Firntec - Webinar - Intrusive Structural Surveys CPD March 2026

This Q&A follows our CPD session on Intrusive Structural Surveys, addressing key questions raised by attendees. It provided practical insight into common challenges, methodologies, and considerations when assessing structural safety in existing buildings. 

Ruben Wood, Structural Engineer, answers questions on topics such as intrusive investigations, material testing, structural risk assessment, and best practice approaches for managing uncertainty in building information. 

Make sure to attend our future webinars to put your questions directly to our technical specialists.

Do you find that the single largest challenge is the lack of historic Golden Thread information (i.e., building of 20 years+ age is unlikely to have local authority documents and information as it is outside of their information retention period? Therefore, the assumption that the structure is ok cannot be taken by the PAP. Therefore, they need to take reasonable and proportionate steps to assess the condition of the structure to identify if risks are present so that they can then take necessary and appropriate risk management measures (i.e., to fulfil duties under S83 and S84 of BSA2022). The lack of information and visible elements without intrusive inspection leads to a greater need for intrusive investigation.

Yes that is correct. Ultimately if you do not have archive information on the as-built structure, you will not have sufficient information to adequately assess the risks to fulfil duties under S83 and S84.  

What is HAC testing?

High Alumina Concrete (HAC) was used extensively in the UK from the 1950’s to the early 1970’s. It typically went into precast units as it developed a high strength quickly. However subsequent research into structural collapses in the early 70’s show that HAC undergoes a change in its mineralogical composition with time, known as conversion, which can effectively half its prescribed strength.

Therefore for buildings built within this timeframe it is worth testing for its presence. 

When does depth of carbonation and chloride content indicate a problem?

Carbonation of concrete refers to the process of carbon dioxide diffused into concrete which, in the presence of moisture, reacts with hydrated cement to form calcium carbonate. If this reaches the embedded steel reinforcement it reduces the alkalinity of the concrete to a level where the steel is vulnerable to corrosion. Therefore when the carbonated concrete levels reach the level of cover the risk of corrosion increases.

BRE Digest 444, BS 5328 notes that the maximum upper chloride limit for typical reinforced concrete is 0.4% by weight of cement. Above this limit, the risk of corrosion of embedded steel is increased under damp conditions.

Where do you look for the building drawings if the client doesn’t have them?

Local building control, original developer or design team. If the building is older than the 2000’s the chances of finding original drawings reduces considerably as they were likely in paper format.

How would you define structural risks e.g. are these only risks related to the building's load bearing capacity, thinking about primary and secondary structures? Based on that, to what extent would the external wall systems and cladding be a structural issue in construction types where they don't contribute to building's load bearing capacity but have to carry their own loads?

Yes they are primarily risks to the primary structure. The cladding panels should be considered as their fixings to the main structure may deteriorate causing a risk of dislodging and falling. However they are not a risk to the superstructure itself. 

If I've understood correctly, you wouldn't recommend a construction audit as a matter of course - only if there is a reason to do so?

Correct – typically if it is a non-standard form of construction or a specific risk is noted.  

BSR asked us about the buildings designed lifespan - we defined the age of the building, the design materials and construction type however, I can't see how this can be answered from its original design and generally buildings aren't really built with an expected lifespan in mind for structural stability (hopefully built to stand), any advice on how to answer that?

Buildings typically are built with a 50-60 year lifespan. This does not mean they are necessarily longer serviceable at the end of their lifespan; however a building owner should and is expected to evidence why it is possible to extend its lifespan. This can be done with Health and Condition durability checks to structure.  

Would this determine cause of subsidence or seasonal movement of a building?

Typically with a visual survey it is possible to inspect for signs of differential foundation movement.

As an owner of High-Rise Blocks what can we do to equip ourselves with sufficient understanding of structural surveys to know that the specialist consultant undertaking structural surveys is taking a pragmatic approach and only recommending further investigations / surveys that are needed. Would Firntec provide a second opinion?

Our recommendation is to build a trustworthy rapport with your chosen consultant. Try with a small sample of buildings first and see the results. Firntec can undertake a sample for comparison.

To the point about primary and secondary loads, would a localised failure of cladding or brick slip fixtures be a building safety risk, particularly one that meets the requirements for an MOR? (ignoring potential fire risks) (appreciate this is difficult to answer without proper context)

It is a building safety risk, not necessarily a risk to the structural integrity.  

Do your surveys investigate locations and condition of horizontal fire breaks within external masonry cladding systems?

That is not part of a structural survey, however my understanding is other branches of Firntec can undertake these.  

