Drainage is the controlled collection and removal of surface water and groundwater from a property, so that water does not pool on the ground, soak into structures or wash soil away. Every building has drainage of some kind, whether that is a roof gutter, a grated channel across a driveway, or a perforated pipe buried in the garden. Good drainage is invisible when it works and expensive when it does not.
This guide explains what drainage is, how the main types of drainage system work, how stormwater and foul drainage differ in South Africa, and how to choose and maintain the right system for your property.
Table of Contents
What Is Drainage?
A drainage system does three things: it collects water at the point where it becomes a problem, conveys it away in a channel or pipe, and discharges it somewhere safe, such as a municipal stormwater line, a soakaway or a watercourse. If any one of those three stages fails, water backs up at the weakest point in the system.
Drainage is usually divided by where the water is sitting:
- Surface drainage deals with water you can see, running across paving, lawns, driveways and yards.
- Subsurface drainage deals with water you cannot see, held in saturated soil against foundations, retaining walls and paved surfaces.
- Roof drainage intercepts rainwater before it ever reaches the ground.
Most properties need a combination. A driveway channel drain will not fix a permanently soggy lawn, and a French drain will not stop a blocked gutter from soaking a wall.
Stormwater Drainage vs Sewer Drainage
This distinction matters more in South Africa than most property owners realise, because the two systems are separate and are regulated separately.
- Stormwater drainage carries clean rainwater from roofs, paving and gardens. It discharges to a stormwater line, a soakaway or a watercourse, and it is generally not treated.
- Foul or sewer drainage carries wastewater from toilets, basins, kitchens and industrial processes to the municipal sewer for treatment.
Connecting a downpipe or a yard gully into the sewer is a cross connection. It overloads the sewer during heavy rain and is a common cause of sewage surcharging in older suburbs. It is also prohibited by municipal by-laws. If you are not sure which line a gully feeds, a camera survey settles it in minutes, as explained in our guide to CCTV drain surveys.
Surface Drainage Systems
Surface drainage intercepts and redirects water flowing across the ground. It is the first line of defence against pooling on paving, in yards and across driveways.
Trench and Channel Drains
A trench drain, also called a channel drain, is a linear channel set into the ground and covered by a grate. It collects water along its full length rather than at a single point, which is what makes it effective across wide, flat surfaces.
- Strengths: handles high volumes quickly and intercepts sheet flow before it reaches a doorway or garage.
- Typical uses: across driveways, at garage thresholds, around swimming pools, and on commercial loading bays.
- Watch for: silt and leaf build up under the grate. A channel drain that is half full of sand drains at a fraction of its rated capacity.
Slot Drains
A slot drain does the same job as a channel drain but presents only a narrow opening at the surface instead of a wide grate. It is more discreet, easier to sweep and less of a trip hazard.
- Strengths: minimal visual impact, simple profile, and in stainless steel it stands up well to corrosion and frequent washdown.
- Typical uses: architectural paving, patios, walkways, and food preparation areas where hygiene matters.
- Watch for: the narrow slot clogs faster than a grate in leafy areas, so it needs a regular flush.
Swales and Bio-Swales
A swale is a wide, shallow, planted channel that slows water down and lets it soak away rather than rushing it off site. It is the low technology option and often the cheapest for a large area.
- Strengths: recharges groundwater, filters silt and pollutants out of runoff, and reduces the load on municipal stormwater lines.
- Typical uses: estate verges, alongside long driveways, at the foot of gentle slopes and across large commercial landscapes.
- Watch for: swales rely on soil that actually absorbs water. On heavy clay they simply become ponds.
Catch Pits and Yard Gullies
Catch pits and gullies are point collectors: a grated opening over a chamber that traps silt and debris before water enters the pipe. They are the workhorses of South African yard drainage and the single most neglected component.
The sump at the bottom of a catch pit is designed to fill with grit. Once it fills to pipe level, everything it was meant to trap goes straight into the drain line instead. Emptying catch pits is a ten minute job that prevents a large share of the blockages we are called out to.
Subsurface Drainage Systems
Subsurface drains are buried below ground to remove water that has already saturated the soil. They are the answer to damp walls, waterlogged lawns and hydrostatic pressure against a structure.
French Drains
The French drain is the most widely used subsurface system: a sloped trench holding a perforated pipe surrounded by washed stone and wrapped in geotextile. Groundwater enters the pipe through the perforations and is carried to a safe discharge point. Our guide to French drain construction covers how they are built properly.
- Strengths: relieves hydrostatic pressure against foundations and retaining walls, dries out chronically soggy ground, and stays completely hidden.
