Key Principles (read this first)

• Size: Make the base the same size as the shed floor, not bigger. Use a gravel margin around the outside for splash protection.

• Level & square: Level within ±10 mm across the footprint; diagonals equal within 10 mm.

• Drainage: Surrounding ground should fall slightly away from the shed. No standing water.

• Ventilation: Keep air circulation under timber floors. Treated bearers improve lifespan.

Which base should I choose?

• Paving slabs on a full mortar bed — Best all-rounder for most gardens; quick, accurate, good value.

• Poured concrete slab — Most robust; choose for heavy workshops, ride-on mowers, and larger buildings.

• Raised timber frame on pads — Fast, minimal digging, perfect for sloped or awkward sites.

Tools & Materials (typical)

Tools: Tape, string line, long straightedge, spirit level, spade, rake, hand tamper or plate compactor, rubber mallet, wheelbarrow, saw, drill/driver, PPE.
Materials (as needed):

• Type 1 MOT sub-base (75–150 mm compacted)

• Weed-control membrane (woven)

• Sharp sand + cement for full-bed mortar (slab base)

• Paving slabs (450×450 mm or 600×600 mm)

• Ready-mix concrete C20–C25 (75–100 mm) & optional A142 mesh

• Pressure-treated timber for frames/bearers (e.g., 45×95 mm or 45×145 mm)

• Gravel (20 mm or pea gravel) for edges and under frames

Method 1 — Paving Slabs on a Full Bed (most popular)

When to use: Standard sheds and summerhouses up to medium size.

1. Set out the exact shed floor size with string pegs. Square it by matching diagonals.

2. Excavate 150–200 mm. Lay a woven membrane on the subgrade.

3. Sub-base: Add 75–150 mm Type 1 in 50 mm layers, compact each layer thoroughly.

4. Level: Screed the compacted sub-base flat. Allow 25–40 mm for mortar.

5. Full-bed mortar: Mix ~1:4 cement:sharp sand. No “dot and dab”.

6. Lay slabs: Start from a straight edge. Tap down with a rubber mallet to one flat, level plane (don’t create a slope).

7. Jointing: When firm, joint with mortar or polymeric sand; brush off residue.

8. Finishing: Keep a 50–100 mm gravel margin around the base. Optional treated bearers beneath the shed for airflow.

Tip: 600×600 mm or 450×450 mm slabs are easier to keep true. Use a 1.8–2.4 m straightedge.

Method 2 — Poured Concrete Slab (heaviest duty)

When to use: Large sheds, workshops, heavy machinery.

1. Set out & formwork: Mark the exact floor size. Fix straight, level formwork to finished height.

2. Excavate for 100–150 mm Type 1 plus 75–100 mm concrete.

3. Sub-base: Compact in thin layers; remove soft spots.

4. Membrane & mesh (optional): Add DPM on wet/weak ground; place A142 mesh on spacers if specified.

5. Pour & finish: Use C20–C25 concrete. Screed level to formwork; float smooth. Keep the slab flat (no slope).

6. Cure: Protect from sun/frost; light loads after 3–7 days, full strength ~28 days.

7. Edge backfill: Remove formwork; backfill edges with gravel for drainage.

8. Install: Ensure slab is fully cured and dead level before the shed goes on.

Note: Never install on “green” (fresh) concrete.

Method 3 — Raised Timber Frame on Pads (great on slopes)

When to use: Uneven/soft ground, minimal digging, quick installs.

1. Pad layout: Corners + pads at 1.2–1.5 m centres. Use concrete pads, ground screws, or stacked/slab pads.

2. Level pads: All pad tops must be at the same level (use laser or tight string lines).

3. Frame: Build a pressure-treated frame (e.g., 45×95 mm or 45×145 mm) with joists at ~400 mm centres.

4. Fixings: Use galvanised screws/joist hangers; treat cut ends; add diagonal bracing against racking.

5. Ground treatment: Membrane on soil; 50 mm gravel under frame for drainage/weeds.

6. Checks: Frame must be dead level and square before shed installation. Maintain at least 50 mm airflow beneath.

Quick Material Estimator (rule-of-thumb)

• Area: width (m) × depth (m).

• Sub-base (Type 1): Area × sub-base thickness (m).

• Example 8×6 ft (≈2.4×1.8 m) with 100 mm sub-base → ≈0.43 m³.

• Slab bedding mortar: Area × 30–40 mm → ≈0.13 m³ (at 30 mm) for 2.4×1.8 m.

• Concrete volume: Area × 75–100 mm → ≈0.32–0.43 m³ for 2.4×1.8 m.

• Slab counts (approx.):

  • Using 600×600 mm: ceil(Width/0.6) × ceil(Depth/0.6)

  • Using 450×450 mm: ceil(Width/0.45) × ceil(Depth/0.45)

Always buy a few extra slabs to allow for cuts/spares.

Common Mistakes to Avoid

• Spot-bedding slabs (“dot and dab”). Always use a full bed.

• Uneven compaction or laying on soft/uncertain ground.

• Oversizing the slab beyond the shed floor (invites water pooling).

• No drainage margin or finished ground falling towards the shed.

• Skipping bearers/ventilation under timber floors.

Safety & Good Practice

• Wear gloves, boots, eye & hearing protection.

• Check for underground services before digging.

• Compact in thin layers; don’t try to compact deep fill at once.

• Use treated structural timber; seal cut ends.

• In cold weather, protect fresh mortar/concrete from frost.

Planning Note (UK)

Small garden sheds typically fall under Permitted Development, but local rules vary. Check height limits, conservation areas, and proximity to boundaries if unsure.