Wide Flat Roofs In Cheetham Hill And Crumpsall

Key takeaways

• Wide flat roofs drain inward through outlets rather than outward over an edge, which makes blockage far more serious.
• One blocked outlet on a large roof can hold a very large volume of water, and water is heavy.
• Overflow provision exists so a blocked outlet shows itself outside rather than inside. Many older roofs have none.
• Upstands, outlets and rooflight kerbs are where these roofs fail, not the open middle.
• For commercial premises the cost is usually stock and business interruption rather than the repair.

Why internal drainage changes everything

A pitched domestic roof sheds water to its edges, where a gutter catches it. A large flat roof usually cannot do that, because the distances are too great and the edges are often parapets rather than open eaves. Instead the roof is laid to fall towards outlets positioned within its area, which take the water down through the building in internal pipework. The design is sound and it has one serious characteristic: the failure mode is inward. When an outlet blocks on a pitched roof, water spills over a gutter onto the ground. When an outlet blocks on an internally drained flat roof, the water stays on the roof and rises.

The weight nobody thinks about

Water is heavy, and a large flat roof holding even a shallow depth of it is carrying a substantial load that the structure was not designed to support permanently. As the roof deflects under that weight, the deflection deepens the pool, which collects more water, which deepens the deflection further. On a well maintained roof this never begins. On a roof with a blocked outlet and no overflow, it can develop over the course of a single prolonged wet spell. This is the mechanism behind the most serious flat roof failures, and it is entirely preventable by keeping outlets clear.

Overflows, and why they matter

Good practice provides an overflow: an opening through a parapet or an additional outlet set slightly higher than the main one, so that if the primary outlet blocks, water escapes visibly over the side of the building rather than accumulating. It is a deliberately ugly solution and that is the point, because water pouring down an external wall is noticed immediately, whereas water quietly rising on a roof is not noticed at all. Many older buildings have no overflow provision. Establishing whether yours does is one of the more useful things a building owner can find out.

Where these roofs actually leak

Rarely in the open middle. The failures are at the details: the upstand where the roof turns up against a parapet or an adjoining wall, the termination of that upstand at the top, the outlet where the covering meets the drainage, and the kerbs around rooflights and plant. Each of those involves the covering being formed around a change of direction, and each takes more thermal movement than the flat area does. When a leak is reported inside a building of this type, the search should start at the nearest detail rather than at the point directly above the drip.

The internal pipework problem

Internal drainage means rainwater pipes running down through the building, often in ducts, cupboards or wall voids. When one of those blocks or fails, water is released inside the structure rather than outside it, sometimes several floors below the roof and some distance from where anyone would look. It also means that a drainage problem can present as a leak with no obvious relationship to the roof at all. On any building with internal downpipes, those pipes and their gullies deserve to be part of the maintenance schedule rather than being remembered only when something goes wrong.

What maintenance actually looks like

For a roof of this type, a schedule rather than a response. Outlets and any gratings cleared at least twice a year and after heavy leaf fall, upstands and terminations inspected, the covering checked for splits at the details, standing water noted and its position recorded so changes are visible over time, and any rooflight kerbs and plant fixings looked at. None of that is expensive relative to the value of what sits below, and all of it is dramatically cheaper than a failure. Keeping a dated photographic record of each inspection is worth the small effort, since it makes change visible.

The real cost of a failure

On a commercial or mixed use building, the repair is usually the smallest part of the bill. Water coming through a ceiling reaches stock, equipment, electrical installations and sometimes tenants, and the cost of interruption, replacement and disruption dwarfs the price of the roof work that would have prevented it. Insurance may cover some of it and will ask about maintenance. This asymmetry is the whole argument for planned inspection on buildings of this type: the maintenance is a small predictable cost and the failure is a large unpredictable one.

Repair, overlay or replace

A defect at a specific detail on a covering with life left in it is a repair. A covering that is generally sound but tired can sometimes take an overlay, though that adds height at every upstand and does nothing about falls or drainage. A roof that ponds persistently, has been patched repeatedly, or has a wet deck beneath needs stripping so the falls and the drainage can be corrected and the structure inspected. On a large roof these are significantly different sums, so it is worth understanding which is being proposed and what evidence it rests on, including whether anyone has established the state of the deck rather than only the surface.

Rooflights, plant and everything else on the roof

Commercial roofs rarely carry only a covering. There are usually rooflights, extract fans, air conditioning units, aerials, cable runs and sometimes access hatches, and every one of those is a penetration through the waterproof layer or a fixing into it. Each needs a properly formed kerb or upstand, and each is a maintenance item in its own right. Plant installed after the roof was laid is a particular risk, because it is often fixed by a contractor whose expertise is in the equipment rather than in roofing, and a fixing driven through a membrane without a proper detail is a leak waiting for its first winter. When any equipment is added to a roof of this type, how the waterproofing will be maintained around it should be established beforehand rather than discovered afterwards. It is worth making that a condition of any installation, and worth keeping a record of who formed each detail, because responsibility for a leak around a fixing is otherwise very difficult to establish later.

Keeping the maintenance record

For a commercial building there is a further reason to document inspections beyond knowing the condition of the roof. Insurers frequently ask what maintenance has been carried out when a claim is made, and an absence of records makes it easier to characterise a failure as neglect rather than misfortune. Tenants and their solicitors ask similar questions when damage affects a let space. A straightforward log of inspection dates, what was found, what was cleared and what was repaired, with photographs, takes very little effort to maintain and materially changes the position if a dispute arises. It also makes handover to a new managing agent or owner far less of a guessing exercise.

Frequently Asked Questions

Why is a blocked outlet on a flat roof so serious?

Because the roof drains inward rather than over an edge, so a blockage means water stays on the roof and rises rather than spilling harmlessly to the ground. Water is heavy, the structure deflects under it, and the deflection deepens the pool, which is how the worst flat roof failures develop.

What is an overflow and does my building have one?

An opening through a parapet or a second outlet set slightly higher than the main one, so a blockage shows itself as water pouring down the outside of the building rather than accumulating unseen. Many older buildings have none, and finding out is one of the more useful checks an owner can make.

Where do large flat roofs usually leak?

At the details rather than the open middle: upstands against parapets and walls, the termination at the top of an upstand, outlets, and the kerbs around rooflights and plant. Those points involve changes of direction and take more movement, so they fail first.

How often should a commercial flat roof be inspected?

At least twice a year, with outlets cleared and after heavy leaf fall. Inspection should cover upstands, terminations, the covering at details and any standing water, ideally with dated photographs so change is visible over time. The cost is small relative to what sits under the roof.

Water is coming in on a lower floor. Is that the roof?

It might be the internal rainwater pipework rather than the roof surface. Buildings with internal drainage carry pipes down through the structure, and a blockage or failure releases water inside the building, sometimes several floors below the roof and well away from where anyone would look.

At what point does a commercial flat roof stop being repairable?

It depends on whether the fault is a detail or the system. A defect at an upstand or outlet on a sound covering is a repair. Persistent ponding, repeated patching or a wet deck means stripping so falls and drainage can be corrected. Ask whether anyone has established the state of the deck, not just the surface.