On a large factory roof, drainage decides whether a Highveld thunderstorm is a non-event or a shutdown. A big roof collects a huge volume of water in minutes. If outlets, gutters, falls and overflows cannot move that water off the roof and away from the walls and foundations, it ponds, finds weak joints and ends up on your production floor. Good drainage is also what keeps a waterproofing system alive for its full design life.
This guide explains what roof drainage does in a large Gauteng factory, what the South African rules expect, where systems usually fail and what to check before the next rainy season.
TL;DR
- Roof drainage on a factory has one job with three parts: collect rain, carry it off the roof, and discharge it where it cannot harm foundations, walls or neighbours.
- SANS 10400-L expects flat roofs to have a defined fall and expects gutters and downpipes to be designed so that, if they block, overflow does not enter the building.
- Most leaks traced to drainage come from small details: shallow falls, protruding outlets, missing weep holes and poorly sealed joins between roof sheeting and membrane.
- Gauteng storms are short and intense, so inlets, gutters and sumps must be cleared and inspected before each summer, not after the first flood.
- Drainage and waterproofing must be assessed together by a competent person, because a perfect membrane fails if water has nowhere to go.
Table of Contents
- What roof drainage actually does in a large factory
- Why Gauteng factories face a tougher test
- What SANS 10400-L expects from a flat roof
- Where factory roof drainage fails in practice
- Old thinking versus current guidance
- A practical drainage check for your factory roof
- Drainage and waterproofing are one problem
- Our perspective
- Getting a drainage-led assessment
- FAQ
- Sources
What roof drainage actually does in a large factory
A factory roof is not just a lid. It is a catchment. A single warehouse-sized roof can shed the rainfall of a small farm in a heavy storm, and all of it arrives at a few points: gutters, valleys and outlets.
Roof drainage does four things:
- Collects water across the whole roof surface, through falls, valleys, box gutters and outlets.
- Conveys it through downpipes or internal stormwater pipes.
- Provides a safe route when something blocks. Overflow paths matter as much as the main system.
- Discharges water so it does not undercut foundations, cause erosion or flood the yard.
The rules point the same way. SANS 10400-L deals with roofs and requires gutters and downpipes, where provided, to be designed so that an overflow caused by a blockage does not penetrate the interior of the building. For a factory with stock, machinery or electrical gear below, that overflow requirement is the clause that matters most.
Why Gauteng factories face a tougher test
Gauteng gets its rain in sharp summer afternoon storms. The roof is dry for days, then receives a large volume in a short burst. CSIR research on stormwater management describes roof drainage as more than simple water removal: it is a component of the wider stormwater system, and rational design by a competent person is needed as high-intensity, short-duration events put pressure on conventional systems.
Three local conditions raise the stakes:
- Large impervious areas. Industrial sites are mostly roof and paving, so there is little ground to absorb runoff. The Department of Human Settlements stormwater guidance notes that roofs and pavements stop rain soaking naturally into the ground, so water runs quickly into storm drains.
- Debris. Dust, leaves, plastic, bird nests and roof-sheet fixings accumulate between storms and end up in outlets.
- Ageing roofs. Many Gauteng factories have been extended in stages. Each extension adds valleys, joins and penetrations that the original drainage was never sized for.
We cover the site-level picture in more detail in our article on why Gauteng industrial buildings flood after storms.
What SANS 10400-L expects from a flat roof
Many large factories combine profiled steel sheeting with flat or low-slope sections, parapet roofs, box gutters and concrete decks over offices or plant rooms. The flat sections are where drainage problems concentrate.
Falls
SANS 10400-L sets a minimum fall for flat roofs towards external gutters, outlets or roof edges. The standard’s text gives 1:80 where there is no interruption in the flow of water, and 1:50 where there is an interruption. A fall of 1:50 means 20 mm of drop for every metre of run. On a 20 m span that is 400 mm of height difference, which is why falls cannot be left to the screed layer’s good luck.
