Concrete Rebar Buying Guide: Choosing the Right Reinforcing Steel Bar
Introduction
Rebar — short for reinforcing bar — is the steel skeleton inside concrete. Without it, concrete cracks and fails under tension. With the right rebar correctly placed, a concrete slab, footing, or foundation can last decades under heavy load. The tricky part is knowing which diameter to use, what concrete cover to allow for, and which accessories ensure the steel ends up in the right position when the concrete is poured.
This guide by Cocus covers the main types of reinforcing steel bar we supply, plus the spacers and accessories that ensure correct concrete cover — so your project meets structural requirements from the ground up.
1. Standard Rebar — 8mm to 12mm Diameter
Smaller diameter rebar (8mm–12mm) is the workhorse for residential and light commercial concrete work. Driveways, garden footings, path edgings, retaining wall bases, and small ground floor slabs all typically use 8mm or 10mm bar. The 12mm bar steps up the capacity for slightly more demanding applications — garage bases, workshop floors, and agricultural buildings.
Reinforcing Steel Bar — 8mm, 10mm & 12mm
Manufactured to BS 8666:2005, Grade B500B — the standard specification for reinforcing steel in the UK. Ready-cut to 450mm or 1000mm lengths; no cutting or bending required on site. 8mm bar starts from £0.96 per single bar; 10mm from £1.48; 12mm from £1.82. Available in single bars or 10-packs for larger pours.
2. Heavy Duty Rebar — 16mm to 20mm Diameter
Larger diameter bar (16mm and 20mm) is required for structural applications where the slab or footing will carry significant load — commercial buildings, heavily loaded floor slabs, bridge abutments, and deep foundations. The step up in diameter substantially increases the tensile and compressive reinforcement the bar provides. At 16mm and above, rebar is typically specified by a structural engineer rather than chosen off the shelf.
Reinforcing Steel Bar — 16mm & 20mm
The same BS 8666:2005 Grade B500B specification as the lighter range, in heavier gauges for structural applications. 16mm single bars from £2.72 (450mm) or £4.72 (1000mm); 20mm from £4.50 (450mm) or £7.44 (1000mm). Order from the structural engineer’s specification rather than on judgement alone for these sizes.
3. Flat Base Concrete Mesh Spacers
Correct concrete cover — the depth of concrete between the rebar and the surface — is critical. Too little and the steel corrodes; too much and the reinforcement is less effective. Concrete spacers hold rebar at the correct depth during the pour, preventing it from sinking to the bottom of the slab. Flat base spacers are specifically designed for use on soft ground, sand blinding, or damp-proof membranes where a standard chair or circular spacer would sink.
Flat Base Concrete Mesh Spacers (Grade Plates)
Designed for use on soft substrates — DPMs, sand blinding, and soft ground where standard spacers would tip or sink. Available in 25/35mm cover sizes. Sold in packs of 25, 50, 100, 250, and 500, from £2.99 for 25 up to £59.80 for 500. Order approximately one spacer per 400mm×400mm grid square for a typical domestic slab.
4. Cartwheel Rebar Spacers
For vertical applications — columns, walls, and pre-cast concrete — cartwheel (circular) spacers are the correct choice. They snap onto the rebar and rotate freely, maintaining consistent cover from every angle regardless of how the bar is oriented.
Plastic Cartwheel Rebar Spacers
25mm cover, suitable for bar diameters 4mm–12mm and 12mm–20mm. Designed for columns, walls, and vertical reinforcement where cover must be maintained in multiple directions. From £2.50 for 25 spacers. Use one spacer per metre of bar in vertical applications, or per engineer's specification.
5. Rebar & Road Pin Protection Caps
Any rebar protruding above slab level or above ground is an impalement risk. CDM 2015 and HSE guidance require all protruding rebar to be capped at all times on site. Caps must be fitted as soon as bar is placed and must remain in place until concrete is poured or the bar is removed.
High-Visibility Rebar Protection Caps
"Mushroom" caps in high-visibility colour, designed to fit over protruding rebar and road pins. Available in 8mm–16mm and 16mm–32mm sizes to cover the full rebar range. From £0.60 each, in packs of 1, 5, 10, 25, and 50. Order one per exposed bar end — there's no compliance argument for leaving them off.