- Typical uses: perimeter drains around a house, behind retaining walls, and across the uphill side of a sloping garden.
- Watch for: the classic failures are no fall, no geotextile and the wrong stone. All three end with a silted pipe within a season or two. See our list of French drain mistakes to avoid.
Subsoil and Agricultural Drains
Subsoil drains use the same principle as a French drain but are laid in a grid or herringbone pattern across a large area rather than as a single line. They lower the water table under sports fields, smallholdings, vineyards and large lawns.
Soakaways
A soakaway is a stone filled or modular crate filled pit that receives collected water and lets it disperse slowly into the surrounding soil. It is a discharge point rather than a collection system, and it is often the only legal option where there is no municipal stormwater line to connect into.
Soakaways only work where the soil percolates. On the clay soils common in parts of the Cape a soakaway that was never percolation tested will overflow in the first serious winter storm.
Roof and Slope Drainage
These systems deal with water before it reaches ground level, or with water that is already moving downhill under its own momentum.
Gutters and Downpipes
Gutters collect rainwater off the roof and downpipes carry it to ground level. A single roof can shed thousands of litres in one storm, so this is the highest volume drainage on most residential properties.
- Strengths: stops water sheeting down walls and keeps it away from the foundation, which is the leading cause of rising damp and lifted paving.
- Typical uses: every pitched roof. Flat roofs use outlets and overflow scuppers instead.
- Watch for: a downpipe that simply discharges onto the ground next to the wall moves the problem by half a metre. Downpipes should feed a channel, a stormwater line or a soakaway well clear of the building.
Slope Drains and Cut-Off Drains
On a sloping site, a cut-off drain across the top of the slope intercepts runoff before it gathers speed, and a slope drain carries it down through a solid, non-perforated pipe to a controlled outlet.
- Strengths: prevents erosion of the slope face and stops a wall of water arriving at whatever is built at the bottom.
- Typical uses: terraced properties, hillside driveways, and any structure built into a bank.
- Watch for: discharging a slope drain onto unprotected ground just relocates the erosion. The outlet needs energy dissipation, usually stone pitching or a small stilling basin.
Retaining Wall Drainage
Water trapped behind a retaining wall is the most common reason retaining walls fail. Saturated soil is far heavier than dry soil and pushes with hydrostatic pressure the wall was never designed for. Weep holes, a drainage layer of stone and a subsoil pipe behind the wall are what stop that pressure from building.
Heavy Duty and Industrial Drainage
Commercial and industrial sites drain larger volumes, carry heavier loads and often carry contaminated water, so the specification changes.
HDPE Modular Trench Drains
High density polyethylene channels are pre-engineered, modular and chemically inert. Unlike concrete they are non porous, so salts, oils and acids do not degrade them, and they install far faster because there is no on site forming.
They are the usual choice for workshops, vehicle wash and service bays, chemical handling areas and food processing floors.
Cast In Place Concrete Channels
The traditional heavy duty channel, formed and poured on site. It offers the highest load bearing capacity and can be built to any profile a project demands, which keeps it in use at ports, airports and heavy plant yards.
Grease Traps and Interceptors
Any kitchen or workshop discharging to the sewer needs an interceptor between the outlet and the drain: a grease trap for commercial kitchens, a silt trap or oil separator for workshops and wash bays. These are drainage components, not optional extras, and a trap that is never serviced becomes the blockage. We cover servicing intervals on our grease trap cleaning page.
Drainage in South African Conditions
Generic drainage advice written for other climates travels badly. Two local factors change the design brief.
- Rainfall pattern. The Western Cape drains a long winter of steady, saturating rain, which is a groundwater and subsoil problem. Most of the rest of the country drains short, violent summer thunderstorms, which is a peak volume and erosion problem. The same property in Cape Town and in Gauteng needs different systems.
- Soil. Sandy Cape Flats soils percolate readily and suit soakaways. Clay soils in parts of the Peninsula and the interior barely percolate at all, so water has to be piped away rather than absorbed.
Building work is governed by the National Building Regulations, with SANS 10400 Part P covering drainage and Part R covering stormwater disposal. Between them they set the requirements for pipe falls, access points, discharge points and the separation of stormwater from foul drainage. The City of Cape Town water and sanitation pages set out where municipal responsibility starts and ends for residents in the metro.
As a rule of thumb, the municipality maintains the sewer and stormwater mains in the road reserve. Everything on your side of the boundary, including the connection itself, is the property owner’s responsibility. That is why a blocked drain inside the boundary is a private job and a surcharging manhole in the street is a municipal one.
