HOW TO AVOID PONDING ON FLAT ROOFS

Ponding is a common occurrence on flat roofs, particularly after heavy rainfall. Even though the integrity of Soprema waterproofing membranes is not affected by temporary standing water (as evidenced by their use in blue and green roofs), we follow the practices formulated in BS 6229:2018 to design flat roofs in such a way that surface water is cleared as swiftly as possible. Among the factors determining this flow, the slope or fall of the roof is the most important.

THE RISKS ASSOCIATED WITH PONDING

Ponding areas increase the risk of a flat roof deck getting deflected. Where the load of the water presses the surface down, dips can appear, leading to increased ponding. Thus, standing water can compromise the structural integrity of the roof in the long run.

Regardless of where they occur, punctures in a waterproofing membrane of course pose a significant problem. However, when a roof is punctured or damaged in a ponding area, the standing water can create a pressure buildup which can then accelerate the flow of water into the building, potentially causing greater damage to both the structure and the interior.

Dirt, leaves and algae tend to accumulate in ponding areas. If this debris dislodges, it can flow towards and block the outlets, thereby increasing the likelihood of additional ponding.

Ponded areas also create a challenge for accessing and maintaining both the roof and any equipment or installations mounted on it. In sub-zero temperatures, areas with standing water will freeze over, posing a significant safety hazard. The freeze/thaw impact on flat roofs with ponding water can also threaten the integrity of the waterproofing.

THE IMPORTANCE OF FALLS TO AVOID PONDING

BS 6629-2018 recommends creating sufficient slopes for effective drainage. We will explain what that means for flat roofs, zero-falls roof and gutters.

1.     FLAT ROOFS

A roof is considered a flat roof if its pitch is between zero and ten degrees. According to BS 6629-2018, a minimum 1:80 finished fall is advisable for both the overall roof area and any internal gutters. To allow for site inaccuracies and minor deflections in the deck, flat roof surfaces should be designed with a fall of 1:40 to ensure the desired finished fall of 1:80. All relevant design factors, such as deflections under load and construction tolerances, should be considered.

When two roof surfaces with different fall directions meet, a so-called cross fall is created. The slope at this junction will be less than the main roof falls. The overall roof design should ensure that the crossfall still meets the minimum slope of 1:80.

BS 6629-2018 further explains that the required fall can be achieved either by sloping the supporting structure or by means of screeding or using a tapered insulation system.

2.     ZERO-FALLS ROOFS

Zero-falls roofs, which are often inverted roofs or protected flat roofs with limited access, have a slope between 0 and 0.7 degrees. Following BS 6629-2018, they should be designed with a fall of at least 1:80 to ensure that water drains freely and to avoid back falls. Inverted roofs with XPS insulation and (most commonly) DUOFLEX structural waterproofing or with ALSAN liquid waterproofing can be designed at zero falls. Both DUOFLEX and ALSAN achieve zero falls BBA approval.

3.     GUTTERS

Gutter beds should be designed at 1:40.

SURVEY, ADVICE AND DESIGN SERVICE

In the design phase of a new build, you can rely on Soprema for advice on how to create sufficient slope. In refurbishment scenarios, we carry out a survey before the work starts and likewise make recommendations, for instance a tapered insulation fall or suggesting stripping the roof back to the substrate to achieve the required slope.

We also direct attention to the overlaps, which should face in the direction of the fall of the roof, and we consider extra imposed loads on the roof that might interfere with the final roof fall. The load should be within tolerance of the compressive strength of the insulation. Otherwise, the insulation might compress and dent, leading to ponding and possibly to a roof fall that is no longer 1:80.

In addition, we make sure that the design of the roof allows for minimum upstand heights of 150 millimeters above finished roof level, in accordance with Building Regulations.

During the work, we carry out site inspections and provide advice. When a project is completed, we do a final inspection and sign-off on the integrity of the waterproofing as a whole.

OTHER MEASURES AGAINST PONDING

Next to ensuring the correct fall, Soprema can help you take other measures to avoid ponding water.

• We take care of the necessary drainage calculations to determine the correct amount of rainwater outlets, with the right size and positioned in the right places (not set too high in relation to the finished surface of the roof covering). We will also propose extra outlets for areas with back falls.
• In the design phase, we make sure that the depth of any sump or gutter is at least 25 millimetres above the maximum water level, to prevent ponding water from overflowing onto the roof slope.
• For areas with interrupted falls, such as spaces directly behind roof lights or plinths for plant installations, we incorporate drainage crickets. These double-triangle structures are designed to divert water around the obstruction and towards the outlets.

For more information on HOW TO AVOID PONDING ON FLAT ROOFS talk to SOPREMA UK