Who this guide is for
- Owners and self-builders reviewing a surface water proposal
- Anyone whose site holds water somewhere nobody expected
- Renovators changing levels, kerbs or thresholds on a drained site
- Facility operators who have inherited a drainage arrangement
- Readers who want the secondary route set out as a component in its own right
The condition beyond the assumption belongs to the design
The attenuation entry lists an exceedance and bypass route among its components, alongside the storage and the flow control. The infiltration entry does the same, and the roof attenuation entry is explicit that the route exists because the restriction can be obstructed and because rainfall can exceed what was assumed.
Read together, the records make one point repeatedly: the surplus is not eliminated by leaving the route out. It is relocated to wherever the finished levels send it, which is a decision made by accident rather than by anyone holding the drawings.
What each arrangement calls its secondary route
The physical form differs by system, which is part of why the route is easy to overlook when several arrangements sit on one site. The question to ask of each is the same one: when this is full and rain has not stopped, where does the next of it go, and has anyone drawn that.
- Attenuation storage: a weir, a bypass, or a shaped surface route across the site
- An infiltration structure: an overflow to another system or a shaped surface route
- A permeable pavement: the outlet arrangement once the store is full
- A rain garden: an overflow within the feature, or a bypass upstream of it
- A roof holding water on purpose: the route once storage is exhausted
- A gutter or a parapet: an overflow outlet set above the working level
Omitting the route does not remove the surplus
The infiltration entry describes the mechanism plainly. Water backs up the inlet, fills the collection system behind it and reappears at whichever inlet sits lowest on the site, which on a sloping plot is frequently the one closest to the building.
That turns an arrangement at the far end of a garden into water at a threshold, by a path invisible in any drawing showing the buried structure alone. The same logic applies to a roof: if no route is provided, the roof finds one, and it will be the lowest point of the perimeter.
The component nobody inspects
The overflow outlet entry makes the difficulty explicit: overflows are rarely used, so they are rarely checked, and blockages in them are found at the worst moment. An overflow that has silted looks identical to one that is clear, because neither is running on the day you look.
Two properties are what make the route worth anything. It has to be independent of the primary drainage, so that a blockage in one does not disable both; and it has to discharge somewhere visible, because a concealed overflow gives up the warning that is half its purpose.
Reading it by what is absent
The junction record for an overflow through a parapet sets out how a secondary route is actually read. Water standing on a roof after rain with nothing running down the external face; a washed strip, staining or growth beneath an opening that is now dry, showing it ran once and no longer does; water held above the level the opening was set to hold.
None of those is a reading anyone can make from a drawing. They are observations made at the building, and they are worth reporting rather than cleaning away, because each of them describes a secondary route that is no longer available.
Finished levels decide the route, not design levels
The attenuation entry asks directly whether the exceedance route has been checked against the finished site levels. That wording matters, because the route runs across the same ground as everything else, and the ground changes after the drainage is drawn.
A kerb removed during landscaping, a raised planter that dams a route, a new fence line across a catchment boundary or a threshold raised during a later refit each redirect the route without anything anyone would call drainage being touched.
What the route crosses on its way
The design question the records put is not only where exceedance water goes but what it passes on the way there. A route across a neighbour's ground, over a retained edge, past a doorway, through a parking area or into a planted feature that was never sized for it are all different propositions.
Where an overflow discharges is part of the same question. An open vessel that is exceeded delivers water onto whatever sits beneath it, and a wall face, a walkway or a terrace taking the wash of every event the primary route has missed is a position nobody chose.
What to establish, and who settles it
The route, its level and its destination are determinations for the drainage designer, made against the rainfall assumptions and consents that apply at the location. What an owner can usefully do is ask for the route to be named, drawn against the levels that were actually built, and handed over.
Recorded, it survives the next change of ownership and the next landscaping contract. Unrecorded, it is indistinguishable from ground that happens to fall that way, and it is removed by the first person who improves the levels.
Exceedance route review checklist
- 1List every arrangement on the site that holds, infiltrates or collects water
- 2For each one, ask where water goes when it is full and rain continues
- 3Ask whether that route is drawn or has simply been assumed
- 4Trace each route across the levels as built rather than as designed
- 5Note what each route crosses: thresholds, retained edges, boundaries, planting
- 6Check that any overflow is independent of the primary route it backs up
- 7Check that any overflow discharges somewhere a person would notice
- 8Look for staining, growth or a washed strip below an opening that is now dry
- 9Look for water standing above the level an overflow was set to hold
- 10Note any kerb, planter, fence or raised threshold added since the levels were set
- 11Ask how the overflow itself is inspected, given it only acts once something else has failed
- 12Ask for the route to be recorded and handed to whoever owns the site next
Common mistakes to avoid
- Treating the condition beyond the assumption as an accident rather than a design output
- Leaving the route undrawn, so the finished levels choose it instead
- Taking an overflow into the same pipe as the primary outlet, so one blockage disables both
- Concealing an overflow discharge, which removes the warning it exists to give
- Checking the route against the design levels rather than the ones that were built
- Adding a kerb, planter or raised threshold across a route nobody recorded
- Reading an overflow that runs as a fault in the overflow rather than a report on the primary route
When to involve a professional
- Where exceedance flow goes is a design output, not a site decision
- Ask what route has been designed and what it crosses on the way
- Ask whether the route was checked against finished levels
- Ask how the secondary route is inspected when it is never in use
- Requirements vary by location and project; verify with your professionals
Frequently asked questions
Questions readers ask about this topic
Why does an arrangement need a route for water it was not designed to take?
Because the surplus does not disappear when the route is left out. It backs up the inlet, fills the collection system behind it and reappears at whichever inlet sits lowest, which on a sloping plot is frequently the one nearest the building.
My overflow has started running. Is that a fault?
The component entry treats a running overflow as the overflow doing its job and telling you the primary route is obstructed or exceeded. The thing worth investigating is the primary outlet, and the thing worth noting is that the secondary route is still clear.
Can the overflow share a pipe with the main outlet?
The published entry is direct about this: an overflow sharing a route with the primary outlet would be disabled by the same blockage. Independence from the primary drainage is one of the properties the secondary route depends on for any of its value.
How would I know whether a route still exists on my site?
Walk it across the levels as they are now rather than as drawn, and look for what has been added since: a kerb removed, a planter dammed across a fall, a fence line on a catchment boundary, a threshold raised during a refit.
Who decides where exceedance water should go?
The drainage designer, working against the rainfall assumptions and consents that apply at the location, and in some cases in discussion with whoever owns the ground it crosses. An owner's useful contribution is asking for it to be named, drawn and recorded.
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