Technical
Durability
Achieving durability with
cross laminated timber (CLT )
In this three-part article, Nick Clifford discusses how the major moisture sources
affecting CLT in situ are currently dealt with by the mass timber industry and other
stakeholders, and the changes being driven by the need to reduce the decay risk.
T
hree major moisture sources CLT might face in-situ
which have the potential to cause severe damage
by wood-rotting fungi are construction phase
moisture, leaking weatherproofing and plumbing leaks.
Part 1: Moisture uptake in CLT
buildings under construction
Although mass timber has obvious benefits from a
sustainability and environmental perspective, further
design considerations are needed for CLT to retain its
status as a popular construction method. Failing to make
changes in the face of a series of substantial decayrelated insurance claims for young CLT buildings in the UK
risks damaging the reputation of mass timber products
over the medium to long-term, because of its vulnerability
to fungal decay in certain circumstances unless significant
moisture uptake is considered in the building design and
factored into the management of the construction site.
Construction phase moisture
The importance of moisture management for CLT projects
during the construction phase is well recognised by
manufacturers, suppliers and erectors. Stakeholders have
sponsored research and published technical guidance
aiming to reduce moisture uptake in CLT panels on-site
during construction. Architects, engineers and contractors
are also more aware of the need for moisture management
planning for CLT projects.
The UK climate makes it difficult to avoid wet conditions
during construction, even with the shortened build times
that mass timber can offer. CLT panels are almost
unavoidably exposed to rainfall, which can later cause
problems if high moisture levels go unrecognised. The CLT
industry has focused on methods to mitigate moisture
12 | Timber Industry Yearbook 2024
uptake during construction and driven the move towards
taped joints, panel edge sealants and regular removal of
standing water – these are (or should be) commonplace in
UK CLT projects by now. There is no doubt that these
measures reduce the rate of moisture uptake in CLT panels,
although claims that they moisture-proof the building during
construction are often overstated.
After a building becomes weathertight, construction phase
moisture dries out passively and for most CLT panels in a
building drying occurs fast enough to avoid a fungal decay
risk. When dried, the performance of the CLT will usually be
largely unaffected, notwithstanding a bit of water-staining
and surface discolouration and possibly some gaps
developing between face planks as they dry.
If panels do get wet, it is not all doom and gloom. There are
many examples in the UK where active drying has proven
successful in removing excess construction phase moisture
from installed CLT. In all those cases a phase of active
drying was utilised because it was recognised that a high
moisture level was present in panels that could have
particularly slow passive drying rates, and this presented a
greater potential risk of fungal decay developing before they
had dried throughout.
Flat roofs and terraces
Evidence gathered1 over the past decade shows that the
position and orientation of the panel in the building is a major
factor in the decay risk. Horizontal panels are more widely
affected than vertical panels by excess moisture uptake and
the potential for fungal decay. For walls and intermediate floors,
reduced exposure to rainfall or better drainage, as well as
absence of impermeable coverings in service, are presumed
to be factors in the lower risk of slow drying rates that can
lead to rot developing. Their position, orientation and service
conditions mean that vertical panels and intermediate floors
tend to be less susceptible to wetting during construction and
Technical
Durability
“The UK climate makes it difficult to avoid wet conditions
during construction, even with the shortened build
times that mass timber can offer. CLT panels are almost
unavoidably exposed to rainfall, which can later cause
problems if high moisture levels go unrecognised.”
have sufficiently quick in-service drying rates, which is a
favourable combination from a decay avoidance perspective.
Roof and terrace panels are a different matter. In the UK, by
far the greatest incidence of construction moisture related
rot in CLT has been found in flat roof and terrace panels,
based on many thousands of inspection locations surveyed
by BM TRADA Technical Timber Consultants. These panels
usually take the brunt of the rainfall leading to standing
water or widespread seeping through joints at panel
junctions and perimeters and parapet upstands, so high
levels of moisture uptake are extremely common in these
particular panels prior to waterproofing works.