Ponding
Water that stays on the roof is a problem on its own. It adds load, accelerates ageing of the membrane and sits against every joint and lap. The same standard requires roof drainage to prevent accumulation of rainwater, so a roof that holds puddles for days after rain is not just untidy, it is non-compliant.
Overflow
A blocked outlet is a matter of when, not if. Overflow provisions such as scuppers through parapets, overflow outlets set above the main outlet, or gutters positioned so that spillage falls outside the building, are what turn a blockage into a nuisance rather than an internal flood.
Stormwater below ground
For water leaving the roof through internal pipes, the national guide on drainage for architects gives minimum stormwater pipe diameters of 200 mm, increasing to 300 mm for pipes under buildings or draining roads. It also expects that stormwater disposal must not undercut foundations or cause erosion. Treat these as starting points to check against your engineer’s design, not a substitute for it.
Where factory roof drainage fails in practice
In our experience of the subject, and in the forensic literature, the failures are rarely dramatic design errors. They are small details that add up.
| Failure point | What happens | What to look for |
|---|---|---|
| Falls too shallow or screed mismatched | Water ponds, load rises, membrane ages early | Puddles 48 hours after rain, staining in dips |
| Outlets protruding above the deck | The outlet becomes a small dam that traps water around it | Standing water ringing each outlet |
| Roof penetrations at low points | Localised ponding and sealing failures | Aircon, conduit or pipe penetrations near outlets or valleys |
| Missing or blocked weep holes | Moisture is trapped in walls and parapets | Damp patches and efflorescence below parapets |
| Joins between sheeting and membrane | Water penetration and corrosion | Rust streaks and stained ceilings along the junction |
| Blocked gutters and inlets | Overflow into the building or onto walls | Silt, leaves and plastic in gutters after storms |
The NHBRC’s forensic report on a Parys estate is a residential case, but its findings apply to any flat roof: it ties cracked walls to flat roofs with insufficient retention depth and poor drainage. The lesson carries over to factories. Water that cannot leave quickly finds its way into the structure.
The sheet-to-membrane junction
Many factories have profiled sheeting meeting a flat waterproofed section. This junction moves with heat, is difficult to flash neatly and is often done by two different trades. If the detail is poor, water enters at the join and corrodes fixings and purlins out of sight. Ask who is responsible for this line before a job starts, and make sure it is written into the scope.
Penetrations and plant
Rooftop extractors, aircon units and cable trays are often added after the roof is finished. Each penetration is a place for a seal to fail. Keep them away from outlets and low points, and re-inspect the seals when new plant is installed.
Old thinking versus current guidance
The traditional rule was to get water off the roof as fast as possible. That still holds for the roof itself, but current stormwater guidance asks what happens next. The stormwater guidance from the Department of Human Settlements promotes managing runoff, in line with sustainable urban drainage principles, to limit downstream flooding and erosion. It also acknowledges that industrial runoff can carry pollutants that need appropriate disposal.
For a factory manager, that means three practical questions:
- Where does the roof water go once it leaves the downpipe: yard, kerb, municipal stormwater, or a retention area?
- Could the yard drains handle a storm that fills every gutter at once?
- Does roof runoff mix with process areas, chemical stores or loading bays?
The CSIR chapter also notes that managers must account for the cumulative effect of climate change. Treat today’s drainage capacity as something to review, not a one-off installation.
A practical drainage check for your factory roof
Before the rainy season (Gauteng’s summer rains build from about October), walk the roof with a competent person. Follow this order:
- Clear every outlet, gutter and valley of debris. Photograph before and after.
- Check the outlets’ level. They should sit flush with the deck, not proud of it.
- Run a hose test on a suspect section and watch where the water goes. Does it reach an outlet, or stop in a dip?
- Inspect weep holes in parapets and upstands for blockage or absence.
- Look at every penetration and sealing detail, especially near low points.
