Concrete Products
Ready-mix concrete grades, rebar and mesh reinforcement, formwork, spacers, and damp-proof membrane for extension foundations and slabs.
Concrete is the first material that goes into an extension, and one of the few you cannot correct once it has set. Before your builder orders a single cubic metre, it helps to know the difference between C20, C25 and C30. Foundations are normally C25/30, as specified by your structural engineer, and if the delivery ticket does not show the right grade, the only way to prove what went into the trench is to core-drill the set concrete and send samples to a lab.
These guides cover everything placed or cast as part of your extension's concrete structure: the ready-mix grades used for foundations, slabs and drainage haunching; the steel reinforcement specified by your structural engineer; the formwork and accessories that hold the pour in shape and keep the steel in position; and the damp-proof membrane that protects the slab from ground moisture. Each page explains what the product does, how to read a specification or delivery ticket, and what to check on site.
Where to start
Read C25 ready-mix concrete first, because it is the grade most extension foundations use and the page explains what the delivery ticket must show. Then read damp-proof membrane, since a membrane that is not connected to the wall damp course is a common cause of damp in a finished extension. Finish with concrete spacers: cheap, easy to leave out, and a frequent reason reinforcement fails inspection.
The guides, grouped by job
Ready-mix grades. C20 for paths, shed bases and drainage haunching, C25 for most foundations, and C30 when your engineer specifies something stronger.
Reinforcement. 12mm rebar for foundations and 16mm rebar for pad foundations and ring beams, plus welded mesh in three weights: A142 for crack control, A193 for floor slabs, and A252 for rafts and heavily reinforced slabs. Tie wire holds the steel together and spacers hold it at the right cover.
Formwork and the slab. Formwork timber for shuttering, including when you can do without it, and the DPM under the floor. The thin layer of blinding concrete that goes under reinforcement is covered with the aggregates.
Where these fit in the build
Concrete and rebar go in at foundations and footings, after the foundation inspection of the open trenches. Mesh, DPM and the slab belong to the ground floor oversite and floor build-up. To size the pour, use the foundation concrete calculator, the foundation cost calculator and the floor build-up estimator.
Concrete Spacers: The Cheapest Thing That Fails the Most Inspections
UK guide to concrete cover spacers for reinforcement. Cover depths, types (castle, clip-on, chair), quantity calculation, building control requirements, and what to buy. From £18.00 – £26.40 per 100.
Damp Proof Membrane (DPM): The Sheet Under Your Floor That Prevents Rising Damp
Complete UK guide to 1200 gauge DPM for extension floors. Correct floor build-up order, DPC connection detail, building control hold points, and prices from £36-72 per roll.
Formwork Timber: When You Need It, How to Set It Up, and When to Skip It
UK guide to timber formwork (shuttering) for extension foundations. Board sizes, staking, bracing, release agent, removal timing, and the critical question: do you actually need it? From £2.40 – £3.60 per metre per metre.
A142 Mesh Reinforcement: Crack Control for Concrete Floor Slabs
UK guide to A142 steel mesh reinforcement. BS 4483 specification, correct placement depth, overlap rules, sheet sizes, and how many you need. From £19 – £30 per sheet.
A193 Mesh Reinforcement: The Steel Grid Inside Your Floor Slab
UK guide to A193 welded steel mesh for extension floor slabs. Specs, sheet sizes, how many you need, overlap rules, spacer placement, and what building control checks before the pour. From £39.60 – £48.00 per sheet.
A252 Mesh Reinforcement: Heavy-Duty Steel Fabric for Raft Foundations and Reinforced Slabs
UK guide to A252 mesh reinforcement. 8mm wire at 200mm centres, from £51.60 – £90.00 per 4.8x2.4m sheet. Specifications, lap lengths, double-layer placement, and what building control checks.
Ready-Mix Concrete C20 (GEN 3): What It's For and When You Need Something Stronger
C20 concrete costs £90 – £115 per m³ per m³ delivered. It's for shed bases, pathways, and drainage haunching, not extension foundations. Here's how to order it right.
Ready-Mix Concrete C25: The Standard Grade for UK Extension Foundations
The definitive UK guide to C25/30 (RC 25) ready-mix concrete. Correct designations, how much you need, what it costs from £90 – £130 per m³, and what your delivery ticket must show.
Ready-Mix Concrete C30: When Your Structural Engineer Says You Need More Than Standard
C30 concrete costs £120 – £145 per m³ per m³ delivered. When your SE specifies it, what to order, and the PAV1 trap that catches homeowners.
12mm Rebar (T12): The Steel Inside Your Foundations
UK guide to 12mm reinforcement bar. Specifications, cover requirements, lap lengths, ordering pre-bent vs cutting on site, and what building control checks before the pour. From £8.40 – £12.00 per 6m length.
16mm Rebar (T16): When Your Foundations Need More Steel
UK guide to 16mm reinforcement bar for pad foundations and ring beams. Specifications, lap lengths, handling weight, and what triggers your engineer to specify T16 over T12. From £11 – £20 per 6m.
Tie Wire: The Cheapest Material That Holds Your Foundation Together
UK guide to rebar tie wire. Black annealed 16-17 gauge, how much you need, tying methods, BS 7973-2 frequency rules, why galvanised wire fails, and what to buy. From £8 – £15 per 2kg coil.