Completing roof or terrace waterproofing as soon as the
dry panels are installed would be ideal but is rarely
achieved in practice. There is almost always an exposure
period of weeks or perhaps months of potential CLT
wetting. Robust moisture management planning is
therefore needed. It should always include checking to
identify wet panels or wet zones within panels, especially in
roof or terrace panels that will be covered with
waterproofing membranes, insulation and vapour control
layers (VCLs). These moisture impermeable layers and
coverings for roofs and terraces all inhibit drying in the
panels that are at greatest risk of exposure and it is crucial
not to close in panels with high moisture levels. >>
Moisture management should be considered early in the planning phase of any CLT project, before construction starts. Photo: Robin Lancashire, BM TRADA
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Technical
Durability
Moisture uptake will almost always be highest in the layers
with lamellae end-grain exposed at any given panel end or
edge and good moisture monitoring recognises this. The
pattern where rot most commonly develops in CLT panels
typically coincides with this higher risk zone at panel
edges and joints. It’s vitally important to be certain those
areas are dry prior to closing up any roofing works.
Drying out construction phase moisture
Passive drying in panels covered with VCL, insulation and
waterproofing membranes is not fast enough to avoid
fungal decay in many cases. Compared with thinner
wood-based panels or timber, drying is much slower in
engineered mass timber products like CLT and glulam that
are essentially planks glued together, because natural flow
pathways for moisture within the wood are broken and
glue-lines are barriers that inhibit drying.
The base of pitched or valley panels can be vulnerable. Photo: Nick Clifford, BM TRADA
Moisture monitoring
The importance of precise, regular moisture monitoring
during construction cannot be over-emphasised. When high
moisture levels have not been recognised or properly
evaluated, fungal decay can and does develop if panels
remain wet for a prolonged period after closing in.
Unfortunately, there are a significant number of cases in the
UK where this occurred in CLT panels, resulting in difficult,
often extensive and expensive repairs or replacements in
CLT buildings, some as little as five years old.
During construction, moisture monitoring best practice for
horizontal panels should focus on panel perimeters/junctions
and at the base of pitched or vertical panels, with consideration
also given to the multiple depths of readings needed to ensure
that high moisture levels, potentially inducing fungal decay,
are not missed. The key value is 20% moisture content (MC)
or wood moisture equivalent (WME), which is the nominal
fungal decay threshold for wood. The panels need to be 20%
MC or below to avoid a decay risk. Checking only surface
lamellae (the surface layer) for moisture would be a mistake.
Unlike typically much thinner wood-based panel products
such as plywood or OSB, mass timber products like glulam
and CLT retain high levels of moisture deeper within these far
thicker panels. Using insulated deep-probes, deep moisture
levels should therefore be checked at multiple depths,
targeted to coincide with the layers of the panel make-up.
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Although wet CLT panels beneath waterproofing can
eventually dry out of their own accord, it’s unwise to rely on
this. The problem is that fungal decay occurs first, and this
can significantly deplete the load-bearing capacity of these
structural panels. Active or forced drying is usually needed
and although unforeseen drying works delay project
scheduling and impact budgets, the alternative of closing
in CLT too soon, with elevated moisture levels, could well
create a greater, more expensive fungal decay problem
that is difficult to remedy later in the building’s life.
To avoid that, active drying is strongly recommended,
using heated air blown across the panel surface on both
faces. Care should be taken not to overheat the wood,
which can itself lead to brittleness and loss of strength in
the panels. But with targeted controlled drying works, likely
to need several weeks at least, moisture levels can be
reduced until a moisture content of <20% (the nominal
fungal decay threshold) is achieved throughout the full
panel thickness (measured periodically with moisture
checks). At this point the panels are too dry for fungal
decay to develop and closing-up works can be completed.
Trapped construction moisture has caused considerable
fungal decay in various UK CLT buildings. Understandably,
to date the industry focus has primarily been on mitigating
construction phase moisture uptake by increasing
awareness of the issue, better moisture management
planning, improved site practices and better moisture
monitoring. However, it is evident that a wider issue
remains for CLT even if trapped construction moisture
is avoided or overcome. The problem is simple, common
and almost inevitable in any building of any age: leaks
in service. >>
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Technical
Durability
Part 2: CLT and leaks in service
plumbing leak or leaking soil vent pipe waterproofing can
potentially devastate CLT panels in a relatively short period.
