Construction of DT 20, 25 Formwork Block Fencing

The method is the same for every block width. The width mainly changes how wide the foundation must be and how much reinforcement goes in.

1. Setting out. Pegs and a string line mark the route along the property boundary. The finished height and the positions of piers and gates are fixed at this stage.
2. Digging the foundation. The trench goes down to frost-free depth. This depends on your local climate and soil, and clay or waterlogged ground usually needs more. Local building practice or the building authority can tell you the usual depth. The bottom is levelled and compacted.
3. Strip foundation. Reinforcement is placed in the trench and concrete is poured. The concrete strength follows the design, for fences often C16/20 or higher. Vertical starter bars must stick up out of the foundation so the wall reinforcement can connect to them later. The strip is usually wider than the block.
4. Laying the blocks. The first course is bedded in mortar and levelled carefully, because it sets the line and height of the whole fence. Further courses are laid with staggered joints.
5. Reinforcement. Vertical bars go into the cores and horizontal bars into the grooves of the blocks. Corners, piers and places where a gate will hang need more steel.
6. Filling with concrete. Concrete goes in gradually, a few courses at a time, and is compacted by rodding or with a poker vibrator so no voids remain.
7. Finishing. Coping stones or a small capping go on top to shed water. The surface is rendered, painted, clad or left as fair-faced block.

Concrete needs time. It reaches its full design strength in roughly four weeks. In hot weather it is kept moist for the first days so it does not dry too fast, and in frost it should not be poured without protective measures.

A wider block holds more concrete and steel, so the wall is stiffer. The blocks are heavier, though, and the foundation must be wider.

• DT 12: low walls, bed edging, small dividing walls in the garden. Not suitable for a taller solid fence.
• DT 15: low fences, plinths and base walls under a light infill, where the wall itself carries little.
• DT 20: a typical house boundary, either as a base wall or as a solid fence of medium height on level ground.
• DT 25: taller solid fences, exposed windy sites, sloping plots and fences with a heavy gate.

Why does height matter so much? A solid fence behaves like a sail. The taller and longer it is, the more the wind pushes against it and the greater the force at the base where it meets the foundation. That is why taller fences get wider blocks and more reinforcement. If the wall also has to hold back soil, it becomes a retaining wall, and that needs a structural engineer whatever block width you use.

For appearance, standard smooth blocks are usually rendered or clad. Some manufacturers also offer blocks with a decorative face, such as a split finish, which need no render. These require very clean work, because every mistake stays visible.

The same length of fence can be a straightforward job on one plot and a demanding one on another. The main factors are:

• Ground: on level ground the foundation is simple, on a slope it has to be stepped and drainage is often needed
• Access: if a concrete mixer truck can reverse up to the trench, pouring is quick, while mixing by hand and barrowing takes far longer
• Height and block width: a taller fence means more courses, more steel and pouring in stages
• Layout: corners, piers, gates and recesses for a letterbox or meter box add cutting and reinforcement work
• Old fence and services: demolishing an existing fence, removing rubble, and locating buried cables and pipes before digging
• Finish: render, stone cladding or a metal infill are separate extra steps

Whether you need permission depends on the height of the fence, whether it runs along a road or the boundary, and on local planning rules. Many areas allow low boundary walls without a formal application but set limits on height next to a road. Do not assume. Check with your local building authority before you start.

The fence must stand on your own land, or on the boundary with your neighbour's agreement. If the line is not clear on site, it is worth having a surveyor mark it. Councils often control the height and look of front fences. For land belonging to a block of flats, the building management or owners' association decides.

You can build a low wall yourself, roughly up to knee height on level ground, for example from DT 15 or DT 20 blocks. Leave a taller solid fence, a fence on a slope or any wall that holds back soil to professionals.

1. Confirm the boundary and have buried services located, then set out the line with string.
2. Dig the trench to frost-free depth, level the bottom and compact it.
3. Place the reinforcement with vertical starter bars and pour the strip foundation.
4. Let the foundation harden for several days, and keep it moist in hot weather.
5. Bed the first course in mortar and check it with a spirit level both lengthways and across.
6. Lay the following courses with staggered joints and keep checking the wall is plumb.
7. Insert vertical and horizontal reinforcement and tie it together at corners.
8. Fill with concrete a few courses at a time and rod each layer with a bar.
9. Fit coping with a fall or an overhang so water drips clear of the wall.
10. Render or treat the surface only after the concrete has cured.

In practice the problem is rarely the blocks themselves. It is what lies under them and inside them.

• Shallow foundation: frost heaves it, and after a few winters the fence leans or cracks
• Reinforcement not tied into the foundation: the wall just sits on the footing and can snap at the base in strong wind
• Filling too high in one go: wet concrete pushes on the blocks and can force them out or split them
• Concrete too wet: extra water makes pouring easier but costs strength
• Uncompacted concrete: air pockets stay in the cores and the wall is weaker than it looks
• No coping: water gets in from the top, freezes and the render falls off
• Long fence without movement joints: concrete shrinks and vertical cracks appear
• Soil banked against the wall: without waterproofing and drainage the wall stays wet and suffers in winter

• The fence follows the marked boundary and the agreed height
• The wall is plumb and the courses run straight with no visible sag along the length
• There are no cracks, pushed-out blocks or traces of leaked concrete on the surface
• Gate piers are solid and fixings sit in the right places
• Coping is firmly bedded and water drips clear of the wall
• A long fence has movement joints
• The ground around the fence slopes so water runs away from the wall
• The contractor has told you what concrete was used and when the wall can carry an infill or a gate