Garage Base Guide 2026

Mark garage outline: Use string lines and wooden pegs to mark exact garage dimensions plus 150mm extra all around for working space. Check square using 3-4-5 triangle method (3m + 4m sides should give 5m diagonal).

Clear vegetation: Remove all grass, plants, topsoil, and organic material within marked area. Topsoil must be completely removed as it compresses over time causing settlement.

Level assessment: Use spirit level and straight edge to assess natural ground level. Decide whether to build up with fill or excavate to achieve level base.

Excavation depth: Dig to minimum 250mm depth for standard base (150mm sub-base + 100mm concrete). Increase to 350mm for heavy-duty bases (200mm sub-base + 150mm concrete).

Level excavation: Bottom of excavation must be level within ±10mm. Use laser level or long spirit level on straight edge for accuracy.

Ground compaction: Compact exposed ground using vibrating plate compactor or hand tamper. Make minimum 4 passes until surface feels solid with no loose material.

Soft spots: Dig out any soft, spongy, or organic areas and backfill with MOT Type 1, then compact.

Material specification: Use MOT Type 1 - crushed limestone or granite (0-40mm graded aggregate) meeting Highways England specifications.

Layer thickness: Install in 75mm layers maximum. For 150mm total depth, install two 75mm layers. Each layer MUST be compacted before adding next layer.

Compaction process: Use vibrating plate compactor (minimum 150kg) making 4-5 passes per layer. Material compacts approximately 15-20% from loose to compacted state.

Level check: After final compaction, check level across entire area. Sub-base should be firm enough to walk on without leaving footprints.

Quantity required: For 18m² single garage, need approximately 3 tonnes MOT Type 1 for 150mm compacted thickness.

Timber selection: Use 150mm × 25mm straight timber (pressure-treated recommended) or proprietary steel formwork for large areas.

Setting levels: Position formwork at exact finished concrete level. Use laser level or water level to ensure all edges are perfectly level.

Securing formwork: Drive 50mm × 50mm stakes every 600mm outside formwork. Stakes should extend 300mm into ground. Screw formwork to stakes firmly.

Edge details: For sloped driveway approach, set formwork at 1:12 to 1:15 fall maximum. Internal corners need diagonal bracing for strength.

Oil release agent: Coat inside face of formwork with release agent or old engine oil for easy removal after curing.

Membrane specification: Use minimum 300 micron (1200 gauge) polyethylene DPM meeting BS 8102 standards. Heavy-duty 1200 micron recommended for workshop garages.

Installation method: Unroll DPM across entire base area, lapping sheets minimum 150mm at all joints. Extend up formwork edges by 150mm minimum.

Joint sealing: Use double-sided DPM tape or hot-air welding to seal all joints. Simply overlapping without sealing allows moisture ingress.

Puncture prevention: Lay 50mm sand blinding layer over sub-base before DPM to prevent puncture from sharp stones (optional but recommended).

Protection: Avoid walking on DPM with boots - any holes compromise moisture barrier. Patch immediately with DPM tape if punctured.

Mesh selection: Standard garage uses A142 mesh (6mm wire @ 200mm spacing). Double garages benefit from A193 mesh (7mm @ 200mm).

Positioning: Support mesh on plastic chairs or bar supports maintaining 40-50mm height from DPM. Mesh should be in middle third of concrete depth for optimal performance.

Mesh overlaps: Overlap mesh sheets by minimum 2 squares (400mm). Tie overlaps with wire to prevent separation during concrete pour.

Edge clearance: Mesh should finish 50mm from all edges. Do NOT allow mesh to touch formwork or protrude through concrete surface.

Alternative reinforcement: Polypropylene fibres (added to concrete mix) provide crack control without mesh (cost: £8-12 per m³).

Concrete grade: Order C25/30 (Gen 4) for standard garages, C30/37 (PAV 1) for heavy-duty use. Specify "garage base slab" when ordering.

Volume calculation: Area (m²) × depth (m) + 10% wastage. Example: 18m² × 0.1m = 1.8m³ + 10% = 2.0m³ total.

Workability: Specify S3 or S4 slump class (75-150mm slump) for easy placement and finishing without excessive water content.

Additional requirements: Request fibres if not using mesh, air entrainment for freeze-thaw resistance (optional), and plasticizer for improved workability.

Delivery scheduling: Book minimum 3-5 days advance. Ensure clear access for truck (4m width, 4m height minimum). Have minimum 3-4 people ready for pour.

Pour method: Direct concrete into furthest corner first, working backward toward access point. Maintain continuous pour without delays preventing cold joints.

Spreading: Use rake or shovel to spread concrete evenly, working around reinforcement mesh. Ensure complete encapsulation of mesh with no voids underneath.

Compaction: Work concrete with rake or shovel to eliminate air voids. For large areas, use poker vibrator (not essential for domestic garages under 50m²).

Leveling: Use screed board (straight 100mm × 50mm timber longer than base width) with sawing motion across formwork to level concrete flush with formwork top.

Timing: Complete entire base in single continuous pour. Typical 20m² garage base requires 2-3 hour work period from first concrete arriving to finishing complete.

Bull float: After screeding, use large bull float (1.2-1.5m magnesium or wooden float) to smooth surface and bring cement paste to top.

Edging: Run edging tool along perimeter against formwork, creating slightly rounded edges preventing chipping.

Final troweling: Once concrete begins to set (1-3 hours depending on temperature), use steel trowel for final smooth finish. Make 2-3 passes for very smooth surface.

Texture options: For non-slip surface, brush finish using stiff broom after bull floating (before final set). Skip steel troweling if textured finish desired.

Control joints: Cut joints to 25mm depth maximum every 3-4m using hand groover or saw-cut within 24 hours. Joints prevent random cracking.

Initial protection: Cover concrete with polythene sheeting immediately after finishing. Prevents moisture loss and protects from rain, frost, and sun.

Curing period: Keep covered minimum 7 days (longer in cold weather). Concrete reaches approximately 70% strength at 7 days, 90% at 28 days.

Moisture maintenance: Keep concrete moist during curing. Spray with water 2-3 times daily if weather is hot and dry.

Cold weather protection: Below 5°C, use insulation blankets under polythene. Frost damage within first 48 hours causes permanent strength loss.

Formwork removal: Remove after minimum 24-48 hours for edges, 7 days if formwork provides support. Remove carefully to avoid chipping edges.

Loading schedule: Light foot traffic after 2-3 days, vehicle loads after 7 days minimum, full loading (garage installation) after 28 days for complete strength.