Excavation, Sub-Base Installation, Membrane & Screed Layer

This section covers the full preparation process up to the point where grids are laid:

  • excavation
  • sub-base installation
  • compaction
  • membrane placement
  • screed layer and levelling

If you follow this in order, you end up with a flat, stable surface that makes grid installation straightforward.

Step 1: Excavation – how deep to dig

Work backwards from the finished surface.

Typical excavation depth (guide only)

For a driveway on medium ground using 40 mm grids:

  • Gravel infill + grids: ~40–50 mm
  • Screed layer: ~20–30 mm
  • Sub-base: ~150 mm

Total excavation is usually around 220–240 mm below finished level.

For lighter-duty bases (paths, shed base on good ground):

  • Sub-base: 75–100 mm
  • Total excavation often 150–180 mm

Always allow a bit of extra depth rather than trying to make it exact.

Step 2: Be aware of services

While excavating, actively look for:

  • waste pipes
  • water pipes
  • electric or gas services
  • drainage runs

Services can be much closer to the surface than expected, especially near buildings. If you uncover something:

  • stop digging locally
  • reassess your depth
  • protect or bridge over it if needed

Never assume services are deeper. In most cases they will be but its always better to be safe than sorry.

Step 3: Manual vs machine excavation

I excavated both my drive and gravel grid base manually. So it can be done, but it’s very hard work.

For anything beyond a small area:

  • a mini digger
  • or mechanical assistance of some kind

will save huge amounts of time and energy.

Even a small machine turns days of digging into hours.

Step 4: Prepare the excavated ground level

Once excavated:

  • remove loose material
  • roughly level the ground
  • remove any soft pockets

If the ground is soft, this is where a woven membrane goes in under the sub-base to provide reinforcement.

For a relatively firm surface, you can run the wacker plate over the ground to add some extra compaction prior to adding the sub base.

Step 5: Install the sub-base (Type 2)

For most DIY installs, MOT Type 2 is the best option.

Why:

  • compacts reliably
  • spreads load well
  • forgiving for DIY work
  • still drains sufficiently

Install in layers

  • Lay in 75–100 mm layers
  • Compact each layer fully before adding the next

Do not dump the full depth in one go.

Step 6: Compaction – take your time

Use a vibrating plate compactor (“whacker plate”). A close-up of a machine AI-generated content may be incorrect.

Good practice:

  • multiple passes
  • different directions
  • extra attention to edges

If the plate sinks or bounces:

  • the layer is too thick
  • or the ground below is too soft

Fix this before continuing.

Step 7: Lay the non-woven membrane

Once the sub-base is fully compacted and tidy:

  • Lay a non-woven geotextile membrane over the entire surface
  • Overlap joints as required
  • Trim neatly at edges

This membrane:

  • stops fines migrating upward
  • allows water to drain through

This layer is essential and should not be skipped.

Step 8: Add the screed layer (this makes everything easier)

The screed layer sits on top of the membrane and directly under the grids.

  • Typical thickness: 20–30 mm
  • Material options:
    • sharp sand (valid option)
    • fine stone (2–6 mm, my preference)

The purpose of the screed layer is:

  • levelling
  • smoothing out small discrepancies
  • giving the grids a flatbed to sit on

Levelling the screed layer (simple but effective)

I used a long straight edge and effectively “sawed” it across the surface.

This:

  • flattened out small highs and lows in the sub-base
  • made the surface far more uniform
  • made grid installation much easier later

This step is well worth taking your time on.

Compaction of the screed layer (don’t overdo it)

I gave the screed layer a light pass with the whacker plate.

In reality:

  • it doesn’t compact particularly well
  • that’s normal
  • its main job is levelling, not structural strength

A light initial pass to settle it is enough. Don’t try to over-compact it.

Where you should be at this point

Before moving on to grids:

  • the sub-base should feel solid
  • the membrane should be flat and intact
  • the screed layer should be level and even

If it looks good now, the grids will sit properly.

Common mistakes to avoid

  • Laying grids directly on the sub-base
  • Skipping the non-woven membrane
  • Trying to compact thick sub-base layers
  • Overworking the screed layer
  • Rushing this stage because “the grids will hide it”. They won’t.

The takeaway

This stage determines how easy the rest of the build will be.

If you:

  • excavate properly
  • compact the sub-base in layers
  • lay the membrane correctly
  • take time levelling the screed

Grid installation becomes straightforward instead of frustrating.