Cookson's Consultancy Ltd

21/08/2026

Not every old roof deck needs to come off.

That was the key question on this one.

We were asked to inspect an existing flat roof formed over a channel-reinforced woodwool slab deck and advise whether the deck could be retained as part of a refurbishment scheme.

The initial information pointed towards a mechanically fixed overlay system, but before anyone committed to a specification, the client wanted impartial advice on what was actually right for the roof.

That is where proper investigation matters.

We reviewed the existing roof externally and internally, carried out core samples, checked the roof build-up, assessed moisture within the system, reviewed the defects and worked alongside a structural engineer to consider the condition of the woodwool deck and fire-affected areas.

The existing waterproofing had clearly reached the end of its serviceable life. The mineral surface had degraded, laps were opening, outlets were blocked in places, previous patch repairs were failing and the fibreboard insulation beneath the felt was saturated.

But the important finding was lower down in the build-up.

The existing bituminous vapour control layer was still doing a job. The fibreboard above it was wet, but the woodwool beneath it was dry at the sample locations.

That completely changes the strategy.

Instead of defaulting to wholesale deck replacement or mechanically fixing through the woodwool, the more appropriate route was controlled refurbishment:

🔹Strip the failed waterproofing and saturated fibreboard back carefully.
🔹Retain the existing vapour control layer where suitable.
🔹Retain the sound woodwool deck where structurally acceptable.
🔹Locally replace fire-damaged or water-damaged areas.
🔹Install a new insulated warm roof system.
🔹Improve the falls and drainage.
🔹Use a bonded waterproofing system in the first instance, backed by a suitable manufacturer guarantee.

Woodwool can sometimes get unfairly treated because it gets associated with other historic deck types, but it is a completely different material. Where channel-reinforced woodwool is dry, stable and structurally sound, it can often be retained successfully.

The value for the client here was not just a condition report.

It was the full route through the problem:

🔹Initial impartial review
🔹Joint roof and structural investigation
🔹Core sampling and moisture assessment
🔹Clear refurbishment strategy
🔹U-value and condensation risk calculations
🔹Outline performance specification
🔹Advice on tapered insulation and drainage improvements
🔹Next steps for manufacturer input and tender documentation

That is the difference between a manufacturer-led specification and a roof-led specification.

The product should suit the roof.

Not the other way around.

30/06/2026

Woodwool slab roofs are often misunderstood.

And sometimes they are confused with Stramit, which can lead to the wrong assumptions being made before the survey has even started.

I have just published a new blog looking at why woodwool slab roofs need proper assessment before anyone decides whether to repair, overlay or replace them.

The key point?

You cannot make a reliable decision from the roof covering alone.

You need to understand the deck beneath it.

Is it channel reinforced?

Is it supported on timber bearers?

Is it pre-screeded?

Has there been long-term moisture ingress?

Is the existing waterproofing bonded?

Would stripping it damage the deck?

What is going back on top?

A local repair is one thing.

A warm roof overlay, tapered insulation, plant, ballast or solar PV is something else entirely.

The blog also looks at why woodwool should not be casually grouped with Stramit or strawboard. They can look similar in some situations, but they are different materials and should not automatically lead to the same recommendation.

Sometimes replacement is the right answer.

Sometimes retention and overlay may be possible.

Sometimes further investigation and structural input are needed before anyone should commit to a specification.

The important thing is that the survey drives the recommendation, not assumption.

You can read the full blog here:

https://bit.ly/4aHSaFi

If you are responsible for an older commercial flat roof and need independent advice before repair or refurbishment, Cookson’s Consultancy can help you understand the construction, risks and options before major decisions are made.

A practical guide to surveying woodwool slab roofs, including deck identification, moisture risk, fragility, overlay options, structural considerations and the key differences between woodwool and Stramit roof decks.

24/06/2026

Woodwool slab roofs need careful surveying.

Not assumptions.

I am seeing more of these roofs now, and the same question keeps coming up:

Can the deck be retained, or does it need to come out?

The answer depends on what you are actually dealing with.