It is an unpalatable fact that simple plumbing leaks and roof
or terrace leaks have caused severe structural damage in
CLT buildings at multiple sites, sometimes destroying roof
panels and parapets as well as intermediate floor panels
and internal wall panels, sometimes down through several
storeys, leaving relatively young CLT buildings in need of
major repairs.
Historically, the CLT industry often pointed to the standard of
workmanship by others as the main issue. Although contractor
workmanship and skill issues are undeniably a factor, this
indicates a reliance on workmanship standards which are
rarely achieved in practice, ie perfect waterproofing.
The issue is not confined to the UK and there are reports of
such cases in Australia, Canada, Germany, Scandinavia and
many other European countries.2 For all its benefits, concern
has been growing about CLT, and this is reflected by an
insurance sector that often requires higher premiums for
CLT compared with alternative construction methods.
The concerns stem from the durability of the product:
specifically, its low resistance to fungal decay in the
presence of prolonged exposure to high moisture levels,
which is a common occurrence when leaks occur. If it goes
unnoticed, even a relatively minor defect like a bathroom
The message has been that the key to reducing the incidence
of fungal decay damage was improvements in skills and
craftsmanship of contractors, to avoid any roof or terrace
waterproofing or plumbing leaks. Admirable as that aspiration
is, it remains an unrealistic target that is not a complete
solution. In addition, it doesn’t make allowances for any
defects the building may experience during its life.
In recent years, we have started to see technical and design
adaptations that will be key to reversing the wavering
confidence in CLT on the part of some insurers, warranty
providers, specifiers and end users. The problem is that
concerns with technical details widely used to date may be
difficult to overcome.
Fungal decay in structural floor and wall panels from a single plumbing leak at ceiling height. Photo: Nick Clifford, BM TRADA
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Technical
Durability
“Fungal spores are carried by rainwater, but fungal decay does
not develop the moment a leak occurs and spores come into
contact with the wood. A panel must remain wet for over a
year before fungal growth becomes a real concern.”
Problems with widely used details for flat roofs
and terraces
Consider flat roofs or terraces in CLT. A detail widely used
over the past decade and still very commonly specified today
is for flat CLT panels to be covered with a VCL (usually either
a bitumen-based or foil-backed membrane) adhered to the
panel surface, then layers of rigid foam insulation (often cut
to provide a fall) covered with single-ply waterproofing
membranes often installed with hot-melt joints or proprietary
sealants. It is increasingly clear that this detail may be at
high risk from fungal decay.
Mass timber is suited to producing large strong panels with
long spans capable of supporting considerable loads, so it
follows that large amounts of mechanical and electrical (M&E)
services equipment are commonly specified for installation on
a flat CLT roof to take advantage of that. Banks of
photovoltaic panels can be commonplace, as are handrails
and balustrades on residential terraces, and other plant
equipment. All require fixing down and all therefore require
waterproofing around their base or feet. On larger roofs there
can be many hundreds of metres of joins/laps in membranes
and hundreds of upstands to waterproof. Even on smaller
roofs it is commonplace to have tens of metres of membrane
seams and numerous difficult-to-achieve waterproofing
details around penetrations. With the best will in the world and
the most diligent of roofing contractors, leaks inevitably occur.
Even if perfect waterproofing was achieved, a busy
construction site can easily damage roofing membranes.
The issue is that rainwater bypassing the waterproofing has no
pathway to drain away or dry out. Even with minor leaks the
water has no other option but to accumulate within the roof
construction. It percolates past the insulation and sits on top of
the VCL, which is not intended to be a fully waterproof layer.
There are numerous cases in the UK where standing water has
been discovered within CLT roofs detailed this way. In some
cases, 15mm or so of water has been found at the bottom of
the insulation layer. The moisture negatively affects thermal
performance of the insulation, but the greater threat is from a
trapped reservoir of water with the potential to cause very
severe but unnoticed fungal decay in CLT when inevitably the
moisture finds a way past the VCL and into the wood.
