Garden Room Bases
Built to last 30 years.
C28/35 ready-mix delivered to your door. From 1m³ upwards. With our volumetric mixers, pay only for what's poured. Get an instant price in 60 seconds.
The Mix You Need
C28/35 High-Strength Mix
- Mix grade
- C28/35
- Slump
- 75mm (workable)
- Aggregate
- 20mm
- Recommended depth
- 100mm
- Sub-base
- 100-150mm Type 1 MOT
- Typical volume
- 1.0-2.5 m³
Volume Guide
How much concrete for your garden room?
Based on a 100mm slab. Use our calculator for exact volume, 10% wastage included.
Small garden office
2.4m × 3.0m
7.2 m² footprint
Medium garden room
3.0m × 4.0m
12 m² footprint
Large garden room
4.0m × 5.0m
20 m² footprint
Extra-large studio
5.0m × 6.0m
30 m² footprint
Why It Matters
Get this right and you won't redo it for 30 years
- Garden rooms are heavy, insulated walls, roof, glazing and furniture add up fast. A weak base will sink or crack within 2 winters.
- A 100mm C28/35 slab on compacted Type 1 gives you a base rated well beyond the loads a timber or SIP garden room imposes.
- Most suppliers recommend the base is 50mm larger than the footprint on each side, we can deliver any volume from 1m³ so you size it right.
- Concrete is cheaper than rebuilding. Getting the base right once means 30+ years with zero movement.
Step by Step
How to prepare & pour
Mark out & excavate
Dig to 200mm below finished level across the full footprint (100mm slab + 100mm sub-base). Keep the dig 50mm wider than the building on each side.
Compact the sub-base
Lay 100-150mm of Type 1 MOT stone. Whacker-plate in 50mm layers until it's rock solid. Shortcut this step and the slab will crack.
DPM & formwork
Lay 1200-gauge damp-proof membrane over the sub-base, overlapping any joints by 150mm. Build timber formwork to your finished level, checked with a spirit level.
Mesh & order concrete
For garden rooms over 15m², drop in A142 mesh on 50mm spacers. Use our calculator to work out your m³, we include 10% wastage automatically.
Pour, tamp & finish
Pour direct from the mixer where possible. Tamp level with a straight-edge, then float for a smooth finish. Cover with plastic for 24 hours if it's hot or frosty.
Cure before you build
Wait 7 days before you start erecting the garden room. Full cure is 28 days but foot traffic and light building loads are fine after a week.
FAQ
Common questions
Does a garden room base need reinforcement?
For bases under ~12m² on well-compacted Type 1, C28/35 at 100mm without mesh is enough. Over 12m², or if you're on clay soil, drop A142 mesh in on 50mm spacers. If your garden room supplier specifies mesh, follow their spec.
Do I need a damp-proof membrane (DPM)?
Yes, always. A 1200-gauge DPM between the sub-base and the concrete stops ground moisture wicking up into the slab and into your timber frame. It costs about £30 for a 4×3m roll, cheap insurance.
How long before I can install the garden room?
You can walk on the slab after 24-48 hours. For installation (heavy modules, SIPs, or a frame build) wait 7 days. The slab will continue hardening for 28 days but will easily take a garden room after 7.
How thick should the base be?
100mm is the standard for garden rooms. If your building is extra heavy (slate roof, masonry skin, or a hot tub inside) go to 125-150mm. More depth is cheaper than more rebar.
How much concrete do I need?
Multiply length × width × 0.1 (for 100mm). A 4m × 5m base needs 4 × 5 × 0.1 = 2.0 m³. Use our calculator, it adds 10% wastage and rounds to what we can deliver.
Can you pump the concrete over my house into the back garden?
Yes. If we can't reach the pour with a chute, we'll arrange a concrete pump. Pumps reach 30m+ horizontally and can go over roofs. Call us on 024 7636 1448 for a pumped price.
Ready to order?
Instant price in 60 seconds. Next-day delivery subject to availability. With volumetric, pay only for what's poured.