Ponding and the Danger it Poses to Flat Roofs

Ponding on flat roof - Photo by crowbert

New owners of flat roofs will be keen to learn how to guard against possible damage and improve the longevity of the roof.

Much of the damage suffered by flat roofs is caused by the environment. Excessive sunlight can cause the surface of a tar roof to blister and crack, as can cycles of freezing and thawing. But perhaps the most egregious threat is that posed by excessive rainwater, or, more specifically, ponding.

What is ponding?

As much as their name might suggest otherwise, flat roofs are invariably inclined slightly, in order to facilitate drainage. Ponding occurs when rainwater collects on a flat roof – most often on roofs that are not sufficiently inclined, or whose surfaces are insufficiently smooth.

Why is ponding so terrible?

Ponding is a particularly troublesome problem because it is self-perpetuating. When a body of water collects on the surface of a flat roof, it will naturally gravitate towards the lowest areas of that roof. If the weight of a body of water causes that area to sink still further, then water will be still more inclined to move toward that area, which in turn will cause more sinking. This is a vicious cycle whose progress will accelerate exponentially if left uninterrupted and ultimately result in a leaking roof.

A further problem is brought about by the fact that ponding also encourages the growth of algae and other plant life. This is problematic for two reasons. Not only does this look unsightly, but it damages the underlying membrane also and so leaves the roof still more vulnerable to water damage.

Finally, ponding can adversely affect a building’s ability to insulate properly. If water is allowed to find its way beneath the membrane on top of the roof, it will inhibit the roof’s ability to retain heat.

What can be done to prevent it?

Now that we’ve established exactly why ponding poses such a threat, we can look to ways of tackling it. Fortunately, there are a few ways in which the risk of ponding can be minimised.

The first of these involves drainage. The easier it is for water which finds its way onto the roof to find its way to the gutter, the better. Flat roofs, in spite of their name, tend to be pitched slightly so as to allow rainwater to flow away. They will typically contain one or more drains, through which water can escape into the guttering system. Should these drains become blocked, however, then this drainage will be prevented from occurring. This is clearly bad news for the long-term health of the roof.

Guttering should therefore be inspected from time to time in order to ensure that it is clear and that water is able to properly flow through the drainage system. These checks should be performed year round, but the danger is obviously greater during the autumn, when deciduous trees are shedding their leaves. If your roof happens to be near to trees, then this danger will obviously be all the greater.

Grass growing in the gutters - Photo by Weston Renoud

If your area has experienced heavy snowfall, your roof may also be at risk. Like water, snow places pressure on the surface of the roof. But unlike water, snow can pile several feet high! It is therefore important to keep your flat roof clear of snow.

Care should also be taken that the roof is not damaged by sudden shock, such as that brought about when a great weight is placed on it. Most flat roofs are not designed to tolerate the weight of a human being, for example. Even if no damage appears at the outset, walking on the top of a roof can produce an uneven surface which may eventually lead to ponding. Falling branches and other objects can have similar effects.

So far we have explored a few of the ways in which a roof can be safeguarded against ponding. But by far the most effective method of achieving this outcome is through properly designing and installing the roof. The former to ensure that the roof is sufficiently sloped and the latter to ensure that the membrane is evenly and cleanly lain.

The advent of new, more easily installed roofing materials, such as EPDM rubber, has led a legion of cowboys to take up tools and try their hand at roofing. The results have, on occasion, been disastrous. If you are considering getting a flat roof installed, whether it be for an entirely new build or a replacement of an existing roof, then you should use a reputable roofer like Marcus Roofing.  Their website can be found at www.marcusroofing.co.uk.