Pipe Bedding & Trench Width: The 75 mm Rule & Why It Matters

Poor pipe bedding is one of the leading causes of long-term drainage failures in Australian homes and commercial buildings — sagging pipes, blockages, root intrusion, and cracks that lead to leaks and expensive rectification orders.

There is no single bedding rule. Each part of AS/NZS 3500:2025 sets its own: water services in Part 1, sanitary drainage in Part 2, and stormwater in Part 3. The numbers differ, and using the drainage rule on a water trench is a common way to get the quantities wrong. Here is what each one actually says.

01There Is No Single Bedding Rule

This is the part that costs people money. Bedding is not one requirement in one clause. Each part of AS/NZS 3500:2025 sets its own, and the numbers are different:

• Water services — AS/NZS 3500.1:2025, Figure 5.11
• Sanitary drainage — AS/NZS 3500.2:2025, Section 5
• Stormwater — AS/NZS 3500.3:2025, Section 6

Apply the drainage rule to a water trench and your trench is 50 mm wider than it needs to be on every metre. Apply it to a stormwater run in concrete pipe and you are using the wrong standard entirely. Take each service on its own.

02Sanitary Drainage — AS/NZS 3500.2:2025 Section 5

Start with the scope, because it is the line most people never read. Section 5 covers pipes up to DN 225. Above that size you are outside it, and the trench comes from the pipe standard instead. More on that in section 04.

Trench width (Clause 5.2.1). Trenches shall be made with a minimum clearance of 100 mm on each side of the drain barrel, measured to the inside of the sheeting or the side of the trench. Figure 5.4.4 states it as B min. = D + 200.

Do you always need a bedding layer? No (Clause 5.4.1). In stable soil a drain may either be laid on a bedding material, or be supported directly on the undisturbed base of the trench, provided that base is free of rocks and tree roots. A bedding layer only becomes mandatory in clay, rock, shale, gravel, or ground containing hard objects. If you price 75 mm of imported sand under every metre of drain in stable ground, you are pricing something the standard does not ask for.

The 75 mm figure, stated correctly (Clause 5.4.4). Sanitary drainage services shall be surrounded with not less than 75 mm of compacted sand or fine-grained soil, with no hard-edged object in contact with or resting against any pipe or fitting. Note the word surrounded. It is an envelope, not just a layer underneath. Figure 5.4.4 breaks it into a bed zone of 75 mm minimum, side support of 100 mm minimum each side, and a pipe overlay of 75 mm minimum above the pipe.

Bedding materials (Clause 5.4.2). Not just any sand:

• Bedding material or similar recycled material of nominal size between 7 mm and 10 mm
• Cement mortar of 1 part Portland cement to 4 parts sand by volume
• Free-running sand passing a 2 mm sieve, containing no clay, organic or other deleterious material
• On a rock or shale trench base at a grade steeper than 20% (1:5), cement mortar bedding at least 50 mm deep measured below the barrel, not less than 75 mm wide, not closer than 20 mm to flexible joints, with pipes supported at no more than 1 500 mm centres before the mortar goes in

Side support and overlay material shall not be inferior to the bedding material (Clause 5.4.3). You cannot bed in 10 mm aggregate and then tip site clay down the sides.

Backfill (Clauses 5.5.1 and 5.5.2). Excavated material may be reused as final backfill provided it is free from rock, hard matter and organic material, and broken up so it contains no soil lumps larger than 75 mm. Backfill must contain no hard matter larger than 25 mm. It must be compacted to restore the trench as near as practicable to the surrounding ground level. This is the clause that decides whether your spoil goes back in the hole or onto a truck, so it is worth reading before you price disposal.

Unstable ground (Clause 5.6). In filled, unstable or water-charged ground, support and bedding must be designed for the conditions. For reactive residential sites, AS 2870 sets the special design considerations.