- Identify the overflow path. If the main outlet blocks, where will the water spill? If the answer is “inside”, get it fixed.
- Follow the downpipes to ground level. Check that discharge points are clear, away from foundations and connected to working yard drains.
- Record everything, and repeat after major storms.
Our waterproofing inspection checklist for South African buildings and maintenance guide go deeper on frequency and record keeping. Working at height on a large industrial roof is a safety matter: use a contractor with proper fall protection rather than sending a maintenance worker up with a ladder and a bucket.
Drainage and waterproofing are one problem
A new membrane laid over a roof with shallow falls and badly set outlets will pond, and ponded water shortens its life. The reverse is also true: new outlets do little if the membrane around them is cracked. A sound approach treats both together:
- Survey falls and outlet positions before choosing a system.
- Correct falls where needed, using tapered screed or insulation, before waterproofing.
- Detail outlets, upstands, parapets and penetrations as part of the membrane scope.
- Provide overflow capacity that is clearly visible and tested.
- Agree a maintenance schedule at handover.
For more on the membrane side, read our guide on how to waterproof industrial roofs and the flat roof waterproofing guide for South African owners. If you want to understand what standards a contractor should work to, see our piece on industry standards for waterproofing.
Storms bring wind as well as water. Our article on roof anchors and Gauteng summer storms explains why fixings need inspection at the same time as drainage.
Our perspective
We see the same pattern in factory roof problems: the owner asks about a leak, and the leak turns out to be a drainage issue. The membrane is blamed, but the real cause is an outlet that sits above the deck, a dip that holds water or a gutter full of silt.
Our view is that a roof should be assessed as a drainage system first and a waterproofing surface second. That means asking where the water goes before deciding what to put on the roof. It also means questions you should put to any contractor: do they check falls and outlet levels, how do they detail the sheet-to-membrane junction, what overflow do they provide, and what warranty covers the drainage detailing as well as the membrane?
Those questions reflect how we help owners: we focus on quality, experience, licensing and warranties when connecting property owners with waterproofing professionals. A contractor who cannot answer them clearly is not ready for a roof of your size.
— Eben
Getting a drainage-led assessment
If your factory roof ponds, leaks at the same spot after every storm or has gutters you suspect are undersized, ask for a drainage and waterproofing assessment together. Through Pro Waterproofing you can submit a quote request for roof, concrete deck, foundation and other waterproofing work on commercial and industrial properties in Gauteng and the Western Cape. We connect you with vetted waterproofing professionals, and the quote form is the starting point. Have your roof plans, photos of problem areas and any previous repair records ready.
FAQ
What is the purpose of a roof drain?
A roof drain collects water from a flat or low-slope roof and sends it into a pipe or gutter system so it does not pond. On a factory, it also has to cope with debris and heavy short storms. Good systems include an overflow so a blocked outlet does not flood the building.
What is the standard fall for a flat roof in South Africa?
SANS 10400-L requires a fall of not less than 1:80 where water flow is uninterrupted, and 1:50 where it is interrupted. A competent person should confirm the right figure for your roof, because details such as upstands and steps affect which applies.
How often should factory gutters and outlets be cleared?
At least before the summer rainy season and after major storms. Roofs near trees, dusty processes or bird roosts need more frequent checks. Keep a dated record so you can show insurers and contractors that the system has been maintained.
Can drainage problems be fixed without re-waterproofing the whole roof?
Sometimes. Clearing blockages, resetting outlets or adding overflows can be done on their own. If ponding comes from inadequate falls or the membrane has already degraded, correcting drainage and waterproofing together is usually more durable. A professional inspection will tell you which applies.
Sources
- SANS 10400-L: National Building Regulations Part L: Roofs
- Guide to Architects concerning Drainage, Water Supply and Stormwater Drainage
- NHBRC report: Parys Golf and Country Estate
- CSIR: Stormwater management
- Department of Human Settlements: Stormwater (Red Book, Section L)