Storm and Rainwater Drainage
A clear guide to how rain is collected from roofs and paved areas and carried safely away — gutters, downpipes, gullies and buried storm drains — and why it is kept completely separate from foul drainage.
Rainwater drainage handles a different job from sanitary drainage: instead of a steady trickle of waste, it must cope with short, intense bursts of water from roofs and hard-paved areas. Even in a dry climate, a single heavy shower can drop a large volume of water in minutes, and that water has to be collected and removed before it ponds, floods entrances or undermines foundations.
The guiding principle is separation. Storm water is clean and arrives in surges; foul drainage is contaminated and flows steadily. Mixing the two would overload the sewer during storms and is not permitted in modern design. This lesson covers how rainwater is captured, conveyed and discharged as its own system.
How it works
Collection at roof level. Flat and sloped roofs are laid with a slight fall toward outlets. Sloped roofs drain into gutters at the eaves; flat roofs use roof outlets (rainwater heads) set at low points. Each outlet has a grating or dome to keep leaves and debris from entering and blocking the pipe.
Conveyance by downpipes. From the roof, water drops through vertical downpipes (also called rainwater downpipes or leaders). These are sized for the roof area they serve and the design rainfall intensity, so they can swallow a heavy burst without backing up and overflowing at the roof.
Surface drainage. Rain that falls on paving, podiums and car parks is collected by surface gullies, channel (trench) drains and catch basins. These inlets have gratings and often a small sediment trap, and they connect into the same buried storm network as the roof downpipes.
Buried storm drains and discharge. Below ground, storm pipes run at a gentle gradient to carry collected water to its discharge point. Depending on the site and authority requirements, that may be a public storm sewer, a soakaway that lets clean water infiltrate the ground, or a controlled outfall. The network is sized so it does not run full and back up during the design storm.
Keeping storm and foul separate. Throughout, the storm system never connects to the foul drainage. This separation protects the sewer from being swamped during rain, keeps clean rainwater available for infiltration or reuse, and is a basic requirement of good drainage design.
Main types
In the UAE
- The UAE’s climate is arid but subject to occasional intense rainfall; storm drainage is therefore sized for short, heavy bursts rather than continuous flow, and ponding on roofs and podiums must be avoided.
- Storm and surface-water drainage is kept separate from the foul sewer, and the discharge point (storm sewer, soakaway or controlled outfall) follows the local municipality and authority requirements for the site.
- Roofs, podiums and basements need reliable rainwater removal and waterproofing detailing so that occasional storms do not flood entrances, ramps or lower levels.
How GPR applies this
GPR designs and installs complete storm and rainwater drainage as part of its MEP and infrastructure scope in Abu Dhabi — roof outlets and gutters, correctly sized downpipes, surface gullies and channel drains, and buried storm pipework to the approved discharge point. GPR coordinates falls, waterproofing and outlet positions with the architectural and structural design and keeps the storm system fully separate from foul drainage.
Frequently asked questions
Why is rainwater drainage kept separate from sewage?
Rainwater is clean and arrives in sudden surges, while sewage is contaminated and steady. Combining them would overload the sewer during storms, so modern design keeps the two systems fully separate.
How are downpipe and gutter sizes decided?
They are sized from the roof area being drained and the design rainfall intensity for the location, so the system can handle a heavy burst without overflowing.
What is a soakaway?
A soakaway is a buried voided pit or chamber that lets clean storm water soak gradually into the surrounding ground instead of discharging it to a sewer.
Why do roof outlets have domes or gratings?
The dome or grating keeps leaves and debris out of the pipe so the outlet does not block, which would cause water to pond on the roof.
Does the UAE really need storm drainage if it rarely rains?
Yes. Rainfall is infrequent but can be intense, dropping a large volume in a short time, so buildings still need properly sized storm drainage to avoid flooding during those events.