Woodwool slabs were commonly used as flat roof decks from the 1960s through to the late 1990s. They are generally formed from shredded timber bound in a cementitious matrix and can be plain faced, pre-screeded or pre-felted.

Some are channel reinforced.

Some are supported on timber bearers.

Some were used as permanent shuttering to another form of deck.

Those differences matter.

Before making any recommendation, I want to understand:

What type of woodwool slab is present?

Is it channel reinforced or supported on timber?

Is it pre-screeded?

How thick is it?

Is there evidence of long-term moisture ingress?

Are there signs of deflection, softening, staining or local breakdown?

What condition is the existing waterproofing in?

Would stripping the waterproofing damage the slab or screed?

Can the existing system be retained, prepared and overlaid?

And just as importantly, what is going back on top?

A local repair is one thing.

A warm roof overlay, tapered insulation, plant, ballast or solar PV is another.

That is often where structural input becomes necessary.

HSG33 is helpful from a safety-first point of view. Roofs should be treated with caution until properly assessed, and woodwool can be fragile, particularly where it is wet, damaged, thin or inadequately reinforced.

But woodwool still needs nuance.

It should not be treated as a single generic deck type.

It also should not be casually confused with Stramit.

Stramit, or strawboard, is a different material altogether. It is compressed straw between facings, typically paper faced. It may look similar in some situations, but it does not behave the same way.

When sampled, woodwool tends to chip and flake because the fibres are held within the cementitious binder.

Stramit tends to pull apart as compressed straw.

That distinction is important.

If I identify Stramit, my view is generally that replacement should be anticipated.

With woodwool, the answer is more dependent on construction, condition, moisture history, span, reinforcement, existing build-up and proposed future loading.

Sometimes replacement is the correct answer.

Sometimes retention and overlay may be possible.

Sometimes the safest answer is that further investigation and structural input are needed before anyone commits to a specification.

The key point is simple:

You cannot make a reliable decision from the roof covering alone.

You need to understand the deck beneath it.

And with woodwool slab roofs, each one needs to be considered carefully.

18/06/2026

The importance of core samples. Or, more specifically, knowing when not to take one.

There is a lot of talk in surveying circles about core samples. And rightly so.

In most flat roof investigations, core samples are incredibly valuable. They can confirm the roof build-up, identify trapped moisture, expose hidden defects, and give the client evidence that a surface-level inspection cannot.

In many cases, I would say they are essential. But not always. And this is where experience and independence matter.

I recently inspected a two-level flat roof that, on paper, had all the ingredients for a more invasive investigation: single-ply membrane, reported woodwool deck construction, ponding water, previous concerns, questions over the build-up, and a scope that allowed for core samples.

It would have been easy to arrive on site, cut holes in the roof and say, “That’s what we were asked to do.” But that is not surveying. That is following a process without thinking.

Once on the roof, the picture was different. The single-ply membrane was generally well installed. The laps were checked and no significant delamination was found. There were no reported active leaks. The moisture mapping did not identify elevated readings to the accessible mapped areas.

The main issues were not hidden within the roof build-up. They were on the surface: blocked outlets, vegetation, debris, a gutter that could not discharge properly, a slipped downpipe partially restricting an outlet, and localised mastic and masonry defects.

In those circumstances, cutting into the roof would not have added value. It would have added risk. Every core sample is an opening through the waterproofing. Yes, it can be repaired, but it is still a deliberate breach in a roof that, based on the evidence, was performing reasonably well.

That is the bit that sometimes gets missed. Core samples are not a badge of thoroughness. They are a tool. A very useful tool, but still a tool. The skill is knowing when to use it, where to use it, and when the evidence says it is not justified.

Had this been a “free survey” from someone with a system to sell or a job to win, I suspect the answer may have been different. The ponding might have become a reason to overlay. The woodwool deck might have become a reason to strip. The blocked outlets might have been treated as a symptom of failure rather than a maintenance issue.

But the independent answer was much simpler: retain the roof, clean it, clear the outlets and gutters, carry out localised repairs, and maintain it properly.