03Water Services — AS/NZS 3500.1:2025 Figure 5.11

Water is a tighter trench and a symmetrical envelope. Figure 5.11 gives:

• Underlay (bedding) depth: 75 mm minimum below the pipe
• Side support: 75 mm minimum each side
• Pipe overlay: 75 mm minimum above the pipe
• Trench width: B min. = D + 150

That is the difference in one line. Drainage gives the pipe 100 mm of side support, water gives it 75 mm, so the water trench is 50 mm narrower for the same pipe. Minimum cover comes from Table 5.10 of the same standard.

Water and drainage in the same trench (Clause 5.3.3.4). It is allowed, with conditions. The water pipe sits on a shelf excavated at one side of the trench, not less than 50 mm from the continuation of the trench, or on compacted bedding, so that the horizontal separation between the two pipes is at least 100 mm and the required cover is maintained. The underside of the water pipe must be at least 100 mm above the top of the drain. Water service pipes must never be laid in or through any drain.

Heated water below ground (AS/NZS 3500.4:2025). Keep pipework associated with heated water services at least 300 mm from electrical cables and gas pipes, or 100 mm where those services are marked and mechanically protected along their length. Communications services need 100 mm. Any crossover must be at an angle of not less than 45 degrees, with at least 100 mm vertical separation, marked and mechanically protected.

04Stormwater and Anything Over DN 225

This is where the 3500 series hands off, and where a lot of civil estimates go wrong.

AS/NZS 3500.3:2025 Figure 6.2.2 sets the vocabulary you will see on drawings: the trench fill zone on top, the embedment zone beneath it made up of the overlay zone, the side support zone and the bedding zone, plus the haunch support zone under the lower quadrant and the springline at mid-height of the pipe.

But for trench width, Clause 6.2.3 sends you elsewhere. Widths are measured at the top of the pipes, between the faces of the unsupported trench walls or the inside face of the sheeting, and shall be not less than those specified in:

• AS/NZS 2566.2 for flexible pipes and fittings
• AS/NZS 3725 for reinforced concrete and FRC pipes
• AS/NZS 2041.1 and AS/NZS 2041.2 for corrugated metal pipes
• AS 4060 for vitrified clay and ceramic pipes

So when a civil drawing shows a concrete pipe trench at OD + 600 with a haunch to 0.3 of the outside diameter and a 300 mm overlay, that detail is coming from AS/NZS 3725, not from AS/NZS 3500. Quoting 3500 back at the engineer will not help you.

Minimum cover (Table 6.2.5). Measured finished surface to top of pipe:

Under a reinforced concrete pavement carrying heavy vehicles, plastics drop to 100 mm and ductile iron to nil, measured below the underside of the pavement. That is a big swing in excavation depth, and it is worth checking before you assume a blanket 600 mm.

05What This Means When You Are Pricing

Five things change the number on the page:

• Use the right trench width per service. OD + 200 for drainage, OD + 150 for water, and the relevant pipe standard for stormwater and anything over DN 225.
• Use the real outside diameter, not the DN. DN 100 PVC is 110 mm OD. A DN 375 concrete pipe is about 467 mm. Trench width is driven by OD, so a DN-based width is wrong from the first metre.
• Check whether bedding is even required. Stable soil with a clean trench base does not need an imported bedding layer under the drainage rules.
• Decide spoil against the backfill clause. Excavated material can go back in if it is free of rock, hard matter and organics, with no lumps over 75 mm. If it cannot, it becomes cartage and imported fill in the same line item.
• Price the zones separately when the drawing does. Bed, haunch, side, overlay and trench fill are frequently different materials at different rates. One blended figure across the trench will be wrong on all of them.

Our free bedding and spoil calculator does this per run. Set the pipe material so the outside diameter is right, set the trench width, and if the drawing shows separate zones you can stack them and give each its own material. It returns the volumes and the order quantities separately, so the aggregate, the sand-cement and the trench fill each come out as their own line.

06Related reading

• Top 5 Estimating Mistakes Plumbers Make (And How to Avoid Them)
• The New 2025 Flexi Hose Rules – Why Every Quote Just Went Up $80–$250
• NDIS SDA Plumbing Requirements – Class 3 Dwellings 2025 Guide
• Testing Plumbing & Drainage Systems – What’s Mandatory in NSW 2025

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