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The nature of a flat roof means retained water is inevitable at some stage.
That’s why the definitions of water that form, and the issues they can create if not dealt with, is an important conversation to be had with building managers.
“We need to raise more awareness and agree on terminology, including acceptable levels of standing and ponding water, and the risks associated with not following guidance and recommendations,” says Richard Clennell, product manager, Reinforced Bitumen Membranes and Insulation for roofing manufacturer Bauder.
There needs to be “buy-in from the industry,” says Clennell, and agreement on how the two are classed. “People are aware of the challenges associated with it, but generally brush it aside. They do not think about the consequences until there is a problem.” That boils down to damage, danger, and crucially knowing the difference for insurers when it comes to signing off the roof.
For Clennell, standing water should be defined as “a small amount of water held behind a lap/detail for no more than 48 hours after the rainfall ends, and not considered detrimental to the waterproofing system.”
Ponding water is “a large amount of water greater than the depth of the laps, accumulating because of poor falls or deck deflection that is held for over 48 hours after the end of rainfall,” he says.
Over time, ponding water may affect the integrity and lifespan of the waterproofing itself, and will in essence, lead to the need for additional maintenance. “The amount of ponding water on the roof could cause deflection of the deck because of the increased load on the roof,” says Clennell.
But, he adds, “there's no clear definition currently, nothing written in the standards to define which one is which, and if some insurers see any water on the roof surface they will not sign off the roof for guarantee.
“There is a lack of understanding of what's going on with the roof, because generally you are going to have waterproofing laps, so you are nearly always going to have a small amount of water sitting on the roof, regardless of how good the falls are.”
British Standards in roof design
The changes to British Standard BS6229 in 2018 helped add some clarity regarding falls. “There’s certainly been a lot more awareness since the changes were introduced,” says Clennell. “Within Bauder’s specifications there are notes to highlight the minimum finished acceptable falls, and gutters have now been added to the requirements.
“Having gutters with no falls used to be deemed as acceptable, but it no longer is. That’s a big change.”
Both BS6229 and BS8217 recommend a notional design fall of 1:40 (meaning that for every metre of roof there is a fall of 25mm) to achieve a minimum finished fall of no less than 1:80 that allows for construction tolerances to minimise the amount of water on the finished roof surface.
Concerns about the definition of ponding and standing water remain, however. “People have been caught out by the warranty insurers when they're not signing roofs off because there is water on the roof surface. It has made people realise they need to sort it out at the design stage rather than to try and counteract any challenges further down the line, which can be more expensive.”
Clennell believes insurers try to be practical when looking at roofs. “They look at it from a risk perspective – if there’s a lot of ponding water on the roof, the probability is that the waterproofing system, or certainly the life expectancy of it, will be reduced. They’re fully aware of that and they are highlighting a potential problem.
During his time in the roofing trade, Clennell has seen the differing types of damage caused by ponding water on exposed roofs. These include roof membrane deterioration, algae and vegetation growth, as well as increased risk of slip hazards due to a lack of safe walkways.
“Inadequate maintenance is generally going to cause problems on a flat roof. On occasion we have even seen trees growing out of outlets,” he says.
Sometimes it comes down to lack of budget allocated for regular maintenance by the building owner. “If they don’t have a leak or other issues on the roof that need to be fixed, someone may not go up there for years,” says Clennell.
An example of standing water not long after rainfall
A simple solution
Clennell says the best way to keep your roof in shape is to call in the experts at the design stage. “This is easier because you can advise the client on how to do the work, and the correct way of doing it,” he says.
This could include improving the roof falls and combining the recommendations within their project documentation. Where roofs also have some form of surface covering like chippings, algae, or moss, Clennell says it may not be clear there is a build-up of water on the roof’s surface if it’s not immediately visible.
An important point is that adding flat board insulation will not improve the existing falls and that water could still be retained if the falls are not improved to 1:80 or better.
In refurbishment scenarios with an existing roof fall that is poor, the easiest way to fix it can be to incorporate tapered insulation to help improve the fall or re-direct the fall to new drainage points where feasible, Clennell explains.
“In these instances, consideration should be given to the existing deck deflection and as to whether the default 1:60 falls will be enough to overcome this, as you may need to opt for insulation with a greater fall, such as 1:40,” he says. “Alternatively, reduce the deflection by using layers of built-up felt, hot bitumen or thin screeds.”
The other option is to put in additional outlets at the low points, although “this may not be a practical solution,” says Clennell. “The main point is to make sure everybody is aware of what the standards and guidance are. Tapered installation is usually the best solution for avoiding ponding water on a roof that has been laid flat.”
Ultimately, the starting point is to get the definitions between standing and ponding water set in stone. Making this happen will mean the work, and maintenance, will be carried out properly. And more importantly, the insurance sector will have a solid explanation that can only benefit the whole industry.
