You should always build a garden wall on a solid foundation of a trench filled with concrete. This is called the 'footing', and if it's substantial and accurate, your wall will last longer and be easier to build. When you mix your concrete, don't skimp on materials by adding large stones or blocks - as this will just weaken your wall.
How to build the foundations for a garden wall
Before you start marking out where your trench will go, check the ground is firm and well drained with no pipes or tree roots. Mark the position of the edges of the wall and footing with builder's lines.
To get your footing straight, you'll need to use timber profiles. Each of these is made from three lengths of 25mm x 50mm timber. Two are sharpened to a point at one end, and the third is nailed to them as a crosspiece. Start by hammering a profile into the ground at either end of the site, about 450mm beyond the area you're going to dig. You'll need to make sure each crosspiece is level and that the profiles are at the same height.
Gently tap two small nails into the top of the crosspiece of each profile to show the width of the wall, and another two beyond these to indicate the width of the footing (in this case, 120mm beyond the wall on each side).
Attach builder's lines to the outer nails on the profile (the ones which mark the edge of the footings) and fix the line with pins in the ground. If the footings have a right-angled corner, set two more profiles at right-angles to your existing ones. Check the lines cross at a true right-angle by using a builder's square.
Hold a spirit level upright against the lines to mark the edge of the footing on the ground. Mark this edge with a builder's line nailed into the ground, spray paint or even a line of nails. Then remove the builder's lines that are above ground, so they don't get in the way while you're digging out the trench.
How to dig and fill a trench for a garden wall
The depth of your footing really depends on the nature of the ground. The table is a rough guide for average soil. Loose and sandy or soft clay soils, or soil on a new site that's recently been dug up and put back, need deeper footings.
If your soil is very soft, you may even need to put a layer of compacted hardcore on top of geotextile permeable fabric in the bottom of your trench. Allow an extra depth of two bricks above the concrete for replacing the top soil, or to give you room to pave against the wall.
Type of wall Wall height Depth of concrete Width of trench Single-skin Up to 1m 300mm 450mm Double-skin Up to 1m 150mm 300mm Double-skin Over 1m, up to 2m 375-450mm 450mm-600mm Retaining wall Up to 1m 150mm-300mm 375mm-450mm
Dig your footing trench, using a spirit level to make sure the sides are level. To check the bottom is level too, put a long, straight piece of wood in the trench and lay your spirit level on top of it.
When you've dug your footing trench, you'll need to fill it with concrete made with five parts mixed aggregate to one part cement. But before you do this, cut pegs from 50mm x 50mm lengths of timber and sharpen them to a point at one end. These should measure the depth of the footing, plus enough to ensure the pointed end is solidly held in the bottom of the trench. Hammer in a peg at one end of the trench until it's level with the finished concrete height. Then use a spirit level to set further pegs at the same height, about one metre apart along the rest of the trench.
Pour the concrete and aggregate mix into the trench until it's level with the tops of the pegs. Leave it to set for 48 hours before you start to build your wall.
Building your garden wall on a slope
If you're building your wall on a slope, your footing needs to be stepped - although the concrete must always be level. The step size will depend on the slope and the material you're using to build your wall - you should work it out in multiples of the depth of a course.
Make each step by putting a length of plywood shuttering board across the trench, securing it with a length of wood attached to pegs on either side, and pouring concrete behind it to make the higher step.
How to make your own bricklaying guides
A gauge rod is ideal for checking that each course of bricks is the right height. You can make one from a length of timber batten, which you should mark every 75mm (65mm for the depth of the brick, plus 10mm for the mortar between each course).
A builder's square is a large set square that helps you make sure your right-angled corners in brick or stone walls are completely accurate. You'll need three lengths of wood about 50mm wide and 20mm thick.
Mark one piece 450mm from one end, the second 600mm and the third 750mm. Nail the first two pieces at right-angles, then align the third piece between the marks and nail this in place, too. After doing this, saw off the excess wood and reinforce the right-angled corner with a piece of hardboard.