Sub-base Preparation - Levelling Course

The levelling course does not carry the load. The weight of people and cars goes into the compacted stone sub-base underneath. The bedding layer has one job: to give an even, accurate bed that the blocks press into slightly when the surface is vibrated, so they settle firmly.

This matters, because the levelling course should not fill big hollows or bumps in the sub-base. If the bed is 1 cm thick in one spot and 10 cm a metre further on, the thicker part compresses more over time and the paving sinks there. The levels and falls belong in the sub-base. The levelling course only fine-tunes them, at the same thickness across the whole area.

The usual choice is crushed aggregate with a fine grading, typically somewhere around 2 to 4 or 5 mm. Sharp, angular grains lock together, let water through and do not shift under the paving. In many countries coarse sharp sand or grit sand is also common for block paving. Some paving manufacturers allow a grading that includes fines (dust), which is easier to screed but drains less well.

• Crushed aggregate 2 to 4 mm: a versatile choice for paths, patios and driveways
• Aggregate with fines or coarse sharp sand: pleasant to work with, better suited to lighter-use areas unless the manufacturer says otherwise
• Rounded soft sand or building sand: the grains slide over each other, so it is not recommended under paving that takes traffic
• Material containing clay or soil: unsuitable, it holds water and frost can lift the paving in winter

If the technical data sheet for your paving names a specific grading and bed thickness, follow it. Manufacturers often take this into account when dealing with warranty claims.

Start only once the compacted sub-base is finished with the correct falls and the edge restraints (kerbs or edgings) are set. The fall for drainage on paving is usually around 1 to 2 %, always away from the house.

Screed rails, either timber battens or steel tubes, are laid on the sub-base and set to height so that their top edge marks the top of the bedding layer. Aggregate is spread between them and pulled flat with a long straight board. "Screeding" simply means drawing this board along the rails to strike off the material to an even level. Once the rails are lifted out, the grooves they leave are filled and smoothed with a trowel.

Nobody walks on the screeded bed after that. The blocks are laid from the area already paved, working forwards. The bed is not compacted separately before laying. It gets compacted together with the paving, when a plate compactor fitted with a rubber or polyurethane mat goes over the surface after laying and jointing. The blocks drop by a few millimetres during this, so you need to allow for it when setting the rail height.

After vibration the levelling course is usually roughly 3 to 5 cm thick. A thinner layer cannot take up small irregularities, while a thicker one compresses unevenly.

A rectangular area with a single, even fall, a well-compacted sub-base and good access for a wheelbarrow is straightforward. The bed can be screeded quickly in one go.

These take more time:

• Areas with several falls, channel drains and gullies where the slope changes direction
• Curves, steps and the edges around manhole covers and house walls
• An uneven sub-base that has to be corrected first
• Poor access, where material has to be carried by hand through a garden
• Wet weather, which soaks the screeded bed so it has to be redone

These same factors usually decide how much labour a contractor allows for in a quote.

• Making up an uneven sub-base with extra bedding: within a year these spots turn into dips
• Walking on the screeded bed: footprints stay under the paving as small hollows
• Screeding too much at once: the whole yard is prepared, then it rains or a cat walks across it overnight and the bed is spoilt
• The wrong material: sand with clay in it, or rounded grains that shift under a car wheel
• Getting the height wrong: forgetting that the paving drops during vibration, so the finished surface ends up below the kerbs or the door threshold
• Vibrating without a protective mat: this damages the surface and edges of the blocks

For a patio or garden path you can prepare the bed yourself, as long as the sub-base is finished and compacted. You will need two steel tubes or straight battens as rails, a long screeding board, a spirit level, a string line and a rake.

1. Check the levels and falls of the compacted sub-base with a string line and a straightedge, and correct any larger deviations in the sub-base first.
2. Lay the screed rails parallel, slightly closer together than the length of the screeding board.
3. Set the rails to height so that, after laying and vibrating the paving, the surface ends up at the planned level and fall.
4. Tip slightly damp aggregate between the rails and spread it out with the rake.
5. Draw the board along the rails with a sawing motion and push the surplus further along.
6. Lift the rails out carefully, fill the grooves and smooth them with a trowel.
7. Prepare only as much area as you can pave that same day.
8. Lay the blocks working from the finished paving and keep off the screeded bed.

Once the paving is down you cannot see the levelling course, so check it before laying and then judge it by how the paving behaves:

• The bed is the same thickness everywhere and free of footprints
• The material is clean, with no soil, roots or larger stones
• A 2 m straightedge laid on the bed, and later on the paving, shows no noticeable gaps
• Rainwater runs off in the planned direction and no puddles remain
• Blocks do not rock or creak when you step on them
• The finished paving lines up properly with kerbs, the threshold and manhole covers
• Joints are filled to the top after vibrating