Flat roof construc­tions: What should definitely be addressed when planning and awarding contracts

A lecture by Professor Zöller from the Aachen Institute for Building Damage Research at the Flat Roof Management Congress of IQDF e.V. in March of this year made me sit up and take notice. Construction work, even if it has been carried out without errors, can be defective.

If freedom from defects is about the fact that a service does not deviate from specified criteria and standards, then freedom from defects presup­poses, in addition to the fulfilment of the criterion of freedom from defects, that the service provided is able to perform the intended function without restric­tions for the service life to be expected according to the general under­standing. The service is therefore free of defects in the law on contracts for work and services if it is suitable for the contrac­tually agreed or, if nothing has been agreed to do so, for the usual use.

Construction methods and uses of flat roofs are subject to constant change. Whereas in the past it was an iron principle to remove rainwater from roof surfaces as quickly as possible in order to avoid damage to the building by running it with a slope as far as possible and by applying as few other layers as possible to obstruct water drainage, the trend today — in a certain way disre­garding earlier safety conside­ra­tions — is towards using roof surfaces more and more as usable areas for greening, photo­vol­taics and urban living spaces. As far as possible, these uses are incre­asingly being expanded to include measures to store rainwater on the roof surfaces, with the aim of preventing overloading of rainwater sewers during the incre­asingly frequent heavy rainfall events and of positively influencing the urban climate by creating evapo­ration surfaces.

Although attempts are made to take into account the risks of damage to the water­pro­ofing associated with these uses by placing increased demands on the robustness of the water­pro­ofing systems and the use of protective layers, the cases of damage that have occurred in practice to date show that, despite these measures, flat roofs have a high risk of leaking during the construction period or of leaking during the usual service life.
In view of the requi­re­ments for freedom from defects outlined at the beginning, this leads to the question of whether a flat roof, which has a signi­fi­cantly increased risk of damage compared to other parts of the building construction, has the suita­bility expected from the user’s point of view (for long-term damage-free use) to be free of defects even if it is executed without defects. In order to answer this question, it seems relevant whether the user, in this case the building owner, must have been aware of the risks associated with the planned construction method and thus accepts them as syste­mi­cally given and customary when he agrees to his archi­tect’s planning or the contract is concluded with the contractor, or whether planners or contractors as experts should have referred to these risks when approving or concluding the contract, namely that

• the water­pro­ofing may leak due to incorrect workmanship and effects caused by construction time,
• the water­pro­ofing may leak during the service life and will leak at the end of the material service life,
• leaks due to the wear layers on the water­pro­ofing cannot be syste­ma­ti­cally detected or located in their location,
• leaks can cause extensive conse­quential damage to the building structure, especially in the case of moisture-sensitive construction methods,
• the elimi­nation of leaks can cause considerable costs if the water ingress point into the roof structure is not known and cannot be deter­mined,
• especially with retention roofs, the risks are further increased, since the increased water accumu­lation on the water­pro­ofing allows water to penetrate the construction much faster and thus in signi­fi­cantly greater quantities than with water­pro­ofing without water accumu­lation,

to ensure that the client decides on the proposed construction method in knowledge of these risks and thus accepts it as an agreed quality.

With regard to the question of the infor­mation obliga­tions existing at the time of planning approval or conclusion of the contract, it seems obvious that planners and contractors, as experts, should also point out to their client as non-experts what possi­bi­lities exist to minimise the damage risks associated with the proposed execution, which in summary consist above all in the fact that leaks in overbuilt water­pro­ofing without a suitable, close-meshed Leak monitoring cannot be syste­ma­ti­cally detected and localized, so that a proper function of the water­pro­ofing as a prere­quisite for its servicea­bility is not ensured in the long term. Advising clients in connection with the execution of flat roofs should therefore also include the topic of real-time monitoring systems, as only they enable rapid detection and good localization of water­pro­ofing damage in overbuilt and used water­pro­ofing surfaces. They represent a tried-and-tested solution for this.

If this infor­mation is not provided by the planner or contractor, disputes between the parties are inevi­table, especially if the building owner suffers damage due to water­pro­ofing damage that was not detected or detected too late or cannot be localized, which would not have occurred or would not have occurred to the extent that had occurred if a real-time monitoring system had been used.

The author is Managing Partner of ProGeo Monitoring Systeme und Services GmbH & Co. KG, Großbeeren and Chairman of the German Association for Leak Detection and Monitoring e.V., Berlin