The four families of unknowns
| Family | What is at stake |
|---|---|
| Structure | actual capacity, earlier alterations, foundations |
| Make-up | wall materials, voids, existing insulation |
| Services | undocumented routes, materials, condition |
| Substances | materials requiring particular treatment |
The first two are discovered on opening up. The next two are sometimes discovered beforehand, by survey, which is the subject of the branch on surveys and preliminaries.
Why structure is the major risk
Three cumulative reasons.
It governs everything else. A structural problem calls into question the feasibility of the brief itself, whereas a services problem can be worked around.
It is discovered late. Loadbearing elements are covered, and their real condition appears only after finishes are removed.
It costs disproportionately. Structural remedial work is not one budget line among others; it may represent a major share of total cost.
Add a sequencing effect: a structural discovery stops the project, because nothing above can proceed until it is dealt with.
Undocumented alterations
A category of unknown often neglected and yet frequent. It concerns services and technical routes.
An older building has generally been altered several times, and those alterations are rarely documented.
Three recurring cases.
An opening formed without adequate support, whose effect has not yet manifested.
A change of loading, through an added storey, a heavy screed or equipment.
A change of use that altered the loads without the structure having been checked.
Those situations are not necessarily dangerous, but they become decisive as soon as the existing balance is altered again.
What investigative openings deliver
They turn an unknown into data, and that is their only purpose. Trial openings are therefore priced as an investment.
They reduce uncertainty on the points investigated, and on those only.
They do not remove the contingency. An opening documents one point; it does not guarantee the homogeneity of the whole.
They have a direct and an indirect cost, the latter from intervening in a building sometimes still occupied.
Their value is measured by the risk avoided, not by the intellectual comfort they provide.
The rule of proportionality
It guides the extent of investigation and avoids both excesses. A trial opening costs less than a stoppage.
Investigate where the stakes are high and uncertainty strong. Structure, if the brief involves loadbearing alterations. Services, if installations are retained.
Do not investigate what will be removed anyway. Knowing the condition of an element due to disappear achieves nothing.
Match the scale of the brief. Heavy renovation justifies investigation that refurbishment does not.
A simple principle follows: the cost of investigation must remain well below the cost of the discovery it avoids.
How to handle a discovery
Four steps, in this order, and the order matters. They run from observation to the decision to open up.
Record and document, with photographs and location, before any intervention.
Have it technically characterised, to distinguish what must be dealt with from what may be.
Price it and decide in writing, the decision preceding execution, covered in the article on addenda and additional works.
Reassess the remainder of the brief, a discovery being liable to absorb the contingency and force choices on what follows.
The commonest error is to execute before deciding, under programme pressure. The question of price then arises afterwards, in the worst balance of power.
What this means for a professional
Four rules.
Distinguish what is known from what is assumed in every document given to the client.
Propose investigation according to risk, explaining what it avoids rather than what it costs.
Provide contractually for handling discoveries, before a discovery occurs, as the article on discovery clauses develops.
Never carry out unforeseen remedial work without a written decision, however urgent it appears.
This article sets out principles of professional method and serves as orientation. It does not replace technical advice or assessment of the individual case.