Stormwater drainage.
Where the small components add up.

What we estimate

We price the full range of stormwater and deep civil work:

Pipe sizes from DN300 up to DN1500. Materials per design specification.

Materials we price

Material choice is made by the designer in line with the WSA Code and the relevant council specification. We price what’s specified, with awareness of why each material gets chosen where it does.

For stormwater, broadly:

• RCP (reinforced concrete pipe) — the workhorse for stormwater. Heavy, durable, available in large diameters up to DN1500 and beyond. Joints are typically rubber-ringed
• HDPE — high-density polyethylene, used where flexibility, lighter weight, or corrosion resistance is required
• PVC — smaller stormwater lines, residential drainage scope, lighter loading
• In-situ concrete — for OSD tanks, large pits, headwalls, and any bespoke structure that doesn’t suit a precast solution
• Precast concrete — standard pits, lids, risers, smaller culvert components

Where most estimates go wrong

Stormwater pricing fails on the small items. The detail we look for in a tender comes from years of pricing this work specifically. The headline pipe quantity is easy to count from the plan. The detail that gets missed lives in the chainages, the pit schedule, and the bedding volumes.

Pipe sizing, depths, and chainages.

Larger catchments mean larger RCP or HDPE pipes. Long pipe runs are broken down into stormwater lines, and each line is made up of multiple chainages based on design levels and slope. Misjudging chainage depths or trench volumes leads directly to cost overruns. We calculate each chainage precisely using the long-section drawings, ensuring accurate excavation, bedding, and backfill quantities for every segment.

Bedding and overlay material.

Wider trenches mean more bedding. Bigger pipes (DN600 and above) need greater bedding width and thickness. Bedding type and compaction class affect the price significantly. We factor in trench widths, pipe diameter, and compaction requirements so nothing is missed in the bedding or overlay allowances.

Pits and pit risers — the hidden stormwater cost.

Every drainage line has multiple pits, each with a designated depth and class. The pit schedule arrives with the design, but turning it into accurate costing isn’t straightforward. Pit depth equals multiple pit risers — the deeper the pit, the more risers required. Bigger jobs typically require Class D pits and grates, which are significantly more expensive than the lighter classes. Many contractors underestimate the riser count or forget to allow for the correct pit class. We analyse the pit schedule in detail, calculate riser requirements precisely, and include the correct pit classes from the start.

RCP pipe cutting.

RCP pipes are supplied in standard lengths but chainages rarely match exactly. That means cutting on site — slow, labour-intensive, and requiring skilled operators. The bigger the pipe diameter and wall thickness, the more time it takes. This step is often forgotten during estimating but shows up later as real labour time and real cost. We include realistic allowances for RCP cutting, especially on larger diameter pipes.

Backfill material.

Excavated material can’t always be reused, especially in roadway alignments or poor soils. Select imported fill is often mandatory. Volumes can be underestimated if trench shape, width, and depth aren’t calculated accurately. Not accounting for backfill properly leaves contractors paying out of pocket for truckloads of material. We calculate precise trench volumes, check backfill requirements, and apply real-world compaction bulking factors.

Machine selection.

A DN600 RCP run at depth needs more than a 5-tonne machine. The choice of excavator must suit pipe size, depth, ground conditions, and required speed. 30-tonne excavators are often the right choice for efficiency and reach on deep stormwater work. Underestimating machine size slows the job and inflates hire costs over the duration of the run.

This is the level of attention stormwater work needs. Every line, every chainage, every pit, every cubic metre of imported fill — properly costed before the bid goes out. The cost of building this capability in-house is one reason specialist estimating exists.

OSD tanks and form-reo-pour structures

Most modern subdivisions and commercial developments require on-site detention (OSD) — a tank or structure that holds back peak stormwater flow before releasing it gradually into the downstream network. Without OSD, the post-development runoff rate would exceed what the receiving system can carry. Council approval is conditional on properly engineered detention.

