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Laying a paving slab base for your shed isn’t just about throwing some squares in the dirt and tapping it with a mallet. If it doesn’t stay entirely level, parts will move out of alignment, doors won’t close, and it might not stay dry.
While many basic DIY guides skip the vital details, this step-by-step method includes the techniques you need to build a base that will keep your garden building dry, stable, and secure for decades.
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The Mathematics of Excavation
Many guides tell you to simply “dig down 6cm.” But the truth is that the depth depends on exactly how high you want your base to sit. Generally, the slabs want to reach slightly above ground level but with their height partially submerged. Not digging deep enough can leave your slabs raised above ground and a frame required to keep the soft materials underneath from spilling out.
Calculate your exact dig depth based on your materials. For a standard heavy-duty base, you need:
- MOT Type 1 Hardcore: 75mm (compacted)
- Mortar Bed: 30mm to 40mm — this will hold your slabs at the same level over time
- Paving Slab: 40mm (standard utility slab thickness)
- Total Depth: ~150mm.
If you want the base to sit exactly 20mm above the grass line (perfect for water drainage without looking like a step), you need to excavate to a depth of 130mm.
You should also familiarise yourself with the planning permission height limits for sheds and ensure your base doesn’t take the building height above these, if you want to avoid planning permission requirements.
Squaring the Base (The 3-4-5 Rule)
Marking out the perimeter with pegs and string isn’t enough. If your base isn’t perfectly square, the floor of your shed will overhang the corners, exposing the timber to driving rain.
Don’t guess—use the 3-4-5 rule to guarantee perfect 90-degree corners.
- Measure 3 feet down your first string line.
- Measure 4 feet down the intersecting string line.
- The diagonal distance between those two exact points should be exactly 5 feet. (For larger sheds, you can scale this up to 6-8-10).
The Math Formula: If you want to check the full diagonal of your base from corner to opposite corner, use the Pythagorean theorem: a² + b² = c². Square your length, square your width, add them together, and find the square root. The resulting number is exactly what your diagonal measurement should be. If both diagonals match this number, your base is perfectly square.
The “Semi-Dry” Mortar Bed
While many professional bricklayers and landscapers prefer laying slabs on a full, wet mortar bed to guarantee maximum solidity, a wet bed can be incredibly difficult for DIYers to work with. Heavy slabs tend to sink, slide, and lose their level in wet mixes.
For a DIY installation, we recommend a semi-dry mix (often called a “dry pack”).
- A 6:1 or 4:1 ratio of sharp sand to cement provides a highly durable bed (though 8:1 is often used successfully for lighter garden patios).
- Add just enough water so that when you squeeze a handful of the mix, it holds its shape without crumbling, but no water drips out.
- This provides a dense, workable bed that allows you to tap the slabs perfectly level with a rubber mallet without them sinking out of place.
The Forgotten Steps: Haunching and Pointing
Most amateur guides stop right after laying the slabs. If you stop there, your base will fail over time.
Haunching the Edges: If you followed the dig depths above, your mortar bed and hardcore sit safely below ground level — simply backfill the working gap around the perimeter with soil and compact it firmly against the slab edges. But if your site forced a shallower dig (sloping ground, tree roots, or buried services) and your base sits proud of the ground, the exposed mortar bed and hardcore around the edges will eventually wash away in heavy rain. In that case, you must “haunch” the edges: using a trowel, apply a 45-degree wedge of wet mortar around the entire exposed perimeter of the base. This locks the exterior slabs and sub-base firmly into place.
Pointing the Gaps: Never leave the gaps between your slabs empty. Water will pool in these gaps, freeze in the winter, and expand—cracking your mortar bed and shifting your slabs. Brush a dry mix of kiln-dried sand and cement (or a specialized resin joining compound) into all the gaps and strike it smooth.
Lay a Damp-Proof Membrane: This sheet prevents any water on the base surface from being soaked up into a wooden shed. If your shed’s floor bearers are pressure treated (which is the industry standard), this is less of a concern as they will already be resistant to rot from water absorption. But the membrane does provide a cheap belt-and-braces protection to keep the floorboards dry, which can never be treated underneath after construction. When you’re ready to build the shed, lay the DPM directly over your finished slabs once the mortar is dried, overlapping multiple pieces if you need to, and place the floor bearers over it to hold it down. Then trim the membrane so that it doesn’t poke out.
By following these steps—calculating true depths, haunching, and laying a DPM—you are building a foundation that rivals professional installations. Your shed will stay dry, square, and structurally sound for decades to come.
