Formwork for foundations and walls

In the price list, formwork is usually measured in square metres of surface that is in direct contact with the concrete. For a wall, both faces are normally counted. A wall ten metres long and one metre high therefore means roughly twenty square metres of formwork, not ten. For a strip foundation, the side faces are counted if they are formed. For a slab, only the perimeter edge counts.

Before you sign a contract, check exactly what the item covers. It usually includes:

• Erecting the formwork, including bracing, spacers and ties
• Striking, which means taking the formwork down after the concrete has hardened
• Cleaning and removal of the formwork material from site
• Material: the contractor's own boards, hired system formwork, or material you supply yourself

In practice, one company often prices striking separately while another includes it. The quotes then look very different, even though they cover the same work. Also ask about openings, internal corners and steps in the wall. A good quote states clearly whether they are included.

Traditional timber formwork

This is made from softwood boards, usually around 24 mm thick, or from water-resistant plywood, fixed to timber bearers, battens and stakes. The crew builds it on site to suit the shape of the building. Its strength is flexibility. It copes with curves, sloping walls, irregular plans and small jobs where bringing in a system would not pay off. On the downside, it takes more labour, uses more material and leaves a rougher surface with board marks and joint lines. For foundations that will end up below ground, that does not matter.

Framed system formwork

This consists of steel or aluminium frames with a facing panel. The panels connect with clamps and are pulled together with tie rods. On building sites you will often see systems from manufacturers such as Peri or Doka, but there are many others. Assembly is quick, the walls come out straight and the surface is clean. The system also handles higher concrete pressure. It makes sense for straight basement walls, longer retaining walls and anywhere the wall will not be plastered. It is usually hired, so good planning matters. Every extra day on site adds to the bill.

Timber beam and large panel formwork

This is a middle option: timber beams, often of the H20 type, combined with three-ply or film-faced panels. It is used for taller walls and where framed panels do not fit the dimensions. On family houses you see it less often.

Permanent formwork

Hollow concrete formwork blocks are widely used. They are stacked dry, reinforced and filled with concrete, and they stay in the wall for good. For basements, insulated concrete formwork (ICF) blocks made of polystyrene, or blocks made of wood-chip concrete, are also common because they insulate at the same time. Nothing needs striking and a smaller crew can manage the work. However, such a wall is usually priced as a separate item, not as formwork per square metre. On firm ground, strip foundations are also often cast straight into the trench without any formwork. This only works in cohesive soil with straight trench sides.

How to decide

For strip foundations in cohesive soil, you often need no formwork at all. Formwork is needed for the part above ground and where the soil crumbles. For basement walls that must be watertight, a cast in situ wall in system formwork is usually preferred, often with waterstops in the construction joints. For a fence base or a low garden wall, timber or blocks are enough. The final decision belongs in the design, set by the structural engineer.

A crew that turns up to an unprepared site loses time, and waiting time usually ends up on your invoice. Before work starts, this should be ready:

• Drawings with structural design: dimensions of foundations and walls, reinforcement, concrete grade and the position of all openings and ducts
• Setting out by a surveyor, especially near the boundary and for the first foundations, where an error shifts the whole house
• A finished excavation at the correct depth, with a base free of softened, muddy soil and standing water
• Blinding concrete under the slab or walls, if the design calls for it, so the formwork has a level base
• Access for the delivery van with material and later for the truck mixer and concrete pump
• Ducts and sleeves from the plumber and electrician for drainage, water, the electrical supply and earthing

The formwork itself does not need any approval. Foundations and walls are part of the building, though, so local building regulations apply. Whether you need planning or building approval, or only a notification, depends on the type and size of the project. Always check with your local building authority rather than relying on what friends tell you. If you live in a block of flats and the work concerns, for example, a cellar wall or a garage on shared land, agree it in advance with the building management or owners' association.

Foundation formwork

First, string lines are stretched between profile boards according to the setting out. The edge of the foundation is then transferred down with a plumb bob. With timber formwork, stakes are driven into the ground and boards are nailed to them. Spacers cut to an exact length are wedged between the two sides to fix the width of the strip. Cross pieces tie the two sides together at the top so the concrete cannot push them apart. The top edge is levelled with a level or laser, because the concrete will be screeded off to that edge. For a ground slab, the crew builds an edge frame and props it from the outside with raking braces.

Wall formwork

Walls follow a similar process, only everything is taller and the forces are greater. One face is erected first, aligned and braced back to the ground with push-pull props. Then the reinforcement goes in, fixed either by the same crew or by steel fixers. Spacers are clipped onto the steel to guarantee the specified concrete cover. Only then is the second face closed. The two faces are pulled together with tie rods and nuts. The rods pass through plastic spacer tubes so they can be pulled out after the pour. The holes are later sealed with plugs or filler.

Checks before the pour

Before the concrete arrives, the crew checks plumb with a spirit level or plumb bob, straightness along the length with a line, dimensions, the height of the top edge, the strength of the joints and the sealing of gaps. Every surface that will touch the concrete is coated with release agent so the concrete does not stick. The inside of the formwork is cleaned of sawdust, soil and bits of wire. With timber boards in hot weather, the formwork is usually dampened slightly so it does not draw water out of the concrete.

