Steel reinforcement — rebar (reinforcing bar) or fabric mesh — works with concrete in tension. Concrete is strong in compression but weak in tension; steel carries the tensile stresses, allowing beams, slabs and retaining walls to span or resist bending forces that unreinforced concrete could not handle. Specifying the wrong size, grade or cover is a common cause of structural failure and is one of the items Building Control inspects during a concrete pour.
In the UK, virtually all structural rebar is grade B500B to BS 4449:2005+A3:2016, with a characteristic yield strength of 500 N/mm². The “B” suffix denotes ribbed (deformed) bar, which bonds mechanically to concrete. Smooth round bar (R-bar) is now rarely used in new construction.
Rebar Grade B500B
BS 4449:2005+A3:2016 defines B500B as:
| Property | Value |
|---|---|
| Characteristic yield strength (f_yk) | 500 N/mm² |
| Tensile strength (f_tk) | ≥ 540 N/mm² |
| Elongation at max force (A_gt) | ≥ 5.0 % |
| Bendability class | B (moderate ductility) |
B500B replaced the old Grade 460 (to BS 4449:1988) in UK practice. The higher yield strength means you can use less steel by area for the same design force — all structural calculations to Eurocode 2 (BS EN 1992-1-1) should use 500 N/mm².
Bar is supplied in standard 12 m lengths and can be cut and bent to schedule by a steel stockist. For domestic work, pre-cut coupons or fabric mesh are often more practical than loose bars.
Bar Sizes: H8 to H25
The “H” prefix (formerly “T” for High Yield in older specs) denotes B500B ribbed bar. Common diameters:
| Bar Size | Nominal Diameter (mm) | Cross-sectional Area (mm²) | Weight (kg/m) | Typical Use |
|---|---|---|---|---|
| H8 | 8 | 50.3 | 0.395 | Links/stirrups, crack control in slabs |
| H10 | 10 | 78.5 | 0.617 | Lightly loaded slabs, ground beams (links) |
| H12 | 12 | 113.1 | 0.888 | Ground beams, retaining wall stems, lintels |
| H16 | 16 | 201.1 | 1.579 | Raft edge beams, pile caps, columns |
| H20 | 20 | 314.2 | 2.466 | Heavily loaded beams, suspended slabs |
| H25 | 25 | 490.9 | 3.854 | Transfer beams, large pile caps, bridge elements |
For domestic extensions, H10 and H12 are the most common bar sizes; H16 appears in edge beams and pile caps. H8 is used mainly for links (shear reinforcement in beams) and as nominal crack-control steel.
Minimum bar diameter is typically 8 mm in a structural concrete element. Maximum bar diameter is usually governed by bond length and congestion rather than supply.
Fabric (Mesh) Reinforcement: A142 to A393
Fabric reinforcement (also called fabric mesh or wire mesh) is welded square-mesh fabric to BS 4483:2005. It is faster to place than loose bars and ideal for slabs.
The designation encodes the cross-sectional area of steel per metre width:
| Fabric Ref | Bar Diameter (mm) | Bar Spacing (mm) | Steel Area (mm²/m) | Weight (kg/m²) | Typical Use |
|---|---|---|---|---|---|
| A142 | 6 | 200 × 200 | 142 | 2.22 | Ground-bearing slabs ≤150 mm, screed reinforcement |
| A193 | 7 | 200 × 200 | 193 | 3.02 | Ground-bearing slabs 150–200 mm, paths, drives |
| A252 | 8 | 200 × 200 | 252 | 3.95 | Raft slabs, lightly loaded suspended slabs |
| A393 | 10 | 200 × 200 | 393 | 6.16 | Heavily loaded raft slabs, industrial floors |
Standard sheet size is 4.8 m × 2.4 m (11.52 m²). For a 20 m² ground-bearing slab, allow two sheets of A142 with 200 mm lap at joints = roughly 2.5 sheets ordered.
A142 is the minimum specified by most structural engineers for a 100–150 mm domestic ground-bearing slab. If the slab is designed to span between piles or over a void, A252 or A393 with loose top and bottom bars is typically required — the mesh alone will not provide the calculated span capacity.
Concrete Cover Requirements
Cover (the distance from the outer face of the concrete to the nearest reinforcement surface) protects steel from carbonation and chloride attack. Insufficient cover leads to corrosion, cracking and ultimately spalling.
Minimum cover requirements to BS EN 1992-1-1 and the UK National Annex:
| Element | Exposure Class | Minimum Cover (c_nom) |
|---|---|---|
| Ground-bearing slab (on DPM) | XC2 | 25 mm |
| Foundation in contact with ground | XC2/XC3 | 40–50 mm |
| External beam/column | XC3 | 35 mm |
| Coastal/de-icing salt exposure | XD1/XS1 | 45–50 mm |
The c_nom value includes a tolerance allowance (Δc_dev) of 10 mm for site work. Concrete spacers (plastic “chairs” or pre-cast mortar spacers) must be used — never use stones or broken tile.
For a domestic strip or trench-fill foundation cast directly in the ground without blinding, 50 mm cover to any reinforcement is standard and required by most Building Control officers.
Cost Per Tonne (2026)
| Supply Form | Approximate Cost |
|---|---|
| Stock length bar (12 m, loose) | £620–£780/tonne |
| Cut and bent to schedule | £700–£950/tonne |
| A142 fabric (per tonne) | £750–£900/tonne |
| A142 fabric (per sheet, 4.8 × 2.4 m) | £35–£55/sheet |
| A252 fabric (per sheet) | £50–£75/sheet |
Prices vary with global steel price. Rebar is traded on commodity markets and costs can shift 10–20% within a year; always get a current quote from your steel stockist. For small domestic quantities, builders’ merchants often charge a premium over steel stockholders — for more than 2–3 tonnes, a direct order from a stockholder with cut-and-bend service is usually cheaper.
Where Rebar is Required in Domestic Construction
Not all concrete elements in a house need reinforcement. Unreinforced concrete is acceptable for:
- Blinding layers
- Mass-fill trench-fill foundations (where the concrete acts in pure compression with no significant bending)
- Hardcore binding
Reinforcement is required or advisable for:
- Ground-bearing slabs (nominal A142 mesh to control thermal and shrinkage cracking)
- Raft foundations (designed reinforcement, usually both top and bottom layers)
- Retaining walls (horizontal and vertical bars)
- Lintels (pre-cast or in-situ, steel in the tension zone)
- Ring beams over large openings
- Pile caps and ground beams connecting piles
A structural engineer will produce a reinforcement schedule (drawing and bar bending schedule) for any designed element. The schedule lists every bar by type, size, shape code (to BS 8666:2020) and quantity — this is what you give to the steel fabricator.
Key Standards
- BS 4449:2005+A3:2016 — steel for reinforcement of concrete; B500B specification
- BS 4483:2005 — steel fabric for reinforcement of concrete (mesh designation system)
- BS 8666:2020 — scheduling, dimensioning, bending and cutting of reinforcement
- BS EN 1992-1-1:2004 (Eurocode 2) — design of concrete structures; cover, lap lengths, anchorage
- BS EN 13670:2009 — execution of concrete structures; workmanship and inspection
