Garden SurveyorGarden Room / Shed Base Calculator
Concrete, timber, gravel grid, paving or ground screw bases.
Habitable / heated rooms need a thicker slab (150 mm), insulation and building-regs sign-off.
Drives default depth, mix grade and mesh recommendation.
10 advanced inputs hidden. Switch to Advanced to fine-tune.
- Base area
- 15m²
- Perimeter m
- 16m
- Base type
- concrete
- MOT Type 1
- 3.94t
- Concrete purpose
- garden-room
- Mix grade
- C25
- Reinforcement
- A142
- Concrete depth
- 100mm
- Concrete volume (incl. waste)
- 1.58m³
- Wastage Pct
- 5%
- Supply route
- site-mix
- Cement bags (25 kg)
- 21kg
- Sharp sand
- 1.34t
- 20 mm aggregate
- 2.05t
- Mesh sheets (3.6 × 2.0 m)
- 3
- Mesh grade
- A142
- DPM (1200 g polythene)
- 18m²
- Formwork (lineal m)
- 16m
- Saw-cut control joints
- 3
- Max joint spacing
- 2.4m
- Wheelbarrow loads (~70 L each)
- 23
- Concrete
- 1.58m³
- DPM (damp-proof membrane)
- 18
Heads-up: Base type drives everything else. Concrete is overkill for a light shed; a heavy garden room or hot tub needs concrete or ground screws — gravel grid will sink unevenly under point loads. Why it matters: Match the base to the load: a 2-tonne hot tub on paving slabs will crack the slabs and the joints.
One continuous pour — line up delivery, access and crew before the truck books in.
Volumetric mini-mix truck
No pump needed
2 people — one placing, one tamping & floating.
Why: No pump needed — the truck chute plus a short 15 m barrow run reaches the pour.
Place and level within ~2 hours of delivery; first floating once the surface sheen has gone.
Pour timeline
1. Before the truck arrives
Formwork fixed and braced, sub-base compacted and dampened, DPM/reinforcement in place, and the discharge point agreed. Have all hands and finishing tools ready.
2. Delivery
Volumetric mini-mix truck backs up to within chute reach.
3. Pour & level
Place 1.6 m³ working away from the far edge back to the exit. 2 people — one placing, one tamping & floating. Screed level off the form rails as you go.
4. Tamp & finish
Tamp to compact and bring up the laitance, then float to the spec finish. Place and level within ~2 hours of delivery; first floating once the surface sheen has gone.
5. Joints
Form or saw-cut 3 control joints — saw-cutting is done within 6–24 h while the slab is green to control cracking.
6. Cure
Keep covered and damp (or use a curing compound) for at least 3 days; reaches ~70% strength in 7 days and full design strength at 28 days. Don't load before then.
This app provides general UK guidance and material estimates only. It is not legal, planning, engineering or building-control advice. Always confirm requirements with your local planning authority, building control, utility providers, manufacturers or qualified professionals.