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The large mass of timber and large panel sizes that are rightly
seen as major benefits of CLT are actually downsides in this
scenario. Large panels mean that water can track across the
VCL for several metres making the location of any leak
difficult to identify. And CLT panels can soak up a huge
amount of water. This means that leaks may go completely
unnoticed for long time in flat CLT roofs or terraces
constructed this way and there are many cases where
extensive fungal decay has developed as a result. Even if a
leaking waterproofing detail is repaired, the CLT remains wet
and at risk from fungal decay until it dries.
Grumbles about the competency of some contractors may hold
true to an extent but the situation could be helped by designs
and specifications that avoid parapets and balustrades, M&E,
and support brackets and upstands – all of which invite a large
number of waterproofing joints or seals, which make perfection
difficult to achieve. It is a fact that leaks occur very commonly
and so, to maintain status as a long-lasting option for roofs or
terraces, CLT needs to be able to deal with the reality of
rainwater bypassing the waterproofing. That will require
significant changes in detailing, or even avoiding specifying
CLT for those applications. Solutions published recently3
include a move away from flat roof or terrace panels and to
recommend a minimum 10° fall in any roof panels.
Although these new recommendations do not specifically state
that users should avoid using CLT panels as flat roofs or
terraces, the intention is for the 10° fall to preclude it. The
introduction of a minimum fall in roof panels is to prevent
rainwater penetration from accumulating on horizonal surfaces
by directing it to the bottom of the falls, where drainage pathways
can be provided. It is a step in the right direction, although work
is still needed on how best to detail those drainage pathways.
Fungal spores are carried by rainwater, but fungal decay
does not develop the moment a leak occurs and spores come
into contact with the wood. A panel must remain wet for over
a year before fungal growth becomes a real concern (it may
initiate much sooner but tends to remain superficial). Eighteen
months to two years are usually required for the fungi to
become well established enough for rot to be potentially
structurally significant in certain timber species – particularly
those used for CLT panel construction. >>
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Technical
Durability
Fungal decay from a green roof waterproofing failure. Photo: Nick Clifford, BM TRADA
Early warnings of leaks are an obvious advantage and
electronic moisture sensors offer a solution, although they
tend to be limited by battery life. They also tend to be fitted
retrospectively. It would be preferable if sensors could be
hard-wired (to rooftop photovoltaics or normal circuits) and
included in normal building management systems to
overcome battery life issues and provide leak warnings
throughout the life of the building. To date, sensors have
usually been installed in cut-outs on CLT panel surfaces.
A further improvement would be to also have a network of
sensors on top of the VCL, where moisture from leaks
accumulates. This would alert users to leaks even earlier.
Early warnings of leaks are important, but to avoid a build-up
of moisture which will eventually find a way into the CLT, a
pathway for drainage and ventilation is critical. This pathway is
worryingly absent from the CLT/VCL/insulation/waterproofing
type details widely used for CLT flat roofs and terraces to
date. A high likelihood of fungal decay should therefore be
expected when leaks in waterproofing occur in roofs built
that way, and this is borne out by experiences across the
UK and further afield.
Providing drainage and ventilation are fundamental design
principles to mitigate against decay risks for all structural timber
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systems and mass timber is no exception. However, incorporating
drainage and ventilation within CLT flat roof and terrace details
presents some difficulties. A pathway for through-ventilation
requires a void. That would be relatively simple to achieve
using battens and plywood or other boards, but ventilating the
void externally would encourage a flow of cold air beneath the
insulation and sacrifice thermal performance. Any void beneath
the thermal envelope would therefore need to be vented to inside
the building. It may be that this could be achieved using slots
or holes in the CLT in various locations, although consideration
must also be given to avoiding loss in structural performance
and compromising any fire performance requirements, perhaps
using intumescent products to close off vents in a fire.
