Structural calculations essential in transformation
The main concern when creating a multi-purpose roof on an existing roof is that the roof and underlying structure must be able to support the additional weight of greenery, water, solar panels and/or people on the roof. In many cases, the existing structure must first be reinforced to do so. A structural engineer can calculate whether this is necessary and what modifications are required. If people will be accessing the roof, safety is also an important issue and the roof will have to be modified accordingly. Think for example of a sufficiently high parapet. This will often require a design.
If there is an error in the calculations or drawings, it often does not come to light until the building is defective. There is much litigation over whether the client or the contractor is responsible for the design. After all, the harmful consequences of the error (defects in the work and consequential damages) are borne by that party. A transformation project may also involve defects in the pre-existing construction that may have been caused by the error in the transformation design. Thus, the question is:
Who is responsible for the design?
It depends on how parties contract. Roughly speaking, three different situations can be distinguished:
Roofing contractor performs design
There is a traditional contracting agreement where the client hires a structural engineer and architect to create a design for a multi-purpose roof for him. In this situation, the roofer performs the design alone. In this case, the client is responsible for the design and therefore for any errors in that design. This allocation of risk follows from Article 7:750 of the Civil Code and paragraph 5 (2) of the UAV 2012. Be careful when in this situation you give a concrete advice about a certain part of the design because then you still draw design responsibility to yourself.
Roofing contractor designs and implements
There is a contracting agreement under which the roofer not only constructs the multi-purpose roof but also makes (or commissions) the design and calculations. In this situation, the design responsibility shifts to the roofer. Think of a design & build contract or a UAV-GC 2005 contract or the situation that you offer a "ready-made" concept based on a design by a supplier or subcontractor. In these cases, you are liable to your client for the design. Before concluding these contracts, weigh carefully whether it is desirable for you to take on the design risk and what should be in return. Also see if you can insure yourself for this. It may go without saying, but only enter into such an agreement if you are capable of carrying out the design work properly or have yourself assisted by expert consultants.
Roofing contractor works in construction team
There is a construction team for the realization of the multifunctional roof. The client, consultants and contractor work together during the design phase to realize the structure. However, they are each responsible for the accuracy of their own designs and advice.
This distinction seems easy to make. In practice, however, it is not always immediately clear which of the parties is responsible for the design. A common example is when the roofer makes working drawings based on the client's design. This does not mean that the design responsibility has thereby fallen to the roofer. The working drawings are merely details from the whole in order to perform the work. For that reason, design responsibility should remain with the client in that situation as well. Consider also the situation where the client, in consultation with the roofer, makes a - in retrospect - wrong decision about a certain aspect of the design. It is then difficult to determine exactly what the roofer's part in the wrong decision was. It is therefore a good idea to put as much as possible in writing so that, if necessary, you can prove how a decision was made.
Alert duty
Incidentally, this risk allocation is nuanced by the duty to warn the roofer in case the design originates from the client. This means that you have a duty to inform your client if you discover an error in the design (Article 7:754 of the Civil Code and paragraph 6 (14) of UAV 2012). A warning must always be given to the client in writing and as specifically and clearly as possible (for example, in a construction meeting report). If you fail to warn when you should have done so based on the circumstances then you may be liable for any resulting damage. An error in the design made by your client may, however, in this case be a reason to lay part of the damage on the client. There is then a question of "own fault" of the client (article 6:101 Civil Code). Even if the client is an expert himself and nevertheless did not discover the error in his design, this may be reason to lay the damage partly at the client's feet.
Conclusion
If you see a good opportunity in multifunctional roofing, make sure that the parties are clear in advance about the costs and risks involved. Lay this down in the contract. If it becomes necessary during construction to deviate from the design, put in writing why this is being done and at whose initiative.
