Even if it’s only for a wall to grace the garden, building a solid foundation is a must. But how deep should you dig?
How wide? And what’s the right thickness of concrete? Here’s an easy to follow explanation of why foundations are so important, how they differ and which one to choose.
All walls need foundations to give them stability, and free-standing garden walls are no exception. The foundation is like a platform, helping to spread the weight of the bricks in the wall onto the earth base below.
Most foundations are made of concrete laid in a trench, and for a garden wall where there’s no additional weight for it to carry (unlike a structural wall, for example, which may also carry part of the weight of a roof) the concrete itself doesn’t need to be very thick -between 100mm (4in) and 150mm (6in) of concrete is guite enough for a wall up to a metre in height. But the thickness of the concrete is not the only thing you have to consider. How deep in the ground you place it is just as important.
For a concrete foundation to provide an effective platform which won’t allow the brick wall to crack, it has to be laid on firm ‘subsoil’. And you won’t find this until you get below the topsoil. The depth of topsoil varies enorm-ously from place to place, so there can be no hard and fast rules about how deep you must dig – but expect anything between 100mm and 300mm (4-12in). Once you’re through to the harder subsoil, you’ve then got to dig out enough for the depth of concrete – at least another 100-150mm (4-6in).
In practice the other big variable is the nature of your soil. Different subsoils have different load-bearing capacities – for instance hard chalky soils can support more weight than clay. but sandy soils can take less. The weaker the subsoil, the wider you have to build the foundation – consult your local building inspector for advice on soil conditions in your area.
There’s another important reason for digging down so deep and that is the effect the weather has on soil. In clay subsoils, for example, a prolonged dry spell will cause the clay near the surface to shrink; then, when it rains, the clay will swell. All this causes considerable movement of the ground and unless a concrete foundation has been laid deep enough it’ll crack up under the stress of constant expansion and contraction. To counteract this, the foundation has to be laid the point at which the weather can cause movement. Again, in different soils, this varies from 150mm (6in) to 500mm (20in) or more down, but it’s advisable to consult your local building inspector to get a more precise figure for soil conditions in your area.
WHAT TO MIX.
– foundations for garden walls don’t need the strongest concrete mix – use the following proportions: 1 bucket cement 3 buckets sharp sand 6 buckets washed aggregate
OR 1 bucket cement 8 buckets all-in ballast
HOW MUCH TO BUY.
– for small quantities buy bags of dry ready-mixed concrete (the bag will say how much it’ll make up).
– for larger quantities, it’s more economical to buy the ingredients separately
– although cement is available in standard size bags, sand and aggregate (or all-in ballast) have to be bought loose – and the minimum quantity you can buy is usually 1A cubic metre.
EXAMPLE:To make 1 cu metre of concrete (enough for a foundation 20 metres long, 500mm wide and 100mm deep) you’d need to buy: 4 bags cement (50kg bags) ½ cu metre sharp sand 3A cu metre washed aggregate
OR 4 bags cement (50kg bags) 1 cu metres all-in ballast
All foundations have to be designed so that they evenly transfer the weight of the wall above to the earth base below. Because of the way the wall’s weight spreads out onto 3 (12in).
The foundation – called the angle of dispersion – the foundation is built so that it is wider than the wall. In fact this load spreading’ follows an angle of 45° and means that the width of the foundation on each side of the wall has to be at least equal to the depth of the concrete. This is a simple rule of thumb which will help you decide how wide your foundation has to be for different wall widths. Of course, if you’re building on a relatively soft subsoil, your building inspector may recommend that you build a wider foundation. Like a raft floating on water, the bigger it is the more stable it will be.
Once you’ve dug your trench, you’ll be faced with another decision: do you just lay the minimum thickness of concrete or lay enough concrete to fill up the trench so you have fewer bricks to lay? In fact, there can be quite a difference in the amount of work involved. If your trench is 500mm (20in) deep and you only lay a 150mm (6in) depth of concrete, it means that just to get back to ground level you’ve got to lay some 5 courses of bricks which ultimately won’t even be seen. Nevertheless, it makes no difference to the strength of the foundation – it simply depends on whether you prefer laying more concrete (and it’ll be quite a lot more) or more bricks. Engineering bricks are recommended for any work below ground, though any special quality brick will do almost as well.
Digging a trench foundation to a depth of about 500mm (20in) is probably the safest rule of thumb to follow if you’re building a wall that’s going to be more than 5 or 6 courses above ground level. (For smaller walls see Foundations for low brick walls).
And to make sure that the line of the trench is straight and the width constant, you have to mark out accurately. For this you need to set up what are called ‘profile boards’ at each end of the trench. All you do is string lines between the boards in the position you want for the foundation.
To make profile boards, use lengths of 50mm x 25mm (2in x 1 in) timber cut a little wider than your trench. For pegs, use 50mm square (2in square) timber about 600mm (2ft) long. You’ll also need nails and string.
Hammer two pegs into the ground at each end of the wall line, and nail the cross-pieces onto them. Next drive nails into the tops of the boards – to mark the outer edges of the foundation -and string lines between them, pegging the string into the ground beyond the profile boards.