Can you please advise the years/decades that separate Post War Reinforced Concrete Frames from Modern RC Frames (i.e., when is modern considered from)

1950’s to 1970’ is considered “post war” typology 5’s. Modern are from early 1990’s.

Where there is no/limited information on the fire strategy, when doing cover meter surveys, in addition to the codes at the time of construction, what other assumptions or considerations are made? Curious as we know design sometimes deviates from codes and there may have been engineered solutions, particularly in newer buildings.

How frequently should structural survey reviews be considered for older high-rise buildings 50-70's if there are no obvious defects? & where original structural drawings or calculations are missing in older high-rise buildings, what methods would you recommend to give confidence that the structure is still performing as designed when constructed.

As part of the structural safety risk management for the building an inspection regime which includes a combination of superficial , regular and benchmark investigations should be implemented. These are described as follows.

Superficial Inspection: Personnel visiting the structure and noting visible indications of potential durability issues. The building manager to have an adequate system for keeping up to date with structural changes to the building and an engagement strategy in place for residents to raise concerns.

Regular inspections: The building is to be surveyed at approximately 5 year intervals by a competent engineer to inspect for signs of movement, structural decay and/or defects.

Benchmark Inspections which will include durability intrusive investigations: Every 15-20 years, when there is a functional change, or when a regular inspection notes a particular issue, whichever comes fists. Note these have just been completed.

If the benchmark investigations have never been completed to a 1950’s to 1970’s building then it is time to commission them, regardless of whether visible defects are present as their may be latent defects.

With LPS are we looking at every tie or just a sample? If feels without looking at all, it is hard to put in the SCR that we have fully evaluated the risk.

A sample. BRE guide suggest 10% however this can be very challenging for access. minimum suggested is 3 units.  

For Typology 5 it seems to have almost become a standard requirement from BSR for a Ferro Scan of the RE Framed Block in the absence of detailed drawings etc. Supported Structural and Fire Resistance matters. We needed them and they are not cheap yet gained BACs- is this Firntec experience?

No. Ferrosans are not always a necessity, only a particular risk is noted. Cover meters and health and condition checks are however.

The Risk Assessment is quite illegible. Is it possible to get a sample of this to understand?

This will be included in our structural appraisal reports and are building specific.  

Would this risk assessment be provided within an intrusive survey?

Yes.

So, the best way to satisfy BSR guidance is to carry out a risk assessment on the documentation that you have received?

Yes however further investigations may be required once you’ve done the initial desk based study.  

Assuming the superficial inspections are done by (internal) staff, what sort of training and information should be provided to staff to ensure they have competency for this task?

A superficial monitoring checklist can be compiled which lists out typical visible defects to structure (cracks, spalls etc.)

The 15–20-year benchmark inspections - is this guidance only? I.e. could you enforce this with a building management third party e.g. PFI contract?

No it can’t be enforced, it is however industry guidance.  

Do you offer peer review service e.g. where developer has self-declared issues and forging ahead with their remediation and we need our own oversight?

We can consider this. Please send an enquiry and we’ll review.

With many buildings undergoing retrofits such as cladding replacement, solar panels or plant equipment, telecoms equipment on roof how concerned should building managers be about additional loadings? is this something that needs evidencing to the BSR?

Most of those items are lightweight and unlikely to severely effect the structure. However, it is wise to check the bearing locations on structure top inspect for signs of overstress.   

Does the recommended frequency of visual or intrusive structural surveys for high-rise buildings depend on the age of the building? For instance, would a new build typically require inspection every five years, whereas an older building might require surveys every three years? We have received mixed feedback from the BSR and would welcome your view based on experience.

The frequency is for RC framed structures, so for both old and new buildings. Note of course the newer buildings are less likely to be due the health and condition checks.

When reviewing existing HRB's what are the most common structural assumptions that turn out to be incorrect?

Fire resistance of structural elements.

Firntec provides Structural Surveying for residential, education, healthcare and commercial buildings. Our expertise covers a variety of structural assessment types, from reinforced concrete frame material testing and cladding fixings surveys to large panel building checks, non-destructive testing, RAAC identification, and comprehensive structural risk assessments. By combining visual inspections, intrusive surveys, material testing, and performance analysis, Firntec ensures the safety, integrity, and compliance of buildings across all these sectors.

If you would like a specific quote, then fill out our contact form with as much detail as possible and one of our team will get back to you.

Thanks again to everyone who attended one of our events this year, we look forward to welcoming you back in 2026 for more discussions, insights and legislative updates.