This section explains what you’re actually buying and why it’s there.

---

Sub-base – why MOT Type 2 works best for DIY

MOT Type 2 is a crushed stone sub-base with limited fines. Depending on where you buy it may be called MOT type 2, or something like 20mm and down. It is essentially hardcore that varies in size from 20mm to dust

Why it works well:

• compacts tightly
• spreads load evenly
• still allows water to pass through
• forgiving if conditions aren’t perfect

For most DIY gravel grid projects, Type 2 is the safest and most reliable choice. That’s why I used it for both my driveway and sauna/shed base.

More free-draining options exist, but they compact less reliably and can move if not well confined.

---

Membranes – woven vs non-woven (this confuses most people)

These are two different products doing two different jobs.

Non-woven membrane (always used above the sub-base)

This sits between the sub-base and the screed layer.

Its job:

• stop fine material migrating upwards
• Stop weeds
• allow water to drain through

Think of it as a filter.

This layer is essential a permeable system, water passes through it but nothing else can penetrate from below.

Woven membrane (only used on soft ground)

This sits under the sub-base.

Its job:

• spread load
• stop the sub-base sinking into weak soil

Think of it as reinforcement, not drainage or weed protection.

You only need this if the ground is soft.

---

Screed layer – what it does and what to use

The screed layer sits directly on top of the membrane and underneath the grids. Its job is to:

• level the surface
• help the grids sit flat
• spread load evenly

You only need a thin layer, typically 20–30 mm.

Option 1: Sharp sand

Sharp sand is:

• easy to source
• simple to work with
• widely used

It’s a valid option, and many installations use it successfully.

The downside is that fine particles can migrate over time and slightly reduce permeability in your non-woven membrane.

Option 2: Fine stone (my preference)

I used 2–6 mm limestone.

I preferred this because:

• it maintains permeability better long term
• it doesn’t hold fines in the same way sand can
• it still levels well when compacted lightly

Both options work.
If sharp sand is easier for you to source, use it.
If you want to maximise drainage and long-term performance, fine stone is a good upgrade.

---

Gravel infill size (important detail)

For filling the grids:

• 6–20 mm works
• 10 mm compacts best (in my experience)

I used:

• 20 mm on my driveway
• 10 mm on my sauna base

If I did it again, I’d use 10 mm everywhere.
Larger stone can bridge inside the grids and doesn’t lock in as well.

---

Drainage – what to realistically expect

Gravel grid systems are permeable by design.

In most domestic situations:

• water drains through the gravel
• into the sub-base
• and disperses naturally

Additional drainage is usually not required unless:

• the ground is heavy clay with standing water
• the area is known to flood
• water has nowhere to disperse

If water soaks away elsewhere in your garden, it can usually soak away here too.

Final confidence check

At this point you should be able to answer:

• which grid thickness you’re using
• how deep your sub-base needs to be
• whether you need reinforcement
• which membrane goes where

If you can answer those clearly, you’re ready to move on.