Intermediate

Permanent Formwork for Foundations: Types, Installation and Benefits

12/04/2026 12 min read

You’re about to pour your foundations and wondering whether you need to erect timber shuttering panels, prop them up, strip them… then do it all over again for the next lift? Permanent formwork is the solution that radically simplifies this stage: instead of removing the formwork once the concrete has set, it stays in place and becomes an integral part of the structure. Less handling, less time, and often better thermal performance. This article explains the different types of permanent formwork, when to use them, and how to install them correctly.

What is permanent formwork

Traditional formwork (timber or metal) acts as a temporary mould: you pour the concrete, wait for it to set, then strip the form. Permanent formwork, on the other hand, is never removed. It serves three functions simultaneously:

Formwork: it contains the fresh concrete during the pour
Protection: it protects the hardened concrete (against moisture, frost, impact)
Insulation (depending on the type): EPS (polystyrene) formwork adds an integrated insulating layer

Permanent formwork vs traditional shuttering

Criterion	Traditional shuttering (timber)	Permanent formwork
Reusability	Yes (3–5 times)	No (stays in place)
Installation time	Long (erection + propping)	Short (simple stacking)
Stripping	Required (24–48 h)	None
Material cost	Low (shuttering timber)	Medium to high
Labour cost	High (fix + strip)	Low
Thermal insulation	None	Excellent (EPS) or none (concrete blocks)
Surface finish	Variable	Consistent

Tip — For a self-builder working alone or in a pair, permanent formwork saves a considerable amount of time. The higher material cost is more than offset by the labour hours saved on stripping, especially in foundations where access is difficult (bottom of trench, suspended floor void).

Types of permanent formwork for foundations

Question

Shuttering blocks (permanent formwork blocks)

A shuttering block is a hollow concrete block without a base, designed to be dry-stacked and then filled with reinforced concrete. It is the most widely used permanent formwork in self-build for foundations and substructure walls.

Characteristic	Value
Standard dimensions	500 × 200 × 200 mm or 500 × 200 × 250 mm (approx. 20 × 8 × 8 in or 20 × 8 × 10 in)
Unit weight	18–22 kg (40–48 lb)
Finished wall thickness	200 or 250 mm (8 or 10 in)
Thermal insulation	None (concrete both sides)
Unit price	£1.50–£2.50
Main use	Substructure, suspended floor void, below-ground wall

Principle: the blocks interlock via a tongue-and-groove system. You place the vertical and horizontal reinforcement bars in the cores, then pour the concrete from above. The result is a continuous reinforced concrete wall with no mortar joints.

EPS insulated panels — ICF (Insulating Concrete Form)

Insulating Concrete Forms (ICF) are expanded polystyrene panels connected by plastic ties. Concrete is poured between the two leaves, creating a wall insulated on both sides.

Characteristic	Value
Standard dimensions	1,200 × 300 mm (approx. 48 × 12 in), variable height
EPS thickness	50–100 mm per leaf (2–4 in)
Concrete thickness at core	150–250 mm (6–10 in)
Thermal insulation	R = 2.5 to 5.0 m²·K/W (R-14 to R-28 approx.)
Price per m²	£25–£50
Main use	Foundation + substructure + superstructure

Best practice — ICF formwork is particularly well-suited to foundations for homes built to current energy regulations: the integrated insulation eliminates cold bridges between the foundation and the wall, a classic weak point in traditional construction.

Closer blocks (concrete planks)

Closer blocks are thin elements (50–70 mm / 2–3 in thick) in concrete, used as external formwork for slab edges, floor perimeters or foundation heads. They do not form a complete mould — they act as a facing on one side only.

Characteristic	Value
Dimensions	500 × 200 mm, 50–70 mm thick (approx. 20 × 8 in, 2–3 in thick)
Use	Slab edge, floor perimeter
Insulation	None (except insulated EPS closer block)
Unit price	£1–£2

Composite steel decking (profiled metal sheeting)

For upper floors and elevated slabs, composite steel decking acts as permanent formwork and contributes to the structural resistance. This is not directly relevant to strip footings, but you may encounter it for an accessible suspended floor void.

