Planning a garden wall, outdoor brick barbecue or patio sub-base means converting square and cubic metres into what UK builders' merchants sell: 215 × 102.5 × 65mm metric bricks, 25kg cement bags and 850kg bulk bags of sand and aggregate.
1. How many bricks and blocks per m²?
With a standard 10mm mortar bed and perpend joint, each UK brick takes up 225mm × 75mm of wall face, so there are 13.3 courses per metre of height and about 60 bricks per m² of half-brick wall.
| Wall type | Bricks / blocks per m² | Building sand per m² | Cement per m² (4:1) |
|---|---|---|---|
| Half-brick single skin (102.5mm) | 60 bricks | ~43 kg | ~11 kg |
| One-brick double skin (215mm) | 120 bricks | ~86 kg | ~22 kg |
| Cavity wall (brick + 100mm block) | 60 bricks + 10 blocks | ~59 kg | ~15 kg |
For your own wall, use the brick calculator, or the mortar calculator if you already know how many bricks you're laying.
2. Building a garden brick BBQ (and why you need firebricks)
Standard clay facing bricks are fine for the outer walls of a U-shaped barbecue or pizza oven base. But the charcoal bed can exceed 600°C, which can make ordinary bricks and cement mortar crack or spall.
Line the charcoal bed and the lower sides of the firebox with refractory firebricks (230 × 114mm) bedded in fire cement. Leave a 2–3mm expansion gap around the steel charcoal tray and grill so expanding hot metal doesn't push the brickwork apart. The brick BBQ calculator works out the bricks, firebricks and base for you.
3. MOT Type 1 sub-base and bulk bag coverage
UK merchants' "tonne bags" usually contain 800–900kg of material. MOT Type 1 crushed stone compacts by roughly 20–25% under a wacker plate, to about 2.1 tonnes per m³ compacted. An 850kg bulk bag covers approximately:
- 50mm compacted (light paths, shed bases on firm ground): about 8m²
- 100mm compacted (patios, BBQ and shed bases): about 4m²
- 150mm compacted (driveways): about 2.7m²
Lay a woven geotextile membrane under the sub-base on soft ground, and fit an HDPE root barrier if building near trees or bamboo. Calculate your exact quantity with the MOT Type 1 calculator.
4. Blockwork: blocks and mortar per m²
Standard UK concrete blocks are 440 × 215mm on face. With 10mm joints, each one covers 450 × 225mm, which is almost exactly 10 blocks per m² whatever the thickness. Blocks use more mortar per unit than bricks but far less per m², because there are fewer joints:
| Block | Blocks per m² | Dry mortar per m² | Approx. weight per m² of wall |
|---|---|---|---|
| 100mm dense | 10 | ~20 kg | ~195 kg |
| 140mm dense | 10 | ~28 kg | ~270 kg |
| 100mm aircrete | 10 | ~20 kg (or thin-joint mortar) | ~75 kg |
The weight column matters for deliveries and for what a floor or footing has to carry. The block calculator gives both counts and weights.
5. Worked example: a garden wall and BBQ together
Say you're building a 4m single-skin wall 900mm high with a BBQ built into one end.
- The wall: 3.6m² × 60 = 216 bricks, plus 10% = 238 bricks.
- The BBQ: a 1.8m-wide, 0.7m-deep U-shape at the same height adds about 190 bricks and around 33 firebricks for a 700 × 500mm fire area.
- Together: roughly 430 bricks. That's about 390kg of dry mortar, or 3–4 bags of cement and about a third of a tonne of building sand, plus fire cement for the firebox.
- Footings: both need a footing or slab on a compacted sub-base, typically 100mm of MOT Type 1 under the BBQ base. That's about half a bulk bag for a 2m × 0.9m base.
Buying for both at once means one delivery and bricks from the same batch, so the colour matches.
6. Ordering, delivery and storage
- Buy bricks from one batch. Colour varies between batches and even between packs. Blend bricks from three or four packs as you lay.
- Bulk bags need crane access. The lorry's crane needs a clear spot within reach of the kerb, away from overhead cables.
- Keep cement dry. Store bags off the ground under cover and use them before the date printed on the bag. Lumpy cement has started to set and should be thrown away.
- Cover sand. Wet sand is heavier to mix and changes the mortar's consistency. Rain also washes fines out of open bulk bags.
7. Weather and curing
Mortar and concrete need time and the right conditions to gain strength. Don't lay bricks or pour concrete when frost is forecast before it sets. Cover fresh work overnight with hessian or sheeting, and in hot, windy weather, dampen very dry bricks lightly and keep new work out of strong sun so it doesn't dry too quickly. Wait at least a week before lighting a new brick BBQ, and start with a small fire.
For a full step-by-step garden wall build with footing sizes and a priced planner, see the garden wall guide. For sub-base depths for every kind of paving, see the sub-base depth guide.