This guide goes beyond the basics. It covers the specific strength rating of GEN 1 concrete, how it compares to the grades either side of it, the most common uses, and how to make sure you order the right amount.
What Is GEN 1 Concrete?
GEN 1 is a standardised concrete mix designed for general applications where the concrete does not need to carry heavy structural loads. The “GEN” stands for general, and the “1” indicates it sits one step above the lowest general-purpose mix.
In technical terms, GEN 1 has a compressive strength of 7.5 N/mm² (Newtons per square millimetre) at 28 days. That might not mean much on its own, so think of it this way: it is strong enough to hold its own weight and resist light pressure, but it is not designed to support the weight of a building or handle heavy traffic.
GEN 1 is covered by the British Standard BS 8500, which sets out the rules for specifying and producing concrete in the UK. Your building control officer or structural engineer may refer to this standard when they specify which grade to use.
How GEN 1 Compares to Other Grades
Understanding where GEN 1 sits in the range of concrete grades helps you decide whether it is the right choice or whether you should go a step up or down.
| Grade | Strength (N/mm²) | Typical Uses | When to Choose It |
|---|---|---|---|
| GEN 0 | 6.0 | Kerb bedding, non-structural fill, blinding | When no strength is needed and you just need to fill a space |
| GEN 1 | 7.5 | Trench fill, mass fill, floor blinding, drainage bedding | When you need a basic level of strength for non-structural work |
| GEN 2 | 10.0 | Drainage works, house floors with no reinforcement, garden paths | When the concrete will bear some light foot traffic or minor loads |
| GEN 3 | 15.0 | Domestic foundations, garage floors, light structural use | When building regulations require a minimum foundation strength |
As you can see, GEN 1 sits between the very lightest fill concrete and the grades that start taking on real load-bearing duties. It is stronger than a simple backfill mix but not designed for anything structural.
Common Uses for GEN 1 Concrete
GEN 1 is a workhorse mix that turns up on building sites of all sizes. Here are the jobs it is best suited for.
Trench fill. When foundation trenches are dug deeper than the structural concrete needs to go, GEN 1 is used to fill the lower section. This brings the trench up to the level where the stronger foundation concrete starts. It is cheaper than using a structural grade for the full depth, and it does the job perfectly well.
Mass fill. Any situation where you need to fill a large void or cavity with concrete, but the concrete is not carrying structural loads. This might include filling old cellars, abandoned septic tanks, or voids left by removed underground storage.
Floor blinding. A thin layer of GEN 1 is sometimes poured over compacted hardcore before the main floor slab is laid. This creates a clean, level surface for the damp-proof membrane and insulation to sit on. It also stops the membrane from being punctured by sharp stones.
Drainage bedding. Pipes and drainage channels often sit on a bed of concrete to keep them stable and at the correct fall. GEN 1 provides enough support for the pipe without being more expensive than necessary.
Packing around posts and poles. Fence posts, gate posts, and other uprights set into the ground benefit from a collar of GEN 1 around their base. It holds them firmly without the cost of a higher-strength mix.
When GEN 1 Is Not the Right Choice
Knowing when not to use GEN 1 is just as important as knowing when to use it.
Foundations for buildings. UK Building Regulations typically require a minimum of GEN 3 (or equivalent) for domestic foundations. GEN 1 is not strong enough to support the weight of a house, extension, or garage. The Planning Portal’s guide to foundation requirements explains what building control expects.
Driveways and paths with vehicle traffic. If cars or vans will drive over the surface, you need at least a C20 or C25 grade. GEN 1 will crack and break up under repeated vehicle loads.
Any load-bearing element. Beams, columns, lintels, retaining walls, and anything else that carries the weight of a structure need a structural-grade concrete, typically C25 or above.
If your builder or engineer has specified GEN 1 and you are unsure why, ask them to explain. There is always a reason for the grade choice, and understanding it helps you feel confident the right material is going into your build.
How Much GEN 1 Do You Need?
Working out the volume is straightforward. Measure the length, width, and depth of the area to be filled, all in metres. Multiply those three numbers together and you get the volume in cubic metres.
For example, a trench that is 10 metres long, 0.6 metres wide, and 0.5 metres deep needs 10 x 0.6 x 0.5 = 3 cubic metres.
It is always worth adding a small allowance for uneven ground and minor variations. A 5% to 10% buffer is usually enough. For help with more complex calculations, our guide on how to calculate exactly how much concrete your project needs walks you through the process step by step.
What to Tell Your Builder
If you are hiring a builder or groundworker, make sure they know the following before the concrete is ordered.
Confirm the grade. If GEN 1 has been specified, check that it is written on the plans or in the specification. This avoids any confusion on delivery day. Confirm the volume. Your builder should have measured up and calculated the amount needed. Ask them to share the figures so you can double-check. Confirm the delivery date. Concrete pours need to be coordinated with other site work. Make sure the trench or formwork is ready before the mixer arrives.
Ordering GEN 1 Concrete
GEN 1 is a standard mix available from most concrete suppliers. If you are using a volumetric delivery service, you only pay for the exact amount mixed on site, which is ideal for fill work where the exact volume can be hard to predict. Get in touch with your supplier, share your project details, and they will confirm the mix, price, and delivery time.