OSD tanks are typically form-reo-pour in-situ concrete structures — bespoke, project-specific, designed to the catchment’s detention volume and the site’s geometry. The three-step process is what the term describes:

• Form — setting up the timber or steel formwork that shapes the concrete
• Reo — placing the steel reinforcement (bars and mesh) inside the formwork to spec
• Pour — pouring the concrete around the reinforcement, then curing and stripping the formwork

We estimate the full scope of in-situ concrete works for OSD and other bespoke stormwater structures:

• Excavation for the tank pit, including shoring where depth requires it
• Base slab — formwork, reinforcement, concrete supply, compliance testing per current spec
• Walls — tall vertical pours typically done in lifts, each lift requiring its own formwork cycle
• Internal baffles — where the design requires flow management within the tank
• Top slab and access lids — reinforced concrete cover with maintenance access points
• Inlet and outlet structures — including the orifice plate or restrictor that controls discharge rate
• Overflow weir — for when the design event is exceeded
• Waterproofing — if specified for the tank’s function
• Backfill — carefully placed around the structure, with compaction managed so the walls aren’t loaded unevenly

The same form-reo-pour discipline applies to large bespoke pits and headwalls where a precast solution doesn’t suit the geometry. These structures take real time and real coordination — they’re not a single line item, they’re a sequence of works that has to be priced as a coordinated build. Our method is built for exactly this kind of coordinated estimating.

What changes the cost on every stormwater project

Beyond the chainage-by-chainage detail above, stormwater jobs have the same condition-stacking that applies to all deep civil work (the same logic applies to deep sewer construction):

• Depth — same logic as sewer. Beyond 1.5m needs trench support; deeper work needs bigger machines; very deep work needs geotech supervision per the relevant authority
• Pit class and grate selection — Class A for non-traffic areas through to Class D for heavy traffic. The class drives both the pit cost and the grate cost — both are real lines
• Concrete supply — volumes climb fast on OSD jobs. Multiple pours, multiple loads, with testing per load per Sydney Water and council specifications
• Working in live road — traffic control, lane closures, working hours, reinstatement
• Adjacent services — stormwater is shallower than sewer but still typically deeper than telecom, gas, electricity, water. Services search runs through the excavation zone
• Discharge point and outlet works — where the stormwater leaves the site. Headwalls, scour protection, council approval for the discharge structure
• Sediment and erosion control — required during construction to prevent runoff carrying sediment to downstream systems

What we deliver beyond the pipe

A complete stormwater estimate includes everything that gets the network from drawing to operational handover:

• Excavation, bedding, and overlay — calculated by chainage, not by averaged length
• RCP cutting and jointing — realistic time allowances for on-site cutting
• Pit construction — precast supply OR full form-reo-pour scope where bespoke
• Risers — counted per pit per the schedule
• Grates and covers — correct class per the design
• OSD construction — full in-situ concrete scope including outlet structure
• Trench support — benching or shoring per depth and ground conditions
• Dewatering — pump selection, discharge route, silt control
• Geotech supervision — where the depth and authority spec require it
• Compaction testing — every 150mm layer of backfill
• Concrete testing — cylinders and slump tests per load on all in-situ pours
• Imported fill — volume, freight, placement, compaction
• Traffic control — when the alignment runs in live road or near it
• Services search — vacuum excavation where machine digging is too risky
• Sediment and erosion control — silt fences, sediment ponds, hay bale check dams during construction
• Scour protection at outlets — rock armouring, energy dissipators, gabion baskets
• Reinstatement — roads, footpaths, kerbs, landscaping
• Council inspection and handover — CCTV reports, as-built drawings, compaction certificates

Every estimate includes the assumptions and exclusions register — what’s been included, what’s been qualified out, what the head contractor still needs to confirm.

Authority fluency — stormwater is council-driven

Stormwater is different to water and sewer when it comes to authorities. Most stormwater networks in Australia are owned and managed by local councils, not by the state water authorities. That means stormwater estimating requires fluency in council-specific specifications, which vary significantly from one local government area to the next.

We estimate to the actual council specification on the project, not a generic template. Across our coverage areas, this includes:

• Sydney metro councils — each with its own engineering standards, pit specifications, and handover requirements. Sydney Water occasionally owns trunk stormwater where it interfaces with their network, but most subdivision and commercial drainage is council-managed
• Hunter Region councils — Lake Macquarie, Newcastle, Maitland, Cessnock, Port Stephens, with their own engineering specifications
• Melbourne metro councils — for stormwater works in the Melbourne area
• Regional Victorian councils — engineering standards published by individual rural and regional councils
• Regional Queensland councils — including those councils where stormwater work is council-managed under their own engineering frameworks

Reading the right council’s specification — including the correct pit class schedule, the correct discharge requirements, and the correct council-specific handover documentation — is part of what we estimate. The wrong spec produces a number that looks right on paper and fails at submission.