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.
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Dense concrete blocks (7N)
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Need ~17 m²
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- 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)
- 21
- 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.
Compliance
Garden rooms within 2m of a boundary commonly need a maximum height of 2.5m for permitted development.
Why it matters: The 2m / 2.5m rule limits overshadowing and overlooking of neighbours when a structure sits close to a shared boundary.
If ignored: Exceeding the height inside the 2m zone removes permitted development, so the building needs full planning permission retroactively.
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.
Quantities pre-filled from your calculation (15.0 m²). Partner links — we may earn a small commission if you buy, at no cost to you.
Essentials for this job
Quantities include a ~10% waste allowance. Guide prices are indicative UK ranges (materials only, inc. VAT) — always confirm with the supplier before ordering.
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Ad · Some links are sponsored. We may earn a small commission at no extra cost to you. Suppliers are chosen for fit, not commission.
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.