External drainage is part of the residence.

One-sentence thesis

A drain is a component. Drainage is the coordinated route from rainfall to a safe, maintainable and lawful destination.

Why the landscape plan is also a water plan

A visually complete garden can still direct water toward the building. Soil levels rise during planting. roots obstruct outlets. leaves fill channels. later paving changes falls. planter irrigation keeps walls wet. pool overflow and roof discharge are added to the same undersized route.

The owner therefore needs a site-water plan that connects architecture, civil works, landscape, pools, retaining walls and operations.

Start with levels

Small level differences can determine whether water:

• crosses a threshold;
• sits against a wall;
• enters a service room;
• drains toward a planter;
• collects behind a retaining wall;
• reaches an inspection chamber;
• escapes safely during an extreme event.

Key levels include:

• internal finished floor;
• terrace finish;
• channel and drain invert;
• external ground;
• planter soil;
• pool overflow;
• road and access level;
• retaining-wall drainage outlet;
• final discharge point.

These should be coordinated before landscape and paving hide the relationship.

Primary water routes

The project may need to manage water from:

• roofs and gutters;
• terraces and balconies;
• courtyards;
• driveways and paths;
• landscape and irrigation;
• pools and overflow systems;
• retaining walls and subsoil drainage;
• AC condensate;
• external showers and wet areas;
• neighbouring or upslope land.

Each source has a different flow pattern, contamination risk and maintenance requirement. Combining them without analysis can create surcharge or backflow.

The downstream question

Moving water away from one wall is not enough if the discharge point:

• floods another part of the site;
• erodes a slope;
• affects a neighbour;
• overwhelms a roadside drain;
• returns toward the building;
• discharges onto unstable ground;
• remains hidden and inaccessible;
• conflicts with local requirements.

The appointed team must verify the lawful and technically appropriate outlet for the actual site.

Primary, secondary and exceedance routes

A useful site-water strategy considers three conditions.

Normal operation

Routine rain is collected and moved through the designed drains, channels and outlets.

Restricted operation

One drain is partly blocked, a chamber is full of leaves or maintenance is late. The site should reveal the problem without immediately sending water into occupied space.

Exceedance

Rainfall or upstream flow exceeds the design assumption. The project needs a safe surface route that limits consequences to people, structure, equipment and neighbouring land.

Not every residence needs complex detention infrastructure. Every residence needs to understand where excess water can go.

Common interface failures

Terrace to interior

External finishes rise during construction. The threshold is reduced. A drain is present but too far from the door. Wind pushes water past the covered area.

Planter to wall

Soil, irrigation and roots remain directly against a sensitive wall. The planter has no verified drainage or overflow route.

Retaining wall

Water pressure builds because drainage is incomplete, clogged or discharged to an uncertain destination.

Roof outlet to ground

A downpipe ends beside the foundation or connects into a blocked underground line with no visible overflow.

Pool and rainwater

Pool overflow, deck water and storm water enter one system without checking capacity or backflow.

Access road and gate

Road runoff enters the property because the gate threshold, channel and external drain were not coordinated.

Maintenance should be designed, not assumed

Ask:

• Can staff lift every grate safely?
• Is the chamber large enough to clean?
• Can a blocked underground line be rodded or flushed?
• Are strainers visible?
• Does an overflow make failure obvious?
• Is the downstream outlet accessible?
• Who checks the system before the wet season?
• Where are the route and maintenance record stored?

A beautiful slot drain with no clean-out access is not a complete maintenance solution.

What DALLEAU would request during feasibility and design

• topographic survey and external levels;
• known flood, runoff and upslope conditions;
• geotechnical and groundwater inputs where relevant;
• roof, terrace, pool, landscape and retaining-wall water sources;
• preliminary drainage concept and discharge assumptions;
• primary and emergency routes;
• maintenance access;
• coordination of thresholds and sensitive rooms;
• specialist triggers for hydraulic modelling, detention, pumping or slope drainage;
• owner decisions on acceptable residual risk and maintenance burden.

What this note does not prove

It does not calculate a design storm, size a drain, approve a discharge point or establish flood safety for a specific property. It does not replace civil, hydraulic, geotechnical, landscape, pool or local-authority review. It does not promise that an extreme event will produce no damage.

Project implication

The owner should be able to follow the water from roof, terrace and landscape to a final outlet, understand the secondary route, and know how the system will be cleaned. If that path is invisible in the design record, it will be difficult to control in operation.