Choosing the right permanent formwork for your foundations

flowchart TD A{What type of foundation?} -->|Strip footing| B{Thermal insulation budget?} A -->|Raft slab| C[Traditional shuttering
or closer blocks at edge] A -->|Substructure / suspended floor void| D{Wall height?} B -->|Standard| E[Shuttering blocks 200 mm
Classic solution] B -->|High thermal performance| F[ICF EPS panels
Integrated insulation] D -->|Less than 1 m| G[Shuttering blocks 200 mm] D -->|1 m to 2.5 m| H[Shuttering blocks 250 mm
+ reinforced rebar] D -->|More than 2.5 m| I[Structural engineer required
Blocks 250 or 270 mm] style A fill:#0F4C81,stroke:#0F4C81,color:#fff style B fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style C fill:#F58220,stroke:#F58220,color:#fff style D fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style E fill:#56C6A9,stroke:#56C6A9,color:#fff style F fill:#56C6A9,stroke:#56C6A9,color:#fff style G fill:#56C6A9,stroke:#56C6A9,color:#fff style H fill:#F58220,stroke:#F58220,color:#fff style I fill:#CD212A,stroke:#CD212A,color:#fff

Installing shuttering blocks step by step

Here is the detailed method for the most common self-build scenario: a substructure wall in shuttering blocks on a strip footing.

Materials required (for 10 linear metres / 33 ft, height 1 m / 3.3 ft = 5 courses)

Material	Quantity	Indicative price
Shuttering blocks 200 mm	100 blocks	£150–£250
Concrete C25/30	1 m³	£100–£150 (ready-mixed)
HA10 vertical rebar	25 bars × 1,500 mm	£50–£80
HA8 horizontal rebar	50 m	£30–£50
Levelling mortar	50 kg	£10–£15
Annealed tie wire	1 reel	£5–£8
Total		£345–£553

Step 1: Prepare the strip footing

The strip footing must be poured and have reached sufficient strength — a minimum of 3 days, ideally 7 days.

Check that the footing is level along its entire length
Starter bars (HA10 or HA12) must project from the footing by at least the full height of the planned wall plus 500 mm (20 in) lap length
Starter bar spacing: every 200–400 mm (8–16 in), in accordance with the reinforcement design

Warning — If you forgot to cast starter bars into the footing before it hardened, do not hammer them into the dry concrete! You must chemically fix threaded rods or rebar into holes drilled with a hammer drill. Resin mortar anchoring provides an equivalent bond to cast-in-place bars.

Step 2: Lay the first course

The first course is the most critical — it determines the alignment and level of the entire wall.

Spread a bed of levelling mortar (20–30 mm / ~1 in) on the footing
Place the first shuttering block by threading it over the starter bars
Check for level (spirit level) and plumb (plumb line or laser level)
Adjust with packers if necessary before the mortar sets
Continue block by block, checking alignment with a string line

Tip — Always start with the corners and high points. Stretch a string line between the corner blocks to align the intermediate blocks. The logic is the same as laying a blockwork wall, but without mortar between subsequent courses.

Step 3: Stack the subsequent courses

Subsequent courses are stacked dry (no mortar) thanks to the tongue-and-groove system
Offset by half a block on each course (running bond) for structural integrity
Place horizontal bars (HA8 minimum) in the recesses every 2 courses
Tie the horizontal bars to the vertical bars with annealed tie wire
Check plumb after every course

Step 4: Pour the concrete

This is the most critical stage. The concrete exerts significant lateral pressure on the blocks — pouring too quickly can cause blowouts.