These strings are then the guide lines for digging the trench, and can be transferred down to the ground using a spirit level. When the trench is dug and the foundation laid, these same profile boards can be used to create the building lines for the wall – you just add more nails and string up as before.
Constructing the trench
Remove the topsoil and dig a trench according to your marking up. To give you a guide for laying the concrete, you’ll need pegs about double the depth of concrete required. These should have tops cut square and should be driven into the centre of the trench at intervals of 600mm (2ft). The tops should all be precisely levelled using a builders level or a straight-edge and spirit level. Soak the trench with water and allow it to drain before the concrete is poured in. This should then be well ‘rodded’ (with a broom handle, for example) to ensure that the entire volume of the trench is filled and the concrete is as high as the tops of the pegs (these don’t have to be removed and will eventually rot). Use a wooden float or suitable piece of timber to level the surface. It needn’t be perfectly smooth as a fairly rough surface provides a good key for the mortar. Cover the concrete with plastic sheeting or damp sacking and leave for 6 days to ‘cure’ – longer if the wall is more than 12 courses high. The slower the curing the stronger the concrete, so don’t try and build a wall on the foundations too soon.
Concrete is ideal for foundations for no other material can take up the precise shape of the subsoil surface at the bottom of the trench and transfer the load so evenly. It should be made of 1 part cement to 3 parts sand to 6 parts aggregate with just enough water to produce a pliable consistency.
Trench foundations can be reinforced with rods or mesh – either will increase the strength of the foundation and whatever is built upon it. Both kinds of reinforcement are actually quite simple to add – you just have to make sure that the steel rods or mesh are bedded in the lower part of the concrete and not exposed at the sides or ends. In some cases, reinforcement is essential – for example, if you’re laying a foundation over a drainpipe. For most garden brick walls, however, going to the trouble of reinforcing a foundation just isn’t necessary – the weight of the wall doesn’t justify it. What is important is that the wall doesn’t crack because the ground underneath moves slightly.
Raft or slab foundations
For small walls of 7 courses or less the simplest concrete foundation is a raft or ‘slab’. This is cast just below ground level (after the topsoil has been removed) in much the same way as you’d lay a concrete path. First dig out to a depth of about 200mm (8in). then add a layer of compacted broken brick or hardcore to provide drainage. Cover this with light polythene sheeting just before concret- ing – this will prevent the concrete drying out too quickly because of water being absorbed by the base. Then lay your concrete about 100mm (4in) deep and tamp to a level surface. Once the wall is built the concrete foundation can be hidden by soil and grass.
For low walls under 7 courses high you could even avoid the expense and trouble of mixing concrete altogether, because a foundation strip of bricks laid cross-ways can be perfectly adequate Lay the bricks on a thin layer of sand which has been well compacted and levelled in a shallow trench. This should be dug to below the level of topsoil – anything between 100mm (4in) to 300mm (12in) below ground level. Grout these together with a slurry’ – a creamy mixture of cement and water. This is called a ‘footing’ course and you can lay bricks on top in the usual manner even before the slurry is hard.
Earth retaining walls
Sometimes walls built in the garden may not be free-standing but used to retain earth on one side – for example, as terraces on a sloping site or to enclose flat areas of lawn or
I flower beds. In such cases the soil behind the wall is constantly trying to push outwards, completely changing the pattern of stress involved.
The simplest solution is to make the structure strong enough to withstand this extra pressure. With a 4 or 5 course wall this can usually be done by building the wall one brick thick (instead of Vfe brick thick) and by providing ‘weepholes’ at regular intervals to drain excess water. These are made by removing mortar from a number of the vertical joints before it sets.
If you find that the surface of an earth-retaining wall is marked by white crusty deposits – called ‘efflorescence’, and caused by water carrying salts through the wall from the soil behind – dig away the earth and coat the inner surface of the wall with bituminous emulsion to create a damp barrier.
Building on a slope
It is visually unsettling and structurally undesirable to lay bricks running parallel to a slope. So, to build a wall that ‘steps’ down a slope, the trench foundations also have to be stepped or ‘benched’ into the slope. Levelling, pegging, pouring and finishing are all carried out in the same way as with a horizontal trench, but in stepped sections. You’ll need form boards to frame the outer edge of each step, but otherwise the width and depth of the foundation is exactly the same as for an ordinary wall. Only the length of each step varies.
The beauty of bricks is that they can add real style anywhere in the garden. The small wall illustrated was built with a random selection of faced and facing bricks to shield a front garden from a concrete driveway. The other wall, built of rustic bricks, was designed to enclose part of a small patio.
In the diagrams the structure and layout of the first wall is clearly illustrated. Approximately 2V2 metres long (just over 8ft), 900mm (3ft) high and a half brick thick (meaning a single line of bricks), each end of the wall needed the added support of a ‘pier’ — achieved by squaring off the ends to make them one brick thick.
The top of the wall could have been finished off with a course of whole bricks laid frog down. Instead, a line of half bricks was laid end on — known as a soldier course. The piers were finished higher than the rest of the wall with three bricks laid sideways on.
The foundation trench had to be 400mm (16in) wide because of the extra width of the piers. It was 400mm (16in) deep — shallower than usual — because the sub-soil was firm. The concrete was laid to within 100mm (4in) of the ground level to save building up to ground level with several courses of bricks.