6. How to Choose the Right Rebar
Rebar selection should be driven by the structural requirements of the pour, not personal preference. Here are the key decisions:
1. Follow the Structural Specification
For any structural application — foundations, load-bearing slabs, retaining walls — rebar size, spacing, and concrete cover should be specified by a structural engineer. In the UK, residential buildings require Building Regulations approval where structural concrete is involved. Use the engineer's schedule as your order list; don't substitute diameters without approval.
2. Diameter for Non-Structural Work
For non-structural domestic work where you're choosing independently: use 8mm bar for light slabs, paths, and footings in well-drained ground; 10mm for driveways and garage bases; 12mm for agricultural building floors and areas subject to vehicle loading. If in any doubt, move up a size — the material cost difference is small relative to the cost of a failed slab.
3. Length: 450mm or 1000mm?
450mm bars are useful for footings, narrow sections, and situations where shorter laps are needed. 1000mm bars suit most slab applications and reduce the number of laps required. Bars can be lapped (overlapped and tied together) to achieve longer runs — a minimum lap length of 40x the bar diameter is standard (e.g. 400mm for 10mm bar).
4. Don't Skip the Spacers
Correct concrete cover is a structural requirement, not an optional extra. Without spacers, rebar sinks to the bottom of the pour and provides almost no tensile reinforcement. Use flat base spacers on DPM and soft ground, cartwheel spacers for vertical reinforcement, and check your cover depth against the engineer's specification before pouring.
7. Concrete Rebar Frequently Asked Questions
Q1: What diameter rebar do I need for a garden or driveway slab?
For a domestic driveway or garden slab subject to pedestrian and light vehicle traffic, 10mm bar at 150–200mm centres is the typical specification. For areas with regular heavy vehicle access (HGVs, plant), 12mm or 16mm bar at tighter centres may be required — consult a structural engineer if in doubt.
Q2: What does BS 8666:2005 mean on rebar?
BS 8666:2005 is the British Standard for scheduling, dimensioning, bending, and cutting steel reinforcement for concrete. Grade B500B specifies the steel's yield strength (500 MPa) and ductility class (B). All Cocus rebar is manufactured to this standard, ensuring it meets UK building regulations and structural specifications.
Q3: What is concrete cover and why does it matter?
Concrete cover is the depth of concrete between the outer face of the rebar and the surface of the slab or element. Minimum cover protects the steel from corrosion (typically 25–40mm for internal slabs, more for exposed or aggressive environments). Use concrete spacers to maintain the correct cover during the pour.
Q4: Can I cut rebar to length on site?
Yes — rebar can be cut with an angle grinder, hacksaw, or rebar cutter. Our rebar is supplied in ready-cut 450mm and 1000mm lengths to reduce on-site cutting. For bespoke lengths or bent shapes (L-bars, U-bars, stirrups), rebar can be bent cold using a bar bender, or ordered pre-bent to a schedule from a steel stockholder.
Q5: Do I need to cap protruding rebar on site?
Yes — HSE requires all exposed rebar ends to be capped at all times. Use high-visibility protection caps from the moment bar is placed until it is covered by concrete. This applies even for very short periods — impalement injuries from unprotected rebar are a significant cause of serious site accidents.
Q6: Can I buy rebar from Cocus?
Yes — we stock reinforcing steel bar in all standard diameters from 8mm to 20mm, in 450mm and 1000mm lengths, with next-day delivery available from our Manchester warehouse. We also stock the full range of concrete accessories including spacers and protection caps.
Conclusion
Rebar is one of those products where getting it right at the start saves significant cost later — a failed or cracked slab is expensive to repair and often impossible to fix without breaking out and starting again. Use the right diameter for your application, maintain correct cover with spacers, cap every exposed end, and follow the structural engineer's specification for anything load-bearing. The material cost is a small fraction of the overall job; it's not worth compromising on.
Browse our full range of concrete rebar and reinforcement accessories at Cocus — including construction site equipment shipped from our Manchester warehouse with next-day delivery available on selected products.