Choosing a drainage subcontractor: the checks that matter
Short answer
Check six things before you let a drainage package: the deepest run the subcontractor has actually built, whether they shore and dewater themselves, who carries testing and adoption, what plant they own rather than hire, whether reinstatement is in their price, and who signs the as-builts.
The short version
- Depth, not diameter, is what separates drainage subcontractors.
- Shoring and dewatering in-house keeps the dig off the hire queue.
- Testing and adoption sign-off belongs in the scope, in writing.
- Owned plant means the programme is not waiting on availability.
- Reinstatement left out of the price always comes back as a variation.
Most drainage packages are not lost on rate. They are lost on depth, on testing, and on who was supposed to put the surface back.
Here is what we would check if we were letting the package rather than building it.
1. The deepest run they have actually built
Diameter is easy. Depth is the thing that sorts the field.
Past about three metres a drainage run stops being a trench and becomes a temporary works problem: shoring design, confined-space procedure, and usually water to get rid of. Plenty of firms will price that. Far fewer have done it last month.
Ask for the job, the depth and the ground, not a capability statement.
2. Whether shoring and dewatering are theirs
If the shoring is hired in and the pumps come from somewhere else, your programme now depends on two supply chains you cannot see.
A subcontractor who owns the trench boxes and the pumps can react to groundwater on the day it appears. One who does not will tell you about it on a Friday.
| Check | Ask for | If the answer is vague |
|---|---|---|
| Depth | Deepest run built, with the site | They have priced it, not done it |
| Shoring and dewatering | Owned or hired | Your dates sit with a hire desk |
| Testing | Who tests, who holds the records | It lands on you at handover |
| Adoption | Schemes taken to sign-off | Expect abortive work |
| Plant | Machines on their own fleet | Programme is availability-led |
| Reinstatement | In the price, itemised | A variation is already coming |
3. Testing and adoption, in writing
Air test, water test, CCTV survey, as-built records, and the inspection by the authority or the water company. Every one of those needs a name against it before the first pipe goes in.
Where a scheme is being offered for adoption, experience of that process is worth more than a keen rate. Work that does not pass inspection gets dug up, and it gets dug up at the end of the programme.
4. Owned plant, not promised plant
Drainage is a plant-led trade. Excavators, dumpers, lifting equipment for the larger pipe and tanks, pumps, compaction. A subcontractor running its own fleet can move resource between runs; one hiring everything in prices the risk or absorbs the delay, and usually does the first.
Depth and access together set how fast the dig can actually go, which is the point we make in how drainage depth drives the programme.
A drainage price that excludes reinstatement is not cheaper. It is unfinished.
5. Reinstatement, itemised
Backfill, sub-base, surfacing, and highway reinstatement where the run leaves the site. These are a real part of what a drainage package costs, and they are the easiest line to quietly leave out.
Inside a Section 278 or Section 38 scheme the reinstatement standard is set by the highway authority, not by what looks tidy, so it has to be priced against that specification.
6. How many interfaces you are buying
Every split between drainage, concrete and surfacing is a line on somebody's programme and a conversation about whose level is wrong.
When the same crews lay the drainage, cast the in-situ concrete and surface on top, the formation level handed over is the one the slab is built on. One contractor, one programme, one set of as-builts.
The short version
Ask about depth, shoring, testing, adoption, plant and reinstatement. Six questions. The answers tell you whether you are buying a drainage package or the first half of one.
Common questions
What is the difference between a drainage contractor and a drainage subcontractor?
In practice the words are used interchangeably, but a drainage subcontractor is engaged by the main contractor as part of a larger build, so the package has to fit somebody else's programme, somebody else's temporary works and somebody else's site rules. A drainage contractor working direct for a client sets its own sequence.
What depth should I expect a drainage subcontractor to work to?
Ask for the deepest run they have actually built, not the deepest they will quote. Anything past about three metres changes the job: it brings in shoring design, confined-space procedures and usually dewatering, and the crews who do that work routinely are a much smaller group than the ones who will price it.
Who is responsible for drainage testing and adoption?
Whoever the subcontract says, which is why it needs saying. Air and water testing, CCTV survey, as-built records and the authority's or water company's inspection all have to sit with a named party. Left vague, they land on the main contractor after the drainage crew has demobilised.
Should reinstatement be in the drainage price?
Yes, and it should be itemised. Backfill, sub-base, surfacing and any highway reinstatement are a real part of the cost of a drainage run, and a price that quietly excludes them is not cheaper, it is incomplete.
Can one subcontractor do the drainage and the concrete?
Yes, and it removes an interface. We self-deliver drainage, in-situ concrete and surfacing with our own plant and crews, so the formation level the drainage leaves behind is the one the slab or the road is built on.