Water Tanks And On-Site Detention

Water leaving a site is a council matter, and water stored on a site is usually a planning condition.

Neither is a factory question, which is exactly why they get underestimated on a modular project.

The two different systems

They are frequently confused and they do opposite things.

**A rainwater tank stores water for use.** Toilets, laundry, irrigation, sometimes more. It is a resource.

**An on-site detention system holds stormwater temporarily and releases it slowly,** so the development does not increase the peak flow into the public system. It is a constraint.

A single tank can occasionally do both, with the detention volume above the retention volume and an orifice controlling the discharge. Frequently they are separate, and the detention system is buried and invisible.

Why they matter on a modular programme

The great advantage of factory construction is that the module is being built while the site is being prepared. Two programmes run in parallel, which is why [modular programmes run shorter](/blog/why-modular-programmes-run-shorter).

That advantage only holds if the site works finish on time.

Tanks and detention involve excavation, structural design for buried elements, plumbing, and frequently a council inspection before backfill. Any of those can hold the site up while a finished module waits.

Which puts them on the critical path more often than people expect, and it is the reason they are scoped at the same time as the [groundworks](/blog/groundworks-and-foundations-for-modular) rather than treated as landscaping.

What gets decided early

**The volume,** which comes from the council requirement and the roof area draining to it. The roof area is known once the design is settled, so this can be resolved early.

**Above ground or buried.** Above ground is cheaper and needs space and looks like a tank. Buried is more expensive, needs structural design for the load above it, and disappears.

**Where it sits relative to the crane.** A buried tank under the crane standing area, or under the path a module has to be flown across, is a sequencing problem. Either it goes in after the lift, or the crane goes somewhere else. Resolving that is part of the [lift plan](/blog/crane-selection-and-lift-planning).

**The pump and its power supply,** which needs a circuit from the switchboard and therefore needs to be known while the module's electrical layout is being set.

**The connection point on the module.** Downpipes and the plumbing stub are built into the module. The site connection has to match.

On sloping and constrained sites

A site with a fall has an advantage and a complication.

The advantage is gravity: a tank positioned downhill of the roof can fill and discharge without pumping.

The complication is that the discharge point has to be legal, which means the connection to the street or to an easement, and on a site that falls away from the road that becomes a real engineering question. It is one of the [site constraints](/blog/site-constraints-that-decide-modular-viability) that decides viability early.

In accessible housing specifically

Two practical points.

**Nothing in the accessible path.** Tank overflows, pump housings and inspection covers cannot sit in a path of travel or create a trip point. On a dwelling designed to the Specialist Disability Accommodation [standard](/standards), the continuous accessible path is not negotiable and the water infrastructure works around it.

**Maintenance access without a ladder.** Filters and first-flush devices need clearing. Where the resident or a support worker will do that, the access has to be reachable from ground level.

The short version

Tanks and detention are ordinary engineering, and they are the part of a modular project most likely to delay an otherwise finished building.

They get designed with the base structure, approved with the development, and built while the module is in the factory.

Discuss a site.

MADEmodular / MADEbetter.