Concrete shuttering block fence, 12 and 15 cm (DT 12/15)

DT stands for a concrete shuttering block, sometimes called a formwork block or hollow fill block. The number gives its thickness in centimetres. A DT 12 block is roughly 12 cm thick and a DT 15 roughly 15 cm. Inside are hollow cores that take the reinforcement and the concrete. The block works as permanent formwork, and the concrete core with its steel does the actual load-bearing.

Thin blocks are mainly suitable for these situations:

• A plinth under fence panels, mesh or timber infill, usually up to roughly 30 to 80 cm high
• A low solid wall along a path, around a flower bed or a front garden
• A fence on a narrow strip along the boundary, where a thicker wall would waste space
• Replacing an old crumbling concrete plinth on the same line

Why does height matter so much with thin blocks? A solid wall acts like a sail. The taller it is, the more the wind pushes against it, and the greater the forces at its foot. A slender wall 12 to 15 cm thick has limited stability. A tall solid fence built from it without piers of thicker blocks, or without a structural check, is a poor idea. The manufacturer states a sensible height for each block. For taller solid walls it pays to ask a structural engineer.

NUMIQ also lists a separate item, the DT 20/25 shuttering block fence. The two look similar, but each suits a different job. The difference is more than thickness. It also affects what the wall can carry, how it is filled and how much work goes into it.

• Purpose: DT 12/15 is typically a plinth or a low wall. DT 20/25 is chosen for taller solid fences, noise barriers, gate piers and places where the wall also holds back a small difference in ground level.
• Cores and concrete: thin blocks have narrow cores, so they need concrete with fine aggregate and more careful filling. Thick blocks take a more ordinary mix and leave room for more reinforcement.
• Foundation: under DT 12/15 a narrower strip foundation is usually enough. Under DT 20/25 it tends to be wider and, for taller walls, more heavily reinforced.
• Weight and handling: one person can carry thin blocks easily, even into a garden with poor access. Thick blocks are heavier, and on longer runs it is worth planning for a pallet truck or machinery.
• Concrete volume: a running metre of DT 12/15 at the same height needs noticeably less concrete. That simplifies logistics and often allows mixing on site.
• Risks: a thin wall reacts more to bulging during filling, to wind and to earth pressure. A thick wall is more robust, but a larger setting-out error is harder to correct on it.

In practical terms: if you want a plinth under panels and the fence will not hold back soil, DT 12/15 is usually enough. If you want a solid wall about head height, if the neighbour's garden sits half a metre higher, or if a heavy gate will hang from the wall, go for DT 20/25, or at least for piers made of thicker blocks.

The boundary and the rules

A fence stands on the boundary, and the boundary should be certain. In practice it often turns out that an old fence is several tens of centimetres off the registered boundary. If you are unsure, ask a land surveyor to mark the boundary, and agree with your neighbour before anyone digs. A dispute over a strip of land is far harder to settle once the concrete is poured.

Whether a fence needs planning permission or a building notice depends on local building regulations. Its height, position, nearness to a road and any conservation or protected zone usually play a role. Do not assume that a low fence never needs any approval. Check with your local building authority before work starts. A fence along a public road may also need the agreement of the road authority. On land belonging to a block of flats, the building management or owners' association has the final say.

Underground services

Water supply pipes, gas, electricity and telecoms cables often run along fence lines. Before digging, have the services located and marked by the utility companies or a cable and pipe locating service. Dig by hand wherever a service runs close. If you damage a gas pipe or smell gas, move a safe distance away at once. Do not use naked flames or switch any electrical devices on or off. Call the gas emergency line or the local emergency number, 112. Do not touch a live cable. Faults are dealt with only by the network operator and a qualified electrician.

Foundation and access

DT 12/15 blocks sit on a concrete strip foundation. It goes down below frost depth. That depth depends on the climate and the soil: in central Europe it is usually roughly 80 to 100 cm, in milder regions less. Vertical steel starter bars are set into the fresh foundation, and they later tie the foundation to the wall. Check whether the price-list item includes digging and the foundation, or only the wall itself. Make sure a pallet of blocks can be delivered, that there is space to store it, and that water and power for a mixer are available.

1. Set out the fence line with a string line between pegs and fix the height of the top edge and the position of gates, side gates and movement joints.
2. Check the strip foundation for level, height and position of the starter bars. If the foundation is uneven, the first course is levelled with mortar.
3. Bed the first course of blocks in cement mortar, exactly to the string line and spirit level. This is the most important step: every error multiplies higher up.
4. Lay further courses dry, in a bond with joints offset by half a block. Use end blocks or cut blocks at ends and gates.
5. Place horizontal reinforcement in the block grooves, usually roughly every second course, and lap the vertical bars as designed.
6. Fill with concrete in layers, never the full height at once. Compact the concrete by rodding or careful vibration.
7. Check plumb and line while the concrete sets, when the wall can still be adjusted slightly.
8. Finish the top: level the concrete, set anchors for panels or posts, and later fit the coping.
9. Apply the surface finish once the concrete has cured: render, cladding, paint, or leave a fair-faced block exposed if the manufacturer allows it.

Filling thin blocks

With DT 12/15, filling is the most delicate part of the job. The cores are narrow and coarse aggregate jams in them. That leaves voids without concrete that cannot be seen from outside. So a concrete with fine aggregate is used, usually roughly up to 8 mm, with a fluid but not watery consistency. The block manufacturer or the designer sets the concrete strength class. It is normally an ordinary structural concrete.

The concrete goes in layers. Manufacturers usually state a few courses at a time, so the pressure of fresh concrete does not bulge or split the thin block. It is compacted by rodding with a bar, or briefly and gently with a poker vibrator with a slim head. Vibrating too long makes the thin wall shake and pushes it out of line.