How to Choose the Right Drainage System
Start from the symptom rather than the product. What the water is doing tells you which system you need.
| What you are seeing | Likely cause | Usual solution |
|---|---|---|
| Water pooling on paving or a driveway after rain | No fall, or nowhere for surface water to go | Channel or slot drain, regraded fall, catch pit |
| Lawn stays soggy for days after the rain stops | High water table or a perched layer of clay | French drain or subsoil drain grid |
| Damp patches on internal walls at floor level | Groundwater pressing against the foundation | Perimeter French drain, downpipe discharge moved away |
| Water sheeting down an outside wall in a storm | Blocked, undersized or missing gutter | Gutter and downpipe repair, downpipe taken to a drain |
| Soil washing off a slope or driveway edge | Uncontrolled runoff gaining speed | Cut-off drain at the top, slope drain with a protected outlet |
| Retaining wall leaning, cracking or weeping | Hydrostatic pressure from saturated backfill | Weep holes, stone drainage layer, subsoil pipe behind the wall |
| Drain that used to work and now backs up | Blockage, collapsed pipe or root intrusion | Camera survey, then jetting or repair |
The last row is the important one. If a drainage system worked for years and has stopped, the design is usually fine and something is in the way. Replacing a system that is simply blocked is an expensive way to solve a cheap problem.
Keeping a Drainage System Working
Drainage fails slowly and then all at once, normally in the first heavy rain after a dry spell. A short maintenance routine prevents most of it:
- Clear gutters and downpipe outlets before the rainy season, and again after any heavy leaf fall.
- Lift channel drain grates and remove the silt underneath rather than sweeping over the top of them.
- Empty the sumps in catch pits and gullies once they are more than about half full of grit.
- Keep tree roots away from drain lines. Roots find hairline cracks in older earthenware and clay pipe and then fill the pipe completely.
- For kitchens, service grease traps on schedule. Congealed fat is the single most common cause of blocked commercial drainage.
When a line is already blocked, the sequence that saves money is to look first and cut second. A camera survey shows whether the problem is a soft blockage, a root mass or a collapsed section, and only then does the right tool become obvious. Our high pressure drain cleaning service clears fat, silt and root intrusion without breaking up paving, and our drain clearing team works across Cape Town on surface, subsoil and stormwater systems. For background on why drains block in the first place, see blocked drains, causes and solutions.
Drainage Questions We Are Asked Most
What are the main types of drainage?
The four groups are surface drainage (channel drains, slot drains, swales, catch pits), subsurface drainage (French drains, subsoil drains, soakaways), roof drainage (gutters and downpipes) and slope drainage (cut-off and slope drains). Industrial sites add heavy duty channels and interceptors such as grease traps and oil separators.
What is the difference between stormwater and sewer drainage?
Stormwater drainage carries clean rainwater from roofs, paving and gardens and discharges it without treatment. Sewer drainage carries wastewater from toilets, basins and kitchens to a treatment works. They are separate systems, and connecting one into the other is a by-law contravention as well as a common cause of sewage backing up during heavy rain.
How do I know if my drainage is blocked?
The usual signs are water standing where it used to drain away, gurgling from a gully or toilet, a slow emptying bath or basin, a smell around an outside gully, and water rising in the lowest fitting on the property when another one is used. Water appearing at a manhole or gully during rain points to the stormwater side; smells and slow fixtures point to the foul side.
Who is responsible for a blocked drain in Cape Town?
As a general rule, the City is responsible for the sewer and stormwater mains in the road reserve, and the property owner is responsible for everything inside the boundary, including the connection to the main. If a manhole in the street is overflowing that is a municipal fault to report. If the problem stops at your boundary, it is a private drainage job.
Can I connect my downpipes into the sewer?
No. Roof water must go to stormwater, a soakaway or a rainwater tank. A downpipe plumbed into the sewer sends the whole roof area into the foul system every time it rains, which is what causes sewers to surcharge into gardens and streets during a storm.
How often should drainage be cleaned?
For a normal home, gutters and catch pits once a year before the rainy season is usually enough, with a second gutter clear if you have heavy leaf fall. Commercial kitchens need grease traps serviced far more frequently, and any line with a history of root intrusion is worth jetting and re-surveying annually rather than waiting for the next blockage.
Getting Drainage Right the First Time
Drainage is not one product. It is a chain that runs from the roof, across the surface, through the soil and out to a discharge point, and it is only ever as good as its weakest link. Identify what the water is actually doing, match the system to that behaviour, and then maintain the collection points, which is where almost all failures start.
If a drainage system on your property has stopped doing its job, blockages are the most likely cause. Drain Blasters has cleared and maintained surface, subsoil and stormwater drainage across Cape Town since 2007. Get in touch for drain clearing or read more about the services we offer.