Pouring and striking

Concrete is placed in layers and compacted with a poker vibrator. Someone from the formwork crew should stay by the wall during the pour and watch for joints that move or cement slurry that leaks out. Side formwork on foundations and walls usually comes off after roughly one to three days, depending on temperature. In cold weather you wait longer. After striking, the concrete needs curing: keep it damp or cover it with sheeting so it does not dry out too quickly.

For the same area, the amount of labour, and so the final cost, can vary considerably. The job is easier with:

• A simple rectangular plan with long straight runs
• Repeated dimensions, so the formwork can be reused several times in a row
• Level blinding concrete to build from
• Good access and space to store material right next to the wall

The job is harder with:

• Curves, sloping walls and many corners, steps and recesses
• Many openings and penetrations that each need their own formwork
• Tall walls that need scaffolding or working platforms with guardrails
• A narrow trench where the crew can barely turn round, or a sloping plot
• Fair-faced concrete that will not be plastered and needs precise joints and high quality panels
• Work in winter or in mud, when the formwork needs protecting and the concrete needs longer curing

Deeper excavations must be secured against collapse, either with trench support or with safely sloped sides. This is a safety matter, not just a matter of convenience, and it belongs in the site preparation.

Formwork sits in the middle of the chain, and almost everything depends on it. Before it come the surveyor, the excavation and the blinding concrete. During formwork, steel fixers place the reinforcement, and the plumber and electrician install ducts and the earthing strip. After the formwork come the pour, striking, then waterproofing, basement insulation and backfilling.

Why does this matter? If a drainage sleeve is forgotten, the wall has to be core drilled afterwards. In a watertight wall, every extra hole is also a leak risk. That is why it pays to have the plumber and electrician check the wall themselves before the second face of the formwork is closed.

An experienced crew usually forms the foundations of a typical family house in one to several days, depending on size and complexity. Basement walls in system formwork are normally built in sections. One section is formed and poured, and once it has hardened the formwork moves to the next. Concrete can be struck after a few days, but it usually only reaches its design strength after about 28 days. Until then, do not load it with backfill or further structures beyond what the structural engineer allows.

• Weak ties or bracing. Concrete presses on formwork harder than it seems, especially with fast pouring and fluid mixes. The result is a bulging wall or, at worst, a burst form and concrete in the trench.
• Unsealed joints. Cement slurry runs out through the gaps and leaves honeycombing in the wall, porous patches of bare aggregate. They are weaker and let water through.
• Missing spacers. The reinforcement rests directly on the formwork and shows on the surface after striking. Over time it rusts and the concrete around it spalls off.
• Formwork out of plumb or off line. A small error at foundation level carries through to the masonry and up to the roof. The bricklayer then has to make up with mortar and adhesive what could have been straight.
• Forgotten openings. Drilling afterwards damages the reinforcement and, in a basement, threatens watertightness.
• A poor top edge. If the formwork is not level, the concrete will not be either. The first course of masonry then needs a thick bed of mortar to correct it.
• Striking too early. Edges break off, the surface tears away with the boards, and in frost the concrete may stay permanently weaker.
• No release agent. Concrete sticks to the boards, the surface tears during striking and the formwork is ruined.

Leave the formwork for load-bearing foundations and walls of a house to a professional contractor working to the structural engineer's design. A low base for a fence, a garden wall or a plinth up to about half a metre high is something you can manage yourself, with basic tools and a helper. Do not work in deep trenches that are not secured against collapse.

1. Set out the centre line of the base with a string line on pegs and check right angles, for example by measuring the diagonals.
2. Dig the trench to the frost-free depth recommended for your fence type by the manufacturer or the design, and level the bottom.
3. Prepare boards or water-resistant plywood and coat the side facing the concrete with release agent.
4. Drive stakes on the outside of the formwork, usually roughly every 60 to 80 cm, and nail the boards to them.
5. Wedge spacers of the exact base width between the boards and tie both sides together at the top with cross pieces.
6. Check plumb, straightness and a level top edge with a spirit level and a taut line.
7. Seal larger gaps and bank soil against the bottom of the formwork so concrete cannot run out underneath.
8. Place the reinforcement according to the fence manufacturer's instructions or the design, and support it on spacers.
9. Set anchors for the posts or a duct for a gate cable, if you will need them.
10. Pour in layers, rod each layer with a bar or compact it with a vibrator, and tap the boards with a hammer.
11. Screed the surface off with a straight batten along the top edge, then keep the concrete damp or covered with sheeting for several days.
12. Strike carefully after a few days depending on temperature, then clean the boards and store them for reuse.

Inspect formwork twice: before the pour, while everything can still be fixed, and after striking, when you see the result. Do the first check with the site manager or supervising engineer, and on a larger build also with the structural engineer, who checks the reinforcement.

• Formwork dimensions and position match the drawings and the setting out
• Formwork faces are plumb and straight along their full length
• The top edge is level and at the correct height
• Tie rods, spacers and braces are fitted at sufficient spacing
• Joints are sealed and the formwork is closed at the base
• Reinforcement sits on spacers and touches the formwork nowhere
• All ducts, openings and earthing are in place as designed
• The inside of the formwork is clean and the faces are coated with release agent
• After striking, there is no honeycombing, visible reinforcement or bulging on the surface
• Edges and corners are intact, not chipped
• Tie holes are sealed and the area is cleared of formwork debris