Who this guide is for
- Owners adding hard surface, a building or drainage to a finished plot
- Renovators whose site drainage no longer copes with what it receives
- Self-builders working within an existing set of levels
- Facility operators adding capacity to an established external area
- Readers who want the constraint set rather than a list of products
What is different when nothing is blank
On a new site, levels, surfaces and drainage are settled together. On an existing one they arrive in the wrong order: the levels exist, the surfaces exist, and the drainage is what has to be fitted between them.
That reverses the sequence the entries recommend, in which the discharge point is established first and the levels are set so they can reach it. A retrofit has to establish what the existing levels permit before it can consider what is desirable.
Fall is the thing that closes options
A gravity network holds a continuous fall so that liquids and the solids they carry travel together, and a run laid flatter than intended is where deposits begin. On a finished site, the depth to achieve that fall is frequently already spent.
Where it is, options that depend on reaching a distant discharge point close, and the arrangements that remain are the ones dealing with water close to where it lands or giving it to the ground. That is a constraint rather than a preference, and it is settled by survey rather than by intention.
The existing network is rarely as recorded
The entry is blunt about this: an existing network's route and condition are established by survey, and assumptions drawn from where covers happen to be visible are frequently wrong. Alterations accumulate, records do not, and the misconnection risk rises with every change made in ignorance.
That makes the survey the first item of work rather than a check at the end. It also makes it the moment to establish which runs are foul and which are surface water, since a retrofit connection made on an assumption is how a silent defect enters a building's history.
What remains available when there is no fall
Several arrangements do not depend on conveying water to a distant point, and they are the ones a retrofit usually ends up considering. Which of them a site can support is a design determination that begins with what the ground will accept and what consents apply.
- Dealing with water where it lands, through a permeable surface construction
- Giving water to the ground, where testing and consent support it
- Holding water and releasing it slowly, where a rate limit rather than a level is the constraint
- Intercepting at the surface, where shaping and inlets can still be altered
- Holding water on a roof, where the structure and the waterproofing have been assessed for it
Opening a finished surface costs differently by construction
What it takes to reach anything buried depends entirely on what is above it. Unbound paving is lifted and reset, and the entry treats that as a normal operation. A bound surface has to be broken, and the repair is a new bound patch bonded to an old one.
A continuous bituminous surface has no unit to lift at all, so every access is a cut and a patch, and each patch adds edges and joints to a construction whose whole argument was that it had none. That is a real part of the retrofit cost and it is decided before anyone starts.
What the existing arrangement was sized around
A retrofit almost always changes what arrives somewhere. A new paved area, a rerouted downpipe, a relaid yard or a resurfaced drive alters the load on an arrangement nobody has touched, and where an existing store or soakaway is present, that arrangement was sized against the old condition.
Surface collection is unusually vulnerable to this. A resurfaced yard, a new planting bed, a raised threshold or a fence line laid across a catchment boundary can send water somewhere it was never intended to go, which is why a clear, unaltered inlet can begin flooding after work done nowhere near it.
Levels you did not choose
The exceedance route runs across the levels as they are, not as they were drawn, and on a retrofit those levels were set by somebody else for other reasons. Tracing where surplus water goes across the built ground is therefore part of the design rather than a check on it.
The same applies at thresholds and level changes, which are where the consequence of arriving water is worst. What interception protects them now, and what a new surface upstream would change about that, is a question for the designer.
The deliverable worth insisting on
A retrofit is the moment a site's drainage record can be improved or lost for another generation. The entries treat the record as what makes later investigation possible at all, and its absence as the reason a buried system is found by trial excavation.
So the output worth asking for is not only the works but the drawing: what was laid, where it runs, what it connects to, what is foul and what is surface water, where access sits and what each feature is. That is what the next owner will be working from.
Drainage retrofit checklist
- 1Commission a survey of the existing network before designing anything
- 2Establish which existing runs are foul and which are surface water
- 3Establish where the site connects and what level is available there
- 4Measure what fall actually remains between the problem and the discharge point
- 5Establish what the ground will accept, where that is part of the proposal
- 6Ask what consents apply to any new discharge, to ground or to a system
- 7Identify what each existing surface is made of and how it would be opened
- 8Identify what the existing arrangement was sized around before adding area
- 9Check whether any later work has merged catchments or moved a low point
- 10Trace where surplus water goes across the levels as they are now
- 11Provide access at every new junction, bend and connection
- 12Ask for a drawing of the finished arrangement to be handed over
- 13Keep the survey and the drawing together with the property
Common mistakes to avoid
- Designing the retrofit before surveying what is already there
- Assuming the visible covers describe the route of the existing network
- Connecting new work to whichever run was found first
- Adding paved area without revisiting what the existing arrangement was sized for
- Choosing an arrangement that depends on fall the site no longer has
- Overlooking what opening a bound or continuous surface will involve later
- Laying a new surface that merges two catchments onto one inlet
- Finishing the work without leaving a record of what was laid
When to involve a professional
- An existing network's route and condition are established by survey
- Any new discharge to ground or to a system is a consent matter
- Ask what fall is actually available before comparing arrangements
- Ask what the existing arrangement was sized around
- Requirements vary by location and project; verify with your professionals
Frequently asked questions
Questions readers ask about this topic
Why does a retrofit start with a survey rather than a design?
Because the constraints come from what is already there. An existing network's route and condition are established by survey, and the entry warns that assumptions drawn from visible covers are frequently wrong, which is a poor basis for a new connection.
What if there is no fall left to reach a drain?
Then the arrangements that remain are the ones not depending on conveying water to a distant point: dealing with it where it lands, giving it to the ground where testing and consent allow, or holding it. Which apply here is a design determination.
I only want to add a patio. Does any of this apply?
Adding paved area changes what the existing arrangement is asked to handle, and it can also move a low point or merge two catchments. The entries note that a clear, unaltered inlet can begin flooding after work done nowhere near it.
Does the surface material matter for future access?
Considerably. Unbound units are lifted and reset as a normal operation; a bound surface is broken and patched; a continuous surface has no unit to lift at all, so every access is a cut that adds edges to a construction that had none.
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