants and their size.


Garden pools


Water is restful and reinforces a garden’s air of calm and tranquillity. A pool of some sort can be chosen to suit most gardens. Concrete pools are difficult to build and often crack. Ready-formed fibreglass is quick and easy to install, but unless well disguised, it tends to look artificial.



Water Features
Click here to view bigger sized image

Pool liners of polythene, PVC or butyl rubber are cheap, easy to put in, and can be made to any size or shape – except formal. Estimate the size of the liner as the maximum length plus twice the maximum depth and the maximum plus twice the maximum depth. Leave a little extra as a precaution.

Dig a hole to the size and shape required. The depth should be 45 centimetres for a small pool, 60–75 centimetres for a large one. A ‘shelf’ to one side, about 20 centimetres deep, is ideal for marginal water plants. The sheet to fit the hole should be the maximum length plus twice the maximum depth, and the maximum width plus twice the maximum depth. Line the hole with a layer of sand; spread the liner; weigh down the edges with bricks or stones, and fill the pool with water.

Around the edges of the pool, the polythene, or PVC, must be hidden from sunlight or it will break down and crack. Usually a pool is associated with a paved area and the paving could be laid to overhang the liner.


Cascades and streams


Cascades, waterfalls or artificial streams can be made with flexible pool liners that take the shape of the excavated soil when they are filled with water. A cascade of little ‘waterfalls’ can be made by having the water fall down a series of overlapping small pools. Unless there is a clean fall over the edge of a stone, water can be lost as it trickles down the face of the step between the small pools. Alternatively, the face of the cascade could be lined as well.

An ‘artificial’ stream can be made of a series of long narrow, shallow pools. In this case, and for cascades, it is essential to hide the liner with sand, gravel or stones. Any sight of the liner destroys the effect of these natural-looking garden features.


Formal pools


Formal pools are always of regular geometric shape, usually straight sided but sometimes circular, or semi-circular. A flexible liner cannot be maintained in a regular shape without rigid support. Concrete is used for formal pools but they are tricky to build without cracks.

The hole is dug out with straight sides and lined with heavy polythene. A solid slab of concrete, well tamped down and perhaps reinforced, is laid in the bottom. As it begins to set, the surface scum is brushed off the edges where the walls will be built. These must be built the following day and the floor kept damp in the meantime.



Water Features
Click here to view bigger sized image

The walls are built of dense blocks with joints of strong mortar and rendered with strong sand and cement plaster. Split blocks or bricks could also be used, without plaster rendering of course. Bonding adhesive can be mixed into the mortar and rendering to increase the bond and help to seal the walls.

Two days later the space between the walls and the polythene lining is filled with general purpose mix concrete, well compacted all round. The edges of the pool are finished with large paving slabs. The concrete surfaces of the pool can be sealed with special pond sealers, painted on.

Alternatively, the pool can be built as described but a flexible pond liner is placed under the pool base. The walls are then built on the base and the liner brought up behind the walls and filled behind before the edging slabs are put in place. The liner seals the pool and the concrete sides give the required formal shape.




Water lilies need deep water, and remember to get some oxygenating plants, such as Canadian pondweed or water milfoil. As these compete with algae for nutrients, they prevent it from clouding the water. Plenty of oxygenators are needed in a new pool.



Water Features
Click here to view bigger sized image

A cascade fed by a submerged circulating pump can be a great addition and is simple to install. Make a concrete course for it to flow down, or else use stones. Strips of pool liner could be used too, but they look a little obvious.