Blocks

Shuttering blocks are made of concrete, some with a smooth face and others with a rougher one. A standard grey block is meant to be rendered or clad. If you want a fence without render, choose a block the manufacturer specifically describes as fair-faced or designed for fences. Besides plain shuttering blocks there are decorative fence systems with a split face, which come with their own pier and capping units.

Reinforcement

Thin blocks take smaller diameter bars, because thick steel with the necessary concrete cover does not fit in narrow cores. The bars must not rest against the block. They should be surrounded by concrete, otherwise they soon rust. For a plinth under panels, simpler reinforcement is usually enough. For a taller solid wall, the structural engineer's design decides.

Top and surface

A coping of concrete or reconstituted stone protects the top edge. It should overhang and have a drip groove so water does not run down the wall. Lay the coping on frost-resistant adhesive or mortar, never on ordinary interior plaster. For the surface, use an exterior plinth render, stone or brick-slip cladding, or a masonry paint suitable for concrete. On any side in contact with soil, a waterproofing slurry is advisable. Otherwise moisture rises and the render flakes off.

The build goes fastest on level ground, with good access and a foundation at exactly the right height. On the other hand, the work gets harder with:

• A slope, where the foundation and wall have to be stepped and more cut blocks are needed
• Many corners, curves, gates and ends over a short length
• Poor access, when blocks and concrete have to be carried through the garden by hand
• An uneven or old foundation that must be levelled or partly replaced
• Demolition and removal of an old fence, or tree and shrub roots in the fence line
• Services in the fence line that force hand digging or a change of route
• Higher finish demands, for example a fair-faced block that must not carry a single drop of concrete

With DT 12/15, the light blocks and lower concrete volume help. But if a thin block is asked to do what belongs to the thicker variant, the cost simply moves into more complex reinforcement, extra piers and repairs.

A block wall is usually only one part of a fence. Before filling, it must be clear which infill system goes on the plinth. Anchors for panels, mesh posts or brackets for timber are set into the fresh concrete or drilled later. Drilling into a thin wall risks spalling the edge, so it is worth sorting out fixings in advance.

Also coordinate:

• Conduits for power to the gate, lighting, doorbell or video intercom, laid before filling, with the wiring done by an electrician
• The place for a letterbox and recesses for electricity or gas meter boxes, whose position the network operator decides
• Piers for the gate and side gate, which for a heavier gate are built from thicker blocks, not DT 12/15
• Drainage along the fence, so water from paved areas does not run to the foot of the wall
• Landscaping, paving and edging that will meet the fence later

• Shallow foundation: after a few winters the fence leans or cracks, because freezing soil heaves it.
• No starter bars from the foundation: the wall is only stuck to the foundation, and wind or an impact can push it over.
• Coarse aggregate in narrow cores: voids stay in the wall, bare reinforcement rusts and rust-stained water seeps to the surface.
• Filling the full height at once: blocks bulge, open up or the wall moves out of line.
• No movement joints on a long run: vertical cracks appear at random points. Joints usually go in roughly every few metres, and the manufacturer gives the exact spacing.
• Coping without overhang or drip: water gets into the top of the wall, the render falls off and the concrete suffers frost damage.
• Ordinary render without waterproofing near the ground: the base stays wet and shows efflorescence and moss.
• A thin wall used as a retaining wall: under earth pressure it tips over, even if it is low.

Laying and filling thin blocks is fairly quick. Digging, the foundation and waiting for concrete take more time. The foundation should set for at least a few days before blockwork starts, and concrete suppliers usually advise loading it only once it has gained enough strength. In summer, protect the concrete in the blocks from drying too fast: keep the wall damp and covered for a few days. In winter, at temperatures close to freezing, concrete is not poured without protective measures.

Fence panels usually go on after a few days. Render or cladding normally follows after several weeks, once the concrete has cured and dried. Concrete reaches its full design strength after roughly 28 days.

A handy DIYer can build a low plinth under mesh or panels alone, provided a finished, cured foundation with starter bars is already in place. Do not take on tall solid walls, retaining walls or any work near underground services yourself.

1. Check the level and height of the foundation and mark the highest point.
2. Stretch a string line along the whole fence between firm pegs.
3. Mix cement mortar and bed the first course exactly to the line.
4. Check each block with a spirit level along and across.
5. Let the mortar under the first course set, usually until the next day.
6. Lay the following courses dry with staggered joints.
7. Place horizontal bars in the grooves and tie in the vertical bars.
8. Prepare concrete with fine aggregate or order a suitable ready mix.
9. Fill in layers and rod each layer with a bar.
10. Check line and plumb after every layer.
11. Set the anchors for the fence infill into the fresh concrete.
12. Cover the wall with sheeting and keep it damp for a few days.
13. Once cured, fix the copings on frost-resistant adhesive.

The simplest check is to look along the wall from one end. The line should be straight, without waves or bulges. Check plumb with a spirit level at several points and the top edge along the whole length. Tapping a block with a hammer reveals places where concrete is missing, because they sound hollow. Look at the block joints: no concrete should have leaked out, and no reinforcement should be visible. On the copings, check overhang, fall and complete joints.

• The fence stands on the agreed boundary and on the agreed line
• The wall is plumb, straight and free of bulges
• The top edge is level or stepped evenly with the ground
• Movement joints are in the agreed places
• No reinforcement or empty voids are visible at the joints
• Anchors for the infill are set at the correct spacing
• Conduits for power are in place and clear
• Copings overhang, have a drip and sit firmly
• The site is tidy, with concrete residue removed from walls and ground
• The contractor has told you when the wall can be rendered and loaded