How to check a drainage plan before you price it

Steve Parker

Sep 19

5 min read

A drainage plan answers 2 questions. How much pipe is there to price? And will the water reach the main? A takeoff answers the first. The second is the one that stops a crew on day 1 of digging.

The drawings in this post are samples we made for illustration, not a real project. Every number below comes from those samples or from the posts we link to.

Quick answer: to check a drainage plan before you price it, measure every run by pipe type, count the manholes, sumps and downpipes, then check the invert levels. Each pipe's upstream invert must sit above its downstream invert by the fall its drawn grade needs. Query any run that fails, in writing, before you price.

The sample stormwater drainage plan

Sample: drainage layout DWG C-201, Rev A. The title block notes the levels are assumed.

A 30.00 m by 30.50 m lot with a 234 m² building at FFL 25.90.
Stormwater in 150 uPVC SN16: 6 downpipes and 4 sumps, collected at MH1 and MH2, then out to the 375 SW main at connection IL 23.95.
Wastewater in 100 uPVC SN16: the building outlet through MH3 to the 150 WW main at connection IL 23.70.
Grades: main stormwater runs at 1:100; sump leads, wastewater and the stormwater outlet at 1:60.

Read the title block first. This one says Rev A and "levels assumed", which means nobody has surveyed the ground or the mains yet. That goes in your written assumptions, whatever else the check finds.

What to measure on a drainage plan

Sample: the same sheet measured. Every run has a length and every structure has a number.

Measure in the order you will price: runs, then structures, then connections.

Stormwater 150: 8 runs totalling 109.5 m (24.3, 23.6, 19.6, 9.6, 8.8, 8.1, 8.0 and 7.5 m).
Wastewater 100: 2 runs totalling 17.1 m (6.5 and 10.6 m).
Manholes, 1050 dia: 3 (MH1, MH2 and MH3).
Sumps: 4 (SP1 to SP4), each with a 150 lead at 1:60.
Downpipes: 6, with 3 down each side of the building.
Connections to existing mains: 2, stormwater at IL 23.95 and wastewater at IL 23.70.

Add the runs back up. If your 8 lengths don't make 109.5 m, a run is missing or doubled, and so is its trench, bedding and backfill. For each of the 2 connections, find out who makes it and what approvals it needs before you price it.

Then read the depths. RL is the surface level at the manhole lid; IL is the invert, the inside bottom of the pipe. RL minus IL is the depth to invert, which sets your trench depth. At MH3, 25.60 − 23.88 = 1.72 m, the deepest structure on the sheet. Our guide to material takeoffs from plans covers the full measuring method.

MH2: where 6 pipes meet

Sample: detail A, the MH2 junction. 6 pipes, 77.2 m, 1 outlet to the main.

MH2 is the busiest point on the plan: 6 pipes and 77.2 m of the 109.5 m of stormwater, 5 coming in and 1 going out.

On price, each of the 5 inlets is a connection that has to be formed and benched. On levels, MH2's invert of 24.10 is the floor for everything upstream. All 5 inlets have to arrive at or above it, and the 8.8 m outlet has to fall from 24.10 to the main at 23.95.

Spot the problem

Here is the same sample sheet with 1 level changed. Before you read on, follow the water from MH1.

Sample: the same sheet with 1 level changed. Follow the water from MH1.

The answer: 1 pipe runs uphill

Sample: MH1 at IL 24.00 sits 100 mm below MH2 at IL 24.10. Query it before you price.

MH1 now reads IL 24.00. MH2 reads IL 24.10. The 19.6 m pipe between them is drawn flowing from MH1 to MH2 at 1:100, but MH1 sits 100 mm lower. Laid to those levels, the pipe falls back towards MH1. Water from 3 downpipes and 1 sump sits in MH1 and the pipe until it has risen 100 mm. That pipe never drains dry, and standing water collects silt.

On the first version of the sheet, MH1 reads IL 24.29 and the levels work. The check is 3 lines of arithmetic:

Fall needed: 19.6 m at 1:100 is 19.6 ÷ 100 = 0.196 m.
First version: 24.29 − 24.10 = 0.19 m. That is 1:100 within the rounding of the levels.
Changed version: 24.00 − 24.10 = −0.10 m. The pipe climbs 100 mm.

Dropping MH2 won't fix it. To fall from 24.00 at 1:100, MH2 would need to sit at about 24.00 − 0.196 = 23.80, below the 23.95 connection at the main. Then MH2 couldn't drain either. So either 24.00 is a typo or the layout needs re-levelling. Both are the designer's call. Your job is to ask, in writing, before your price goes in.

You are not redesigning anything. The designer works to Building Code clauses E1 (surface water) and G13 (foul water) and to AS/NZS 3500. Your check is narrower: do the flow arrows, the grades and the 3 manhole inverts all say the same thing?

Why levels belong in the pricing check

A reverse fall on paper costs 1 email. Found on site, it costs a stood-down crew, an open trench and an argument about who pays. In a published pre-tender audit of a drainage subbie's rates, the rebuilt crew day rate for deep in-road work came to about $5,500–6,000. A shallow residential crew costs less, but a day waiting on new levels is still a full day, and it is rarely in anyone's price.

Levels drive other lines too. Depth sets the trench rate, and every grade sets how deep the far end of its run gets. If the levels move, your price moves.

A plan also can't show what it left out. On one civil job with no stormwater attenuation drawn, the stormwater code of practice still required it, and it went into the price as a provisional allowance of about $35–45k.

Next step: Takeoff Verification or the free quote check

Takeoff Verification: $490 + GST per trade, back in 48 hours. We re-measure the quantities off your drawings and mark each line as confirmed or out, in writing. How Takeoff Verification works.
Free quote check. Send the drainage plan and 1 quote. We check it line by line and send back the gaps and drawing queries in writing, each tied to the sheet it came from.

Both start at trueworks.co.nz/contact.

FAQ

What is an invert level on a drainage plan?

The invert level (IL) is the level of the inside bottom of the pipe, usually given at each manhole and connection. On the sample, MH2 has a lid RL of 25.35 and an IL of 24.10. Water flows from the higher invert to the lower one.

How do I check the fall on a drainage run?

Divide the run length by the grade and compare it with the difference between the 2 inverts. A 19.6 m run at 1:100 needs 0.196 m of fall. If the upstream invert isn't higher by about that much, query it.

What should I count on a drainage plan?

Every manhole, sump, downpipe and connection to a main, plus each run by pipe type and diameter. The sample gives 3 manholes, 4 sumps, 6 downpipes and 2 connections.

What if the plan says levels are assumed?

Price on the drawn levels, say so in your quote, and ask for surveyed levels before anyone digs. On the sample, all 3 manhole depths hang on assumed levels.

Before you price the next drainage job, follow the water through every manhole, not just the metres. Send the plan and your quote through trueworks.co.nz/contact and we'll check it line by line, in writing, before you commit. Your first one goes through as the free quote check, NDA available.