Never pour more than 3 to 4 courses at a time (600–800 mm / 24–32 in of height)
Use C25/30 concrete with a S3 or S4 slump class (fluid mix) to fill the cores properly
Compact with a poker vibrator 25 mm diameter (the 200 mm cores are narrow)
Vibrate in short passes (10–15 seconds per point) without touching the blocks
Wait a minimum of 24 hours before pouring the next lift
If the wall is more than 4 courses high, repeat in lifts

Warning — Never let the poker vibrator touch the shuttering block directly: it can crack or displace the block. Plunge the poker into the centre of the core, between the reinforcement bars. If you do not have a 25 mm poker vibrator, use a rebar rod to rod the concrete manually — it is slow but effective.

Conseil

Best practice — Order your ready-mixed concrete with a long chute (or a concrete pump) to pour directly into the cores from the top of the wall. Filling by wheelbarrow and bucket is possible but slow and tiring. For 10 linear metres at 1 m height, expect approximately 1 m³ of concrete — half a mixer truck is sufficient.

Step 5: Finishing and waterproofing

Finish the wall with a horizontal ring beam at the top (U-shaped lintel blocks, reinforced and concreted)
After a minimum of 7 days’ curing, apply a waterproofing coat (bituminous or resin-based) to the below-ground face
Fix a studded drainage membrane (e.g. Delta-MS type) against the buried face to protect the waterproofing
Install perimeter drainage (100 mm / 4 in perforated drain pipe, gravel, geotextile)

Special case: ICF formwork for foundations

EPS panel formwork (ICF) follows a different logic to shuttering blocks.

Assembly

Panels arrive pre-assembled (two EPS leaves connected by plastic ties)
Stack courses offset by half a panel (like LEGO bricks)
The ties create the void for the concrete and serve as supports for the reinforcement
Place horizontal and vertical bars between the leaves before pouring
Pour concrete from the top in lifts of 1,200 mm (48 in) maximum

Specific points to watch

Bracing is mandatory: EPS panels are light and concrete exerts high lateral pressure. Install vertical props and diagonal braces every 600 mm (24 in)
Service penetrations: drill pipe and conduit openings before the pour — EPS cuts easily with a knife or saw, but is impossible to cut once the concrete has hardened
Mechanical protection: on the buried side, the EPS must be protected against puncturing by stones during backfilling. Use a studded drainage membrane

Tip — The brands Euromac2, Omnidea and Nudura are among the best known in ICF. Each system has its own ties and accessories — do not mix brands, as the interlocking details are not compatible.

Common mistakes and defects

Mistake	Consequence	Solution
Pouring too fast	Block blowout, deformation	Pour in lifts of 3–4 courses
No vibration	Honeycombing, voids in concrete	Vibrate each core, 10–15 s
Badly positioned rebar	Insufficient structural capacity	Check before pouring — impossible after
First course not level	Entire wall misaligned	Levelling mortar + laser check
No waterproofing on buried face	Water ingress, rising damp	Bituminous coat + drainage membrane
ICF without bracing	Deformation, panel blowout	Props every 600 mm

Regulations and standards

DTU 21 (execution of concrete works): applies to concrete pouring in permanent formwork
BS EN 15435: European standard for concrete shuttering blocks
Technical appraisal: ICF systems should hold a valid technical appraisal (equivalent to a CSTB avis technique) to be covered by structural warranty insurance
Current energy regulations: ICF formwork directly contributes to the thermal performance of the building fabric

Warning — If you use an ICF system for the foundations and the walls, verify that the manufacturer holds a valid technical appraisal. Without this, your structural warranty insurer may refuse cover. Standard concrete shuttering blocks, on the other hand, are covered by DTU 21 without any specific technical appraisal.

Comparative cost for 10 linear metres (height 1 m)

Solution	Materials	Estimated labour	Estimated total
Timber shuttering + poured concrete	£200–£300	8–12 h	£400–£600
Shuttering blocks	£345–£553	4–6 h	£500–£750
ICF formwork (EPS)	£500–£800	3–5 h	£650–£1,000

Permanent formwork costs more in materials, but the time saving is significant — especially when working alone. For a self-builder who values their time, the extra cost is often justified from the very first project.