Turf Underlay Calculation

To calculate turf underlay, measure the lawn area, choose the required depth, then multiply length × width × depth. This gives the amount of turf underlay needed in cubic metres.

For many new lawn areas, a turf underlay depth of around 75–100 mm is a practical guide, especially where the existing soil needs improvement. The right depth depends on the site, soil condition, drainage, levels and the type of turf being installed.

Turtle Nursery supplies turf underlay soil, soils, manures and landscape supply products for Sydney homeowners, landscapers and trades, with pickup and delivery options available.

What Is Turf Underlay?

Turf underlay is a soil blend used to prepare the surface before laying turf. It helps create a more suitable growing layer for new lawn roots.

Good turf underlay can help with:

• Lawn levelling
• Root establishment
• Soil preparation
• Moisture movement
• Surface consistency
• Filling shallow uneven areas

It should be spread and levelled before the turf is laid.

Why Turf Underlay Is Important

Turf needs contact with a suitable growing layer. If the ground is too hard, uneven, compacted or poor quality, new turf may struggle to establish.

Turf underlay helps prepare the area so the turf can knit into the soil more evenly.

It is especially useful where existing soil is clay-heavy, compacted, disturbed by building work or uneven.

Simple Turf Underlay Formula

Use this formula:

Length × width × depth = cubic metres required

Make sure all measurements are in metres.

For example, if your lawn area is 8 metres long, 5 metres wide and you want 100 mm of turf underlay:

8 × 5 × 0.1 = 4 cubic metres

If using 75 mm depth:

8 × 5 × 0.075 = 3 cubic metres

How to Measure Your Lawn Area

For a simple rectangular lawn, measure the length and width in metres.

For irregular areas, break the lawn into smaller rectangles or sections, or circles, then add the areas together.

For example:

• Front section: 5 m × 4 m = 20 m²
• Side section: 8 m × 2 m = 16 m²
• Total area = 36 m²

Then multiply the total area by the underlay depth.

What Depth of Turf Underlay Is Needed?

For many lawn projects, around 75–100 mm is a practical turf underlay depth.

A shallower layer may be suitable where the existing soil is already good and only minor levelling is needed. A deeper layer may be needed where the ground is poor, uneven or short on soil depth.

A practical guide:

• Minor levelling: around 50 mm
• General new lawn preparation: around 75 mm
• Poorer soil or better preparation: around 100 mm

The site should still drain properly. Adding underlay alone will not fix serious drainage problems.

Factors That Affect Turf Underlay Quantity

The amount of turf underlay needed depends on:

• Lawn size
• Required depth
• Existing soil quality
• Ground levels
• Drainage
• Compaction
• Whether soil has been removed
• Whether extra levelling is needed

Uneven ground usually needs more underlay than a smooth, level site.

Can You Lay Turf Over Existing Grass?

It is generally better not to lay new turf directly over existing grass.

Old grass can create an uneven layer, block root contact and break down under the new turf. This may cause poor establishment, soft spots or patchy growth.

For best results, remove old grass and weeds, prepare the soil, level the surface and use suitable turf underlay before laying new turf.

Turf Underlay vs Topsoil

Turf underlay and topsoil are not always the same thing.

Topsoil is often used for general filling, levelling and soil depth. Turf underlay is specifically used to prepare the surface for new turf.

A suitable turf underlay should create a workable, level growing layer for lawn establishment.

For related soil information, see Turtle Nursery’s guide on when to use topsoil vs garden soil.

Common Mistakes When Ordering Turf Underlay

One common mistake is ordering too little. Uneven areas, settling and spreading can all increase the amount needed.

Another mistake is using too shallow a layer on poor soil. If the existing ground is clay-heavy or compacted, a thin layer may not be enough.

Some people also forget to check drainage before adding underlay. If water pools in the area, the problem should be addressed before turf is laid.

How to Prepare the Area Before Turf Underlay

Before spreading turf underlay:

1. Remove old grass and weeds.
2. Clear rocks and debris.
3. Check drainage.
4. Loosen compacted areas if needed.
5. Spread underlay evenly.
6. Level and rake the surface.
7. Lightly firm the area.
8. Lay turf as soon as practical.

The surface should be smooth and even, not loose and lumpy.

Sydney Turf Underlay Advice

Many Sydney lawns sit over clay-heavy or disturbed soil. In newer homes and landscaped blocks, the existing ground may not be ideal for turf without preparation.

Turf underlay can help create a better lawn base, but it should be used with proper site preparation.

For larger projects, Turtle Nursery’s delivery options can help with bulk turf underlay, soil, mulch, gravel, sand and other landscape supplies across Sydney.

You can also browse Turtle Nursery’s soils and manures range for lawn and garden preparation products.

Summary

To calculate turf underlay, multiply length × width × depth in metres to work out cubic metres required. For many new lawn areas, 75–100 mm is a practical depth, depending on existing soil condition and preparation needs. Turf underlay helps create a level growing layer for new turf, but drainage, compaction and site preparation still matter.

Key Points

• Calculate turf underlay using length × width × depth.
• Use metres for all measurements.
• Many new lawns need around 75–100 mm of underlay.
• Uneven or poor soil may need more material.
• Turf underlay helps prepare the surface but does not fix drainage by itself.
• Remove old grass before laying new turf.

FAQs