Not every roof with ponding water needs replacing. Not every roof with a concerning build-up needs cutting open. Not every survey needs to end with the most invasive option.

Good surveying is not about doing the maximum amount of investigation possible. It is about doing the right amount of investigation, based on evidence, risk and judgement.

Sometimes that means taking core samples. Sometimes it means knowing when not to.

12/06/2026

The client didn’t want a full roof replacement.

The roof didn’t care.

That was the situation on a recent high street retail property where we were asked to provide an independent flat roof assessment.

The question was simple:

Can it be repaired, refurbished, or does it need replacing?

Getting the answer was less simple.

The building was in a pedestrianised town centre location, with no suitable rear access for a MEWP. Before we could inspect the roof, we had to liaise with the local authority, arrange out-of-hours access, coordinate a cherry picker, and carry out the survey after trading hours.

That is often the unseen part of roof consultancy.

Once on the roof, the defects were familiar:

Aged built-up felt.
Poor falls.
Vegetation around outlets.
Defective perimeter details.
Failed upstands.
Ponding.
Internal evidence of water ingress.

There was also a woodwool slab deck beneath the waterproofing, which meant any recommendation had to be carefully considered.

This was not a roof where “patch the leaks” was a sensible long-term answer.

But it also was not automatically a full strip and replacement.

Core samples were dry at the locations tested, which meant the deck may still be suitable for retention.

The technically preferred solution was a warm roof overlay, subject to structural confirmation, incorporating a vapour control layer, insulation, improved falls and a new waterproofing system.

That would deal with the roof properly.

But, the client had lease constraints and needed to understand whether targeted repairs could reduce the immediate risk of water ingress in the short term.

So we prepared a targeted repair specification and pricing schedule.

Not “price some repairs”.

A defined scope:
Outlet refurbishment
New perimeter trims
Felt repairs
Upstand detailing
Gutter works
Access requirements
Caveats and exclusions

Because vague scopes lead to vague prices.

And three vague prices are almost impossible to compare.

A clear specification protects the client, helps contractors price fairly, and makes the risk visible.

That is where independent roof consultancy earns its keep.

Not by telling every client they need a new roof.

And not by pretending tired waterproofing can be magically fixed with a few patches.

But by giving a clear, evidence-based route forward that reflects the roof condition, construction, budget, lease position and risk.

Sometimes the answer is replacement.

Sometimes it is refurbishment.

Sometimes it is a carefully caveated interim repair strategy.

The skill is knowing which is which.

If you are responsible for a commercial roof and need independent advice before committing to major spend, Cookson’s Consultancy can help you understand your options.

11/05/2026

Water ingress during major roof refurbishment works is not always entirely avoidable, particularly on older industrial roof systems already suffering from age-related deterioration and historic defects.

The important thing is how those issues are investigated, understood, and addressed.

As part of ongoing dilapidations works to an industrial unit in Telford, we recently carried out joint flood testing and specialist dye testing in conjunction with RPMG following reports of water ingress during rooflight replacement works.

The building was already experiencing leaks prior to the commencement of the wider remedial programme, which includes:

🔹rooflight replacement
🔹replacement fixings
🔹cut edge corrosion treatment
🔹localised roof repairs

Controlled water testing was undertaken as part of the contractor’s due diligence process to identify the precise source of ingress and to confirm whether water was tracking internally through the existing roof construction.

In one area, dyed water applied to a single defect within the roof sheet resulted in two separate internal leak points, confirming water tracking along the purlin before discharging internally.

Further investigation also identified loose fixings and localised perforations around previously deteriorated cut edge corrosion areas, allowing additional remedial works to be targeted before completion.

This is exactly why proper investigation matters on refurbishment projects. Industrial roof systems rarely fail in one simple location, and understanding the true route of water ingress is critical before repairs are finalised.

Good roofing projects are not about pretending defects never occur during complex refurbishment works. They are about identifying issues quickly, understanding the cause correctly, and implementing appropriate remedial action before completion.

Cookson’s Consultancy regularly supports landlords, contractors and surveyors with independent investigations, defect diagnosis and refurbishment advice on ageing industrial roof systems.