Why flat roofs and box gutters leak
A flat roof relies on gentle falls, clear outlets and an unbroken membrane, and when one fails, water sits on the surface until it finds a way in. Box gutters sit inside the roof line, so when they overflow, the water runs straight into the building.
In Cape Town, flat roofs take a lot of rain in a short season, roughly May to August, then months of strong sun that dries out and cracks exposed membranes. The south-easter fills outlets with dust and debris, and near the coast, along the Atlantic Seaboard, in Blouberg and up the West Coast, salt air is hard on metal gutters, flashings and fixings.
The usual causes we find are:
- Ponding water, where the falls are too flat or the slab has settled.
- Blocked or undersized outlets, which turn the roof or gutter into a shallow pond in heavy rain.
- Cracks in the slab or screed, from movement, heat and age.
- Failed membranes, blistered, split, lifted at the seams or worn through in the sun.
- Failed details at upstands, parapets, pipes, plant bases and edges, where most leaks actually start.
Standing water a day or two after rain means the falls or drainage need attention first, or the new system will age early in the same places.
How we waterproof a flat roof
Most failures come from the preparation and the details, not the membrane itself, so we fix the cause before the system goes on.
- Inspection. We trace where the water is getting in, check the falls, test the outlets and look at the existing membrane.
- Falls and outlets. Where water ponds, we correct the falls with a new screed. Blocked outlets are cleared, and undersized or badly placed ones dealt with.
- Cracks and joints. Cracks are repaired and expansion joints sealed so they can move without opening up.
- Preparation. Failed layers are removed, and the surface is cleaned and dried. Torch-on laid on a damp or dusty slab won’t bond, a main reason membranes blister.
- Primer and membrane. A primer goes on, then the membrane, with overlaps that run so water flows over them, not into them.
- Upstands, outlets and edges. The membrane is turned up walls and parapets and finished under a flashing or into a chase, and outlets are dressed into the drain. These details get the most time.
- Finishing. A reflective coating or mineral-surfaced top sheet protects the bitumen from the summer sun and keeps the roof cooler.
- Sign-off. Andre or our foreman, Randall, checks the finished roof with you.
Torch-on waterproofing
Torch-on is a modified bitumen membrane on a reinforcing core, laid over a primed surface and heated with a gas torch so it bonds to the roof and the overlapping sheet, forming one continuous waterproof layer. It’s tough, handles our winter rain well when properly detailed, and can be repaired in sections. It suits flat or nearly flat, solid, fairly simple surfaces: flat concrete roofs on factories, warehouses and office blocks, concrete box gutters, parapet tops, and slabs over garages, plant rooms and stairwells.
Because it’s laid with an open flame, we first check what’s near the work area: timber, insulation, plastic vents, gas lines and openings into the roof space. Where a flame isn’t sensible, we use a cold-applied system for that section.
Torch-on or a liquid membrane?
Torch-on usually suits large, open, simple flat roofs and box gutters. A liquid-applied membrane is usually better where:
- there are many pipes, vents, brackets or other penetrations
- the shape is awkward, with lots of corners, steps or curves
- an open flame isn’t safe, for example next to timber or inside a busy building
- an existing system needs a compatible coating rather than another sheet
On many roofs we use both: torch-on on the open areas and a liquid-applied membrane or Soudal sealants on the details.
Box gutters
A box gutter is a channel built into the roof rather than hung from its edge: between two roof slopes, behind parapet walls or along the edge of a flat roof. They’re usually concrete on older and larger buildings, and galvanised or coated metal on many factories, warehouses and newer roofs. For gutters that hang on the outside edge of a roof, see gutters and fascias.
They leak when outlets block, metal rusts where water sits, joints split, the gutter or outlet is too small for the roof, or concrete cracks under the lining. We recommend the option that lasts longest:
- Repair suits a sound gutter with a few failed joints, a damaged outlet or a local crack. We clear it, seal the joints with Soudal sealants or a compatible membrane, and fix the outlet.
- Relining suits a gutter whose structure is sound but whose lining has failed, or a metal gutter with surface rust. Concrete gutters are usually stripped, repaired and relined with torch-on or a liquid membrane.
- Replacement suits metal gutters that have rusted through, lost their shape or are too small. We replace them with Brink box gutters sized for the roof area they drain.
Many box gutter leaks are simply blocked outlets, so clearing the outlet and downpipe is often our first step, and emergency call-outs are available if water is coming in. Clearing outlets before each winter is the cheapest protection, and we can include it in a roof maintenance plan where your building needs one. Where a gutter runs behind a parapet, we check the parapet and its flashings too; see waterproofing in Cape Town.
Roof terraces and slabs over occupied space
Chairs, planters, braais and foot traffic will damage torch-on or a thin coating left exposed, so a terrace membrane is usually protected under tiles or paving, or finished with a coating designed for foot traffic.
Podium slabs over parking, slabs over shops and offices, and apartment roof slabs have people, cars or stock underneath, so we plan the work in sections, agree access with the building manager or trustees, and pay particular attention to expansion joints, often the weak point.
When waterproofing isn’t enough
If a flat roof has been patched too often, or has moved too much, we’ll tell you honestly if replacing it is the better spend. See roof replacement.
Commercial, body corporate and home flat roofs
- Commercial and industrial: large flat roofs and long box gutter runs over stock, machinery or offices, worked around your operations by working-at-heights compliant teams, with rope access where scaffolding isn’t practical. See commercial and industrial roofing.
- Body corporates: roof slabs above top-floor units, podium slabs and box gutters behind parapets, quoted area by area. See body corporate roofing.
- Homeowners: flat roofs, roof decks over garages and box gutters on modern houses. See roofing and waterproofing for homes.
The guarantee
Our waterproofing workmanship is guaranteed for 1 to 4 years, depending on the job. Where we replace a gutter, your quote sets out the guarantee that applies, along with any manufacturer’s guarantee on the materials.