The contractor’s duty to warn: was the risk clearly reported in time?

1. Identify the problem that required a warning

IPLO mentions errors in the instructions, unsuitable items supplied by the client and mistakes in plans or calculations, among other things. First identify the specific risk. Is it an unsuitable base, incorrect dimensions, material unfit for its purpose or a technically incorrect design? The eventual fact that something broke does not by itself establish which earlier signal was recognisable.

Collect the original drawings, data and material choices with version dates. Record where they came from and when the contractor received them. A subsequently revised drawing must not silently be treated as the original. Ask an expert what information was relevant to performing the instructions at that time and what knowledge was reasonably available to the contractor.

2. Establish when the risk could have been known

The duty to warn can arise when entering the contract and during its performance. A problem visible only after opening up a structure requires a different timeline from an error in the first drawing. For each signal, identify who saw it, what they said and which work continued afterwards.

Also ask what inspection was offered or carried out. A contractor has not automatically examined every concealed component fully in advance, but cannot dismiss every known signal without assessment either. The right boundary depends on the instructions and circumstances. An expert report must therefore distinguish observation, knowledge and conclusion, rather than use later damage to portray everything as certain beforehand.

3. Check the substance of the warning

According to IPLO, the warning must be sufficiently specific and unambiguous and identify consequences for proper performance in time. Its purpose is to enable the client to choose what is needed to prevent damage. An isolated phrase “at your own risk” may not explain the technical problem or the actual choice requested. Read the whole message in context.

A useful assessment considers the problem, possible consequences and the moment you could respond. Was there still an opportunity to change the design or material? Was something merely described as difficult, or was the resulting execution risk also made clear? Keep attachments and earlier questions. A general reservation below every quotation does not automatically replace a warning directed at the specific problem.

4. Investigate writing and the relevant timing

IPLO identifies paper, email and app messages as possible written forms. A verbal conversation on site should therefore not be treated as sufficient without further examination. Collect what was confirmed afterwards, by whom and with what content. A note written only months later is different evidence from a message sent to the client beforehand.

For these strengthened warning requirements, IPLO specifically states there is no transitional law: they also apply to existing contracts from before 1 January 2024. Do not confuse this with transitional rules for other Wkb components. For older events, have the precise timeline legally assessed. The contract date alone does not decide every warning question; when the warning should have occurred also matters.

5. Assess your response and any expert assistance

If you asked for execution despite a warning, collect your exact response and what you knew then. Accepting a specific risk may matter, but a general agreement to a schedule does not automatically accept every technical consequence. Ask the contractor to identify the message on which their assertion of your consent is based.

If an architect or structural engineer was involved, provide their role and instructions too. Having your own adviser does not automatically make the warning question redundant. Several responsibilities may need investigating. Let experts describe the technical cause and the lawyer assess the legal allocation of tasks, so a report does not silently become a general liability judgment without context.

6. Separate consequential damage, repair and cost warnings

A possible breach of the duty to warn does not automatically require full reimbursement of every later amount. Investigate what a proper warning would have changed and which part of the damage is connected to that. Distinguish costs that would also have been necessary with proper execution from additional repairs or delay. Keep estimates and alternative solutions separately.

A message saying something will cost more may also concern additional work. The technical duty to warn of an error or unsuitability and the price-related warning are not the same test. Have both questions considered separately when the same message is relied upon for them. This keeps clear whether the dispute concerns a change in instructions, a technical risk or both.

7. Fictional example: client-supplied tiles on an unsuitable floor

Suppose a client supplies their own tiles and the contractor sees during preparation that the existing floor may lack sufficient stability. They report only that laying will take some extra time. After cracks appear, they say all risks were the client’s because the client chose the materials. There is an app message but no further explanation about the floor.

A useful investigation collects material data, the floor’s condition, available knowledge and the actual warning. It can then assess which risk was recognisable and what a proper notification might have changed. This fictional example identifies no liable party. It shows why the source of the material does not answer the entire duty-to-warn question.

8. Checklist for your warning file

First ask a lawyer to assess the specific duty, available knowledge and warning. Have the expert substantiate cause and repair separately. Report immediately if works are continuing and a decision is needed to prevent further damage.

• Original instructions, drawings and material data, including versions.
• Timeline of discovery, execution and possible warnings.
• Full emails, app messages, attachments and responses.
• Architect or structural engineer’s role and factual expert findings.
• Repair estimate distinguishing necessary original costs from additional damage.