Ultimately, the issue of coping with waterproofing leaks must
be addressed technically because the current approach
where CLT is commonly specified for flat roofs and terraces
is not sustainable from a decay avoidance perspective. If we
do not resolve the design challenges described above it
may be advisable to avoid specifying CLT panels for flat
roofs and terraces at all, because the high incidence of
decay in those panels will continue. However, once they
become widely adopted, the new recommendations where
technical details include minimum falls in roof and terrace
panels would be expected to help significantly. >>
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Technical
Durability
Part 3: CLT buildings in service:
plumbing leaks
The third major source of in-service wetting of CLT buildings
may well be more troublesome to overcome. Plumbing leaks
in wet rooms have caused severe fungal decay in some UK
CLT buildings. In multi-occupancy residential properties
there are cases in the UK where multiple apartments have
suffered fungal decay because of leaks, usually in bathrooms
with repairs required. The need for repair or replacement is
concerningly common.
Pipework is hidden for obvious reasons and waterproof
coverings are needed in wet rooms, therefore leaks can easily
go completely unnoticed for a long time, particularly because
CLT can suck up so much water before anyone knows. Warm
temperatures, high moisture levels, large volumes of timber and
lots of time are the perfect ingredients for a decay outbreak and
severe damage can and does occur relatively quickly.
Facilitating replacement of a full-sized CLT floor panel
within a building potentially needs significant disruptive and
expensive works. In CLT buildings floor panels often provide
structural support to the walls above, and this can present
structural load-pathway problems when designing a repair to
a panel that may well have very limited access pathways.
To repair rotten CLT floor panels, in-situ decayed wood can be
routed out and replaced (for example with layers of structural
plywood fixed to the sound CLT). It sounds straightforward, but
this is only an option if the damage is within certain limits. An
important factor is the thickness of sound timber that remains,
which must be able to receive fixings to their required depth.
Because site-applied adhesives cannot be included in structural
calculations for timber repairs, structural engineers have to rely
fully on mechanical fixings, usually screws (although site-applied
adhesive is sometimes included for good measure). For timber,
structural calculations that use screws include an embedment
depth. If there is not enough sound timber left to receive the
fixings and achieve that depth, repair becomes difficult to
calculate structurally. In summary, calculating and detailing
in-situ repairs to structural panels presents difficulties that
require careful consideration by suitably qualified professionals.
Wet room floors
The industry is aware of the issues with CLT wet room floors
and is considering various ways to mitigate the risk from
plumbing leaks by finding better repair and replacement
options. For example, using smaller panels for wet rooms can
make replacement easier. Another way to improve reparability
is by hanging wet room floor panels from the walls in a similar
20 | Timber Industry Yearbook 2024
Plumbing leak damage in a school WC. Photo: Nick Clifford, BM TRADA
manner to conventional joisted floors, rather than using them
to support the wall panels above, which is commonplace for
CLT. If there is a need for repair or replacement, the damage
has already been done and not only to the CLT panels.
Confidence in mass timber in the minds of the end-users
and other stakeholders is also affected. And, crucially, in the
eyes of insurers too, if another potentially substantial claim
has transpired as a result of a relatively minor and commonly
occurring leak in a CLT building wet room.
Only focusing on replacing or repairing decayed panels in
a better way does nothing to avoid a reoccurrence if a leak
occurs again if it is simply repaired or reinstated using the
same details. The same decay risk is simply built back in, and
unfortunately some UK CLT buildings have been repaired
using exactly this approach, which fails to mitigate that risk.
Far better to reduce the likelihood of structurally significant
decay in wet room floors, although achieving this will also
require design changes that may seem unpalatable.
Mitigating damage from plumbing leaks
The industry has started to offer solutions that potentially
reduce the risk of leaks, which is a positive move. However,
some of the proposed solutions are thin on practical details and
need further work if they are to provide meaningful reduction in
the decay risk. For example, there are proposals for ‘tanking’
CLT wet room panels to prevent moisture from leaks coming
into contact with the CLT. But those proposals do not currently
provide key information such as how to detail tanking at a wet
room door threshold to prevent overflow, or how to otherwise
allow moisture to drain away. Penetrations through tanking will
also still be needed, for example to fix the WC to the floor.
A ‘tanking’ approach also places even greater reliance on
contractor skills and workmanship. It’s likely that tanking will be
bypassed by moisture. It may be preferable to avoid specifying
CLT for wet room floors and to use other materials.
Technical
Durability
Of course, decay could
potentially occur if
wet room floors were
traditional timber
joists instead, but
that damage would
be much easier and
cheaper to repair.
Wet room floor joists
within an otherwise
CLT structure can be
detailed so as not to
provide support to any
structural CLT panels
above, so decay in
joists would also be less
Decay can develop behind wet room wall
linings. Photo: Nick Clifford, BM TRADA
likely to present wider
structural concerns than
for CLT, which often supports other panels. This also makes
repairs far simpler and less disruptive. Leaks in traditional
timber joisted floors also tend to be spotted much more
easily and quickly than in mass timber panels, so any decayinducing wetting is far less likely to go unnoticed or unreported
for prolonged periods. Being able to spot leaks much earlier
would bring a reduction in severe damage incidents.
The use of wood preservatives to make CLT panels in wet rooms
more resilient to fungal decay for longer is potentially beneficial.
However, it would be difficult to achieve any meaningful increase in
decay resistance on site using brush or spray applied preservative
treatments, and it is not clear how such treatments would be
specified for site application to CLT, in terms of preservative
loadings and penetration requirements. For preservative treatments
to provide a robust and consistent increase in decay resistance
for wet room CLT floor panels it is likely that factory-applied
pressure impregnation treatments would be needed, and some
manufacturers are starting to provide treated CLT panels.
Whatever the solution is, without making changes in the way
we specify CLT around wet rooms there is unlikely to be any
reduction in the number of rot problems in CLT buildings for
the foreseeable future.
Addressing the challenges
Mass timber has so many positive aspects. Compared to
other construction methods it surely leads the way in terms
of sustainability and environmental credentials. There are
many fantastic CLT buildings and imaginative mass timber
projects already and hopefully it will rightly become the obvious
choice for many more specifiers going forward. A weakness
of CLT though is its durability and that absolutely must not
be overlooked in the design. As a community, mass timber
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stakeholders need to give much closer attention to the challenge
of decay risk mitigation. Hoping for improvements in contractor
competency and better communication between all parties is
not enough. Improved mass timber repair and replaceability
methods may do nothing to avoid a reoccurrence. Well-informed
technical changes are needed in the use and specification of
CLT for roofs, terraces and wet rooms if we are to enable mass
timber to reach its full potential in the longer term. n
About the author
Nick Clifford
Senior Technical Consultant
BM TRADA
Nick Clifford is a senior technical consultant providing expertise
in assessing the condition of timber structures, flooring, cladding
and decking for the BM TRADA timber consultancy team.
References
1. BM TRADA, Research Summary: Cross-laminated timber
and moisture (2021), https://issuu.com/warringtonfire/
docs/bmt_research_summary_cross-laminated_timber_
moistu?fr=xKAE9_zU1NQ
2. Shirmohammadi, M., Leggate, W. and Redman, A., (2021)
Effects of moisture ingress and egress on the performance
and service life of mass timber products in buildings: a
review. Construction and Building Materials. 290. https://doi.
org/10.1016/j.conbuildmat.2021.123176
3. Waugh Thistleton Architects, The New Model Building, https://
timberdevelopment.uk/resources/new-model-building-guide/
Further information
• CROSS, Rotting of cross-laminated timber (CLT) roof panels,
(2019) https://www.cross-safety.org/uk/safety-information/crosssafety-report/rotting-cross-laminated-timber-clt-roof-panels-852
• CROSS, Water ingress to cross-laminated timber structural
frame, (2022) https://www.cross-safety.org/uk/safetyinformation/cross-safety-report/water-ingress-crosslaminated-timber-structural-frame-1124
• Structural Timber Association, Moisture management
strategy: Process guidance for structural timber buildings,
Version 1.0, (July 2022) https://www.structuraltimber.co.uk/
wp-content/uploads/2022/09/STA-Moisture-ManagementStrategy-v1-July-2022.pdf
• The Alliance for Sustainable Building Products, Mass Timber
Insurance Playbook, https://asbp.org.uk/project/masstimber-